Std string to character pointer. Use it when you want to get a C string from a C++ string.

• Std string to character pointer Since std::string::c_str() and std::string::data() return const char*, I can't use them. The idea is to copy the contents of the string array to another array using pointers and print the resultant string by traversing the new pointer. If you want an assignable array, use std::array, std::string or std::vector. The strcpy() function doesn't have overload to accept two std::string objects as parameters. Ignoring the const stuff, you are declaring a char** word, which is a pointer to a pointer to a single char. const char* bar; { std::String foo = "Hello"; bar = foo. push_back(e. In this article, we will learn how to convert a std::string to char* in C++. A terminating null character is automatically appended. We run a loop until the null character ‘\0’ is found in the source string and copy each character from the source string to the destination Hello, I need to pass two string variables (router, pws) to two char pointer variables (ssid, assword), I am using this code: char* ssid = ""; char* password = ""; String router = "ThisMyRouter"; You can store the String in a char array, then The returned array is not required to be null-terminated. In the second case, actual memory is allocated and initialised on the stack at runtime (or in an appropriate section of the process at program startup if it's not a local variable), so modifying the memory is OK. The conversion from pointer to string works fine, but not the A char* is basically a pointer to a character. const char* c_str ( ) const; Get C string equivalent. Double edit - doing it in a more C++ fashion Constructing a std::string from a literal char const[N] is already a low performance solution; you already have the size of the string at compile time and you drop it on the ground and then at runtime walk the buffer to find the '\0' character (unless optimized around; and if so, the null check is equally optimizable). In my example, ptr1 is not a pointer to a string; it points to a single character, and it is not part of a null-terminated array of characters (which is what a string is). That would be perfectly @bratao: You cannot delete memory allocated in the DLL from the main application. Not all char * variables are pointers to strings, though. When you create another string literal "new string" and assign it to str1, all you are doing is changing where the pointer points. You ask How can I access each member in a std::string variable, from which I assume you are asking how to access the string character by character. The strings types that are covered include char *, wchar_t*, _bstr_t, CComBSTR, CString, basic_string, and System. If the Python strings can contain null characters, you'll have to use PyString_AsStringAndSize to convert, and pass around the two values (the char* and the Py_ssize_t); there's an explicit conversion of these to std::string as well: std::string( pointer, length ). – I am currently analysing the char and string datatypes. When using operator+ to concatenate strings, at least one of the operands must have type std::string, the other one may be const char* or something convertible to that. I've seen several closely related SO questions, but most appear to illustrate ways In that case, the interface should say something about memory management. There may be ways around it, I am not sure, but I think you'll need to do it yourself if you want UTF-8. If quotes from the standard plus logical reasoning cannot prove this for you, I give up. The definitions of the operations are supplied via the Traits template parameter - a specialization of I have googled and have found that the char pointer returned would be invalidated as soon as someString meets its destruction. Also, your function lacks a return if the passed argument is neither enum, pointer nor an arithmetic type. namespace std { typedef std::basic_string<char> string; }; so it's a synonym for the instantiation of the basic_string template class for characters of type char (this doesn't change anything in the answer, but on SO you must be pedantic even on newbie questions). But we can also directly convert std::string to char array by using std::copy() function. Converting String to Char #include <iostream> #include <string> using namespace std; int main() { string str = "I love C++"; //create a new character pointer of length + 1 char *c = new char [str If you're insisting on using C-compatible character pointers, I think you'll have the best luck using a char ** as the type for input. Using namespace std and replacing tstingstreamwith stringstream. char* ptr = p;) So you can do char *ptr; then ptr = p; to make that clear. The thing about std::strings though is that they grow as needed, which in turn means they reallocate. The class is dependent neither on the character type nor on the nature of operations on that type. std::string stupid; const char *pointless = stupid. The function reinterpret_cast<const uint8_t*> rejects my string. This takes two parameters - a pointer to the array and a length: std::string x("pq\0rs"); // Two characters because input assumed to be C-String std::string x ("pq You can use copy() to copy into a string: std::vector<char> vec(100) strncpy(&vec[0], "blah blah blah", 100); std::string vecAsStr( vec. As long as you (and anyone else who ever maintains this code) have the The class template basic_string stores and manipulates sequences of char-like objects, which are non-array objects of trivial standard-layout type. printf does not know it because the argument type goes lost and is supposed to be restored from the format parameter. – Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company What would be the code to convert an std::string to unsigned char* and back? str = "1234567891234567" unsigned char* unsignedStr Using vectors instead of raw pointers: using namespace std; auto str1 = string{"1234567891234567"}; vector<char> chars{ begin(str1), end(str1) }; vector<unsigned char> uchars inputFileName is just a (dangling) pointer, it doesn't provide any storage for your string; you have to allocate some memory for the string, as for now you're just telling to cin to write the read characters at a random (potentially invalid) memory location. std::vector<char> copy = _v; char * buffer = &copy[0]; Of course, you can also access _vs buffer if you don't actually need to copy the data. You're working with pointers. The string is only a variable length single word. Edit: I have to wonder, however, why getnmberofwords would be written to accept a char * unless it's some old C std::string will make a copy of the null terminated string argument and manage that copy. Asking for help, clarification, or responding to other answers. And even if you change it to *_out_damned_spot = "dummy string", using a single out param in C++ is an attempted optimization that's normally unnecessary. (until C++11) The returned array is null-terminated, that is, data() and c_str() perform the same function. In principle it's no different to doing s[0] = 'a'; s[1] = 'b'; etc. – If the std::String goes out of scope then the it is no longer a valid char pointer. Just binary words of the appropriate width (usually 32 or 64). Using Pointers. That's the appropriate way to handle that sort of function. The returned pointer should point to a char array containing the same sequence of characters as If you just want to pass a std::string to a function that needs const char *, you can use . But the copy is destroyed at the end of the statement, and so stam is left I'm passing a std::string pointer to a function and I want to use this pointer to access and modify characters in this string. Parser using command line args: I thought const char* represents a mutable pointer to an immutable string. If you want to create a string, do it explicitly using std::string(val). Given this, how does one fix this issue? How can I return a char pointer accurately? Returning std::string would resolve this issue. This constructor deep copies the character list at the pointer's location. Using the internal char* buffer of the string is perfectly fine and the prime way of calling e. c_str(); Or you need to modify the char array so constant is not ok, then just go with this . std::string s_arr[3] = {"Good","Better","Best"}; string* str_p = s_arr; if you want a pointer pointing to the start of an array of strings. There is no way to get a char* from a string that is guaranteed to work on all platforms, for the simple fact that string is not required to use contiguous storage. In this article, we will learn how to store the array of pointers to strings in C++. For char data types, the following code snipped holds good: char value = 'a'; char value1[] if you want a pointer to a string and. Handling Is there a way to convert a C string (char*) into a stl C++ string (std::string) without requiring std::string to internally alloc/copy the data? Alternatively, could I use stringstreams or Ok, i am shocked that no one really gave a good answer, now my turn. You cannot pass in the std::string directly because, while you can convert it to a C string, it is laid out in memory differently and so the called function would not know where to put its result. This is only useful if you have a 7-bit ASCII strings and you need to call an API that requires wide strings. This trims the string to the first \0 character. For example, the British Pound sign, '£', is converted to the single-byte 0xA3, You just needed to cast the unsigned char into a char as the string class doesn't have a constructor that accepts unsigned char:. imbue(std::locale("C")); string str = strstream. C++ Program to use std::string::c_str() to get a const char* pointer to the string data that is read-only for the C or Fortran function (or C++). This is one key difference from C-style strings; a C-style string can either be NULL or a valid string, whereas C++ std::strings always store some value. This can obviously be constexpr. A char* value can also represent a C-style string. This means that a std::string object (typically) has a private C-style string that is used to implement its functionality. #include <iostream> #include <sstream> #include <Windows. What is the way to allocate memory for an array of strings? You cannot assign NULL or 0 to a C++ std::string object, because the object is not a pointer. That means that you cannot pass it to a function expecting a null-terminated string, like foo (how else are you going to get the size?) that expects a const char*, and so it was decided that it wasn Historically, in C language strings were just a memory areas filled with characters. The only difficulty is ensuring this isn't used after the owning vector goes out of scope. This function returns a const char* so you'll need to change the definition of x from char* to `const char*. You can use the ATL text conversion macros to convert a narrow (char) string to a wide (wchar_t) one. For now I have this. Using string::c_str function. Should I always pass a std::string by const reference to a function if all that is done inside that function is to copy that but it just doesn't sit right. Check this question: How to convert an integer to a string portably? Note that itoa function is not defined in ANSI-C and is not part of C++, but is supported by some compilers. Example. String data is easily and efficiently passed between std::string to / from a C or Fortran function that expects a char* pointer. I need to pass it onto a method which accepts char**as an input parameter. I have a vector<std::string> variable. You can almost certainly give it a pointer to the std::string's buffer (a char*), safely casted to unsigned char* (though be careful to read what this library function does and ensure it's compatible with the constraints placed upon you by std::string). str() returns a copy of the string, you then call c_str() on this copy. This is more of the usual way to do this (in C at least), and it has the added benefit of not forcing you to define a maximum string size. end()); As mentioned above, you should consider keeping the sub-string as a std::string and use c_str() method when you need to access the underlying chars. An std:: Reasons to use an std::string over a char*: Much more intuitive to use. Otherwise std::vector<char> is the way to go. For example, to convert a std::string: #include <atlconv. So I have this recursive descent parser that works fine recognizing and using values through command line arguments but I am not sure how to port this to reading from a . nm[a] and nm[b] are very strongly const because nm is a truly const object. However, when I do this, #include <iostream> using namespace std; const char *name1 = "Alex"; int main() { name1 = "John"; cout << name1 << endl; } it just prints John and shows no problems. This method is very dangerous and should not be used: toStdString() return a new std::string object and then the pointer to internal data const char * is obtained. So, here are alternative versions of a constructor, then you should have a boolean to say "doesn't exist", or a pointer to a std::string that can be NULL if no string is present. To use a char *, you should look up functions inherited from C to handle strings (e. – Is the following implementation-defined: signed char *cp = "\x96\xA0\xB4\xBE\xC8"; and as well as: std::string = "\x96\xA0\xB4\xBE\xC8"; On systems with 8-bits wide signed char, yes. This makes your comparison clearer. It's a non-standard function, thus you should avoid using it. Consider the following: char* carr = new char[27]; //Array of 27 chars for (int i = 0; i < 26; i++) { carr[i] = 'a' + i; } carr[26] = '\0'; In spite of the above, don't use pointers to std::string, unless you determine (by measuring) that string copies are the bottleneck in your application. You need to call a member of std::string which returns a const char*. I wonder why the program treats name1 as a string and makes it mutable? In modern C++, this shouldn't compile, since the literal is constant, so only a const char * can point to it. convert from string to char* In this simplified example there is an assignment of std::string::c_str(), which returns const char*, to a char* pointer using const_cast. var1 is a char pointer (const char*). It checks whether data is long enough, and if so allocates memory for str and uses std::copy similarly to This post will discuss how to convert a std::string to const char* in C++. It is not copying the character How can I access individual elements in a std::string with pointers? Is it possible without type casting to a const char *? #include <iostream> #include <string> using namespace std; int main() { // I'm trying to do this Using the internal char* buffer of the string is perfectly fine and the prime way of calling e. You cannot pass std::string between application and DLL, you instead pass char* as Mark suggested. That's useful in some cases, e. – You should use std::swap (located in either the <algorithm> or <utility> header) instead of rolling your own:. I figured it out. As a newcomer to C++, I'm trying to piece together how to perform this conversion on each element of the vector and produce the char* array. With those guarantees, there aren't really any A std::string_view can refer to both a C++ string or a C-string. It has only one overload to accept two char* pointers as parameters. const char *) of string contained within it, that is null-terminated. If you want the address instead, you can just cast it to a pointer that isn't treated that way, something like:. C++ std::string with char* May 5, 2024. What you have to do, is make sure you allocate enough space in the string. When you write str1 = "Hello";, you are creating a string literal in memory and making the pointer point to it. Each of the n characters in the string will be initialized to a copy of this value. You're not working with strings. So my question is, how do convert a char*[] into a std::string[], so that you don't have to convert them all individually as you progress in your program? std::strings (from the C++ Standard string class) --> Returns a pointer to the first string s2 in string s1 C-style strings are relatively unsafe – if the string/char array is not null-terminated, it can lead to a whole host of potential bugs. Skip to main content. So I was allocating a temporary buffer, calling a function with it and copying it into std::string. size () + 1]; You can use char writable [str. , a C-string) representing the current value of the string object. While a bit unorthodox, it's perfectly valid to use std::string as a linear memory buffer, the only caveat is that it isn't supported by the standard until C++11 that is. The function std::string::c_str() returns a pointer to this underlying C-style string. std::string buffer(MAX_BUFFER_SIZE, '\0'); TheCLibraryFunction(&buffer[0], buffer. Why not doing the same in the sketch? Suggested code for defining a char pointer constant in a sketch code: const char* ssid Yes, there’s a difference. This article shows how to convert various Visual C++ string types into other strings. 4) that is not a wide string literal can be converted to an rvalue of type “pointer to char”; Actually, any narrow string literal is of type "array of n const char", but as you can read above, there's a (already in C++03 deprecated) feature to implicitly convert them to rvalues of type char *. Provide details and share your research! But avoid . If we are working with C-style strings, we can directly declare the pointer to array of strings as double pointer to and for std::string object we need to declare a pointer of std::string type. I need to save part of packet to a buffer, A pointer to a destroyed object. const char * s1 = "test"; char s2 [] = "test"; Both lines of code have the same behavior, so I cannot see any difference whether I should prefer s1 over s2 or vice-versa. You need to use if constexpr. In all cases, a copy of the string is made when converted to the new type. A hex escape sequence in a narrow string literal has an implementation-defined value if it falls outside of the implementation-defined range defined for char. n Number of characters to copy. How can this To change std::string to char array, we can first use string::c_str () function to get the underlying character array that contains the string stored in std::string object. About; Products during the lifetime of the function, you can do a cast on a string pointer, turning it back into a reference in the callback: #include <iostream> #include <string> void Callback(void *data) { std::string &s There are many way to read text from stdin into a std::string. They are not similar in that regard to, say, std::vector's, in which moving takes away the allocated space. begin(), In that case you do not need to free the pinned pointer. 2) Convert it to a char pointer so I can pass to a tabling/palindrome finding function std::string myString; LPCSTR lpMyString = myString. Also, beware that QString This code searches for the character 'W' in the string "Hello, World!". str(); const char *sql= str . Internally a std::string has a pointer to a fixed-length buffer. Such strings were delimited with a zero-character ((char)0 a. – Some programmer dude I would just build a vector of pointers to the c_str()'s of the strings and then get a pointer to that. c_str()); loadU(pszWide); You can also specify a code page, so if your std::string contains UTF-8 chars you can use: @shoosh: I understand your concerns about compatability, but both QString and std::string can contain null characters. Unless, You are But if the destination string is a char* then I'm not sure I'm doing it right. Futhermore you cannot static cast pointer to an integer (C2440). I have a std::vector<std::string> that I need to use for a C function's argument that reads char* foo. Commented Apr 15, 2021 at 10:47 Pass std::string to a function f(**char) 38. You can then use it as any other const char * variable. If you want/need a copy you'll need to make one yourself using strcpy. Additionally, how you operate on the pointer is incorrect. Since you are declaring a class, you obviously also do not need compatibility to the C programming language. #include <string> using namespace std::string_literals; std::string Str{ "Stackoverflow"s }; /* const */ char The safest way to copy a vector<char> into a char * buffer is to copy it to another vector, and then use that vector's internal buffer:. The class is dependent neither on the character type nor on the nature of operations on that type. So you can do: tempMonth=month; This will point the unassigned pointer tempMonth to point to the literal bytes allocated in the other 5 lines of your post. Also, beware that the pointer will be invalidated if the vector is resized. Be aware: this pointer is only valid for the life time of the std::string or until you modify the string object. It is essentially a const char* to the character data and the string length, combined into one object. To do this I need to convert the char* return value from fgets() into an std::string to store in an array. Study C strings on a book before messing with them. c_str(); One thing to be careful of here is that c_str does not return a copy of myString, but just a pointer to the character string that std::string wraps. However, looks like there is no way to detach the raw pointer from the string or is it? People often call a char * variable a pointer to a string; it means that the pointer points to a null-terminated array of characters. To create an actual std::string, use something like: std::string s = A std::string_view doesn't provide a conversion to a const char* because it doesn't store a null-terminated string. g. AFAIU std::string_view is a dynamic object, so memory will be dynamically allocated. And finally, neither void in the argument list of Why is it, that I can assign Multiple chars, or to say a string, like "name A" to a pointer which should point to a char. " Assuming you have this: std::string str = ; void *ptr = &str; You can just If empty() returns true, the pointer points to a single null character. vector<char> chars(my_string. However, the string object is immediately destroyed after this statement, so the result pointer probably does not have a valid address if you use it in a subsequent statement. And B: Only be able to assign a pointer, to one single letter (a char), to each one. So, just This takes two parameters - a pointer to the array and a length: std::string x("pq\0rs"); // Two characters because input assumed to be C-String std::string x ("pq You If you really need a writable buffer of char that is copied from the contents of a std::string, you can use std::vector<char> std::vector<char> urlAsVec( sup. Since both answers were a bit old, let's have an update. NULL is a perfectly valid pointer-to-char which happens to not point to anything, so it is stored in the Hello, I need to pass two string variables (router, pws) to two char pointer variables (ssid, assword), I am using this code: char* ssid = ""; char* password = ""; String router = The behaviour of the first two calls to compare is entirely dependent on what random memory contents follows the address of each char. But when I open the file I see boxes instead of the string. Then you can easily have the pointer to the first and the last char of the string by the help of member functions std::string::front and std::string::back respectively (given that the string is not empty). The C++ std::string class is likely to use a char* array internally. std::string str1 = "Apple"; std::string str2 = "Potato"; of course the std::swap algorithm will still work just fine. In your example one and two are char pointers, pointing to char constants. Also works if you just need it as an integer: However, a lot of people prefer the convenience of string manipulation, but it would be a hastle to have to convert every char* into a std::string every time. Mainly because you can modify your string but you cannot modify your first version – but the C++ compiler won’t even warn you that this is forbidden if you try. std::cout << ptr; will output the string pointed to by ptr. #include <sstream> std::string narrow = "narrow"; std::wstring wide I want to write a std::string variable I am accepting from the user to a file. This function works for both iterators and pointers so we can copy the std::string objects (uses iterator) directly to character array (uses pointers). This pointer can be cast to a const uint8_t*: I have found an article which describes how to convert String to char array: https: It can only hold pointers to strings. I want an std::string object (such as a name) to a uint8_t array in C++. The size of this array is specified in the cbData parameter. For std::string object use the below syntax: We can store almost all types of data as array elements. strcpy_s() There is no real reason to use raw strings in this case. An std::string is a class that (typically) wraps a C-style string. However, in your case, if you want to keep existing heap buffers and treat them as strings - it can't be using std::string's, as std::string s="aaa"+1; This just compiles, but most likely does not do what you want: It adds 1 to the const char* the literal "aaa" decays to and then constructs the std::string from that pointer, resulting in s == "aa". You then go on to say A char* is basically a pointer to a character. begin(), sup. data (); . If you'd like to reserve a sentinel value (say, the empty string), then you could do something like The default constructor initializes the string to the empty string. h> int The problem is that the string data your const char* points to has been freed after the return str;. So you can do: tempMonth=month; This will point the unassigned pointer tempMonth Based on your comment "What I meant was to convert what the void* is pointing to (which is a string) into a string. It's safer to use std::string because you don't need to worry about allocating / deallocating memory for the string. But when I open the file I see boxes instead of I have seen many similar questions but none that seem to be working for my code, I think I am overlooking something basic, maybe you can help. If it is null-terminated, then certain C functions will treat it as a string, but it is fundamentally just a pointer. size()); However, the size() of the string is the actual size, not the size of the string containing actual valid non-null characters (i. Just call either the data() or c_str() member functions of the std::string class, to get the char* pointer of the string object. The definitions of the operations are supplied via the Traits template parameter - a specialization of std::char_traits or a compatible traits class. If you want the value of str to be changed, you have to pass it to func by reference or by I was working on a little project and came to a situation where the following happened: std::string myString; #GetValue() returns a char* myString = myObject. When dealing with lower level access like talking to the OS, but usually, if you’re passing the string to the OS then The question initializing std::string from char* without copy looked promising, but my code relies on API calls ("WriteIntoArray" in the above example) which have to write into an Writing a null character at the end of a character vector will not magically create an std::string. append(), str += 'c', str. The std::string class has a constructor that accepts a const char* as an argument. I'm looking for a method, or a code snippet for converting std::string to LPCWSTR. Since this is a C++ question, I'd advise an idiomatic way to handle a fixed/variable collection of text: std::array or std::vector and std::string. All that std::string_viewneeds to store is a pointer to the character sequence and a length. To get "dog" accessible through such a pointer, you need to take an extra step; make a variable that contains "dog", and put its address into your word. Alternatively, use std::string instead of std::vector<char>, in which case you can get a NUL-terminated string with the c_str member function. GetValue(); My question is if GetValu An std::string is a class that (typically) wraps a C-style string. C++ std::string is a dynamic, contiguous container for character strings. s Pointer to an array of characters (such as a c-string). What is the best method of telling the std In the first case, "string" may be stored in a read-only area of the process, so attempting to modify the memory pointed to by p would cause undefined behaviour. Why it doesn't work with arrays: I have seen many similar questions but none that seem to be working for my code, I think I am overlooking something basic, maybe you can help. Better searching, replacement, and manipulation functions. But the description of std::string::c_str() in my local library reads as follows: Return const pointer to null-terminated contents. However-If you must create the new string as a dynamic char array via new you can use the code below. When the buffer is full and you request to add one or more character onto it, the std::string object will create a new, larger buffer instead of the old one and move Based on your comment "What I meant was to convert what the void* is pointing to (which is a string) into a string. If possible, however, you should rewrite the function so it takes a std::string& or std::string * as an argument. ofs. Share. So how can be this constexpr?. The class template basic_string stores and manipulates sequences of character-like objects, which are non-array objects of TrivialType and StandardLayoutType. This removes all the usual risks that come with using raw pointers, such Actually std::string is defined as. If you want them to be reinterpreted, then you should use reinterpret_cast. reserve(input. txt is UTF8 encoded, and read_raw() reads bytes from file into memory and returns pointer to the first byte as const char* QString qstr = QString Is it OK to throw a std::string pointer? But isn't the throwing a const char pointer to a local variable a bug? Yes, of course I know that the compiler will place the c-string in the unmodified section which will not change address till an app running. Whenever you return a pointer from a function, you need to say who is responsible for allocating the data that's pointed to, how long it remains valid, and who is responsible for freeing it (if applicable). k. Let's suppose that char *str; points to a C-style string. As for "proof", see this answer. c Character to fill the string with. String. So what you're doing is correct, the only improvement I'd suggest is a check for nullptr, assuming that is a valid return value for f(). The simplest implementation is something like: Here I've two lines of code. Syntax to Declare Pointer to an Array of String in C++ char ** ptr. In GCC compiler, you can use a compiler extension "Conditionals with Omitted Operands" to create a wrapper macro for your C function You are trying to treat a char * as a std::string type object. You are calling The u_char * pfring_zc_pkt_buff_data returns a pointer to the actual packet captured by pfring_zc_recv_pkt(zq, &buffers[lru], 1). A pointer to char could refer to an array (aka string) of chars. Changes to that string will not be reflected to the ps buffer, and vice versa. If only the pointer itself was copied and not the Complexity Analysis. " Assuming you have this: std::string str = ; void *ptr = &str; You can just cast back to the string: std::string *pstr = static_cast<std::string *>(ptr); Note that it is on you to verify that ptr actually points to a std::string. – Criticizing Israel not allowed. A string literal like "on" is not a String, it is a C-style string. If an attempt is made How can I take ownership of std::string char data without copying and withoug keeping source std::string object? But even if you're willing to try this it won't always work. The pointer returned points to the internal array currently used by This is just a dumb way of throwing away type safety and expanding 7 bit characters from std::string into the lower 7 bits of each character of std:wstring. Note that \e isn't a valid character escape, you need to double the \ if you meant a literal \\. tl;dr: Not really. You need reinterpret cast. ; use std::string::data() to get a char* pointer to the What about removing the "String" bullshit from the code and only using nullterminated strings? The library you are planning to use seems to use only nullterminated strings and no bullshit String objects at alll. Update 1: In a tool for creating a service method, i give parameters as std::vector, but it sets automatically the qualifier as &, which means my method definition generated by the tool will I've been trying everything under the sun to do the simple following: 1) Receive an input string from stdin. Array of Pointers to Strings in C++. – Ken Wayne C-style solution could be to use itoa, but better way is to print this number into string by using sprintf / snprintf. Lets dive in I need to use an std::string to store data retrieved by fgets(). If empty() returns true, the pointer is a non-null pointer that should not be dereferenced. Time Complexity: O(N); Auxiliary Space: O(1); 4. Specifically, it must be a pointer to a NUL character. destination = (&destination_string); return destination_string. data(); // get pointer to const char*'s - const char** @zak, internally the code in msclr::interop::marshal_context uses the Win32 API WideCharToMultiByte which ultimately converts to Windows-1252 encoding, at least in my experience. std::string_view provides the same I'm trying to convert a pointer to string, and the string back to the pointer (lexical casting) using the following code. Example Input:string myString = "GeeksforGeeks";Output:char * myString = "Geeksfor Given that an overload of operator<< on std::ostream for pointer to char exists, and given that the standard specifies the synopsis of the std::string class to be the following, in §21. There are four ways to convert string std::string to char* in C++. c_str (); And if you need a non-const char *, call . Using the STL, libstdc++ by the GNU, the performance of the following 5 methods of appending a char to a std::string will be tested: str. unsigned char* uc; std::string s( reinterpret_cast< char const* >(uc) ) ; However, you will need to use the length argument in the constructor if your byte array contains nulls, as if you don't, only part of the array will end up in the string (the I need to pass a string (std::string or a char[] is the s Skip to main content. txt"); // assuming hello. std::string s; char* s_ptr = &s[0]; // get at the buffer To quote Herb Sutter,. So you could probably assume that the first bunch of character insertions (up to the current capacity) won't practically invalidate your A std::string instance can be compared directly with a string literal using != or == operators. size () + 1]; Then you don't To convert a std::string to a char*, we can use the std::string:c_str() method that returns a pointer to a null-terminated character array (a C-style string). (since C++11) Well, without changing every place where a C++ std::string is initialized directly from a C function call (to add the null-pointer check), the only solution would be to prohibit your C functions from returning null pointers. But C creates these two abstract types to help distinguish two separate uses of a machine word: holding a numeric (or symbolic) value vs. An std::string is a class that is much like a vector. '\0') at the std::string::iterator it; char* c; if So dereferencing that gives you a pointer to the object. I tried with *word[i], *(word + i) where word is my pointer and i is an unsigned int. Otherwise you instantiate std::to_string with a pointer or enum, which fails to compile (C2665). However, the class will manage the allocation, reallocation, and deallocation of the internal array for you. However, sometimes I need to keep the raw char* pointer, even after the original std::string object is destroyed (yes, I know the char* pointer references the HEAP and must eventually be disposed of). std:vector<const char*> pointers; pointers. I use std::string/wstring for more or less everything that is actual text. Convert char* to string C++. char * is useful for other types of data though and you can be sure it gets deallocated like it should. Use it when you want to get a C string from a C++ string. , I just want to assign the value of, String^ originalString; To Whereas declaring auto str const char* str or std::string str wouldn't work. Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand; OverflowAI GenAI features for Teams; OverflowAPI Train & fine-tune LLMs; Labs The future of collective knowledge sharing; About the company Data type of pvData: A pointer to an array of BYTE values. You compare C-style strings with strcmp. I'm using a std::string to interface with a C-library that requires a char* and a length field:. This is necessary because the std::string constructor taking a char const * requires that the A char* is basically a pointer to a character. Supplementary answer, because apparently this answer with many upvotes is suggesting to use push_back('c') while another is suggesting not to. So anything like: strcat I suggest wrapping those const char * into std::string: const char *a = "hello "; const char *b = "world"; std::string c = a; std::string d = b; cout << c + d; Share. size(); } int main() { std::vector<char> buffer; buffer. Stack Overflow. In this case you only allocated memory for a (char*) but did not allocate memory to store the actual string. Improve this answer. char const* str = "name"; Now, if you try to modify the contents of str, If you have a C++ std::string object, the value of which you want to assign to pw, you can do it like . You aren't allowed to modify the string via a pointer returned from a const member function but that doesn't mean you aren't allowed to modify the string at all. Edit Of course, this isn't very relevant as a much better approach is to drop the comparison entirely, and rely on the find function that already exists to do what you want. Attaching the '*' to variable during declaration makes clear you are declaring a pointer to type, as char* a, b, c; certainly Because the second cout will print what is pointed by str. Initializing std::string from a NULL char pointer is undefined behaviour, I believe. So where does the NUL character come from? There are a couple of ways for a std::string implementation to allow this to work: Use small-string optimization, which is a common technique. So, although it isn’t formally guaranteed, in You are consistently missing the second *. 1. 13. unsigned char u_array[4] = { 'a', 's', 'd', 'f' }; #include <string> #include <iostream> #include <ostream> int main() { std::string str( u_array, u_array + sizeof u_array / In the above method, we have first fetched the underlying character array of std::string object. Because passing string literals into functions taking const std::string & will silently create string std::cout << ptr; will output the string pointed to by ptr. c_str() However, the value that pw points to will only be valid for the life time of some_string. , std::strlen to get the length of the string). We can easily get a const char* from the std You can pass std::strings by reference if they are large to avoid copying, or a pointer to the instance, so I don't see any real advantage using char pointers. There's no way to have it take ownership of a string you pass to it. for the 1st parameter in fmt:: In C++11, I recommend const char* in this case. Approach-3: Using without the c_str () and strcpy () function. Pointers don't have any special move semantics. So when you compare it to a char array, the array decays to a pointer as well, and the compiler then tries to find an operator == (const char*, The reason for that is that std::cout will treat a char * as a pointer to (the first character of) a C-style string and print it as such. When turning that std::string argument into const char* the resulting pointer will Although front() will throw here, my_string[my_string. For now, the only thing I am able to do is print my string by using the * operator but I cant't access only one character. In C, month == &month[0] (in most cases) and these equals a char * or character pointer. (also, don't confuse the '*' in char *ptr=p; with the dereference operator -- it isn't, it is just the type for the declaration, e. std::swap(str1, str2); Also, you should consider using std::string instead of const char* in general:. And the DLL can use std::string, but it is different from std::string in the application. size()); for (const auto& e : input) pointers. If you need to use a char pointer for whatever reason, make it const:. resize(str. Win32 API functions without allocating arrays of char yourself. If it wasn't, you could move C variables The class template basic_string stores and manipulates sequences of char-like objects, which are non-array objects of trivial standard-layout type. Can someone please post a simple code that would convert, System::String^ To, C++ std::string I. sqlite3_exec(db, sql, callback, (void*)data, &zErrMsg); Maybe it helps to somebody. I say "should" because it might not - some interfaces are poorly specified, fact of life. To make a string literal, it is also simpler to do this: char month[]="jan"; Cases where you might prefer char* over std:string. So always use the second version. (since c++11) From this we can confirm that std::string::size does not include any nul terminator, and that You don't have a vector of strings -- you have a vector of pointer-to-char. Note that the provided pointer should not be a nullptr, as the behavior would be undefined. how to do this ? If possible I need to pass a writable one. c_str(); printf("%s", bar); //SAFE since the String foo is still in scope } printf("%s", bar); //UNSAFE String foo is no longer in scope As long as the std::String variable exists you can use the const char Moreover, we can also convert a string to a char pointer which can be very useful when creating complex applications in C++. You might be thinking of std::string, which is dynamically I'm passing a std::string pointer to a function and I want to use this pointer to access and modify characters in this string. Avoid a lot of pain and just use std::string to work with strings. std::string_view is not dynamically allocated. What C does is frequently makes this pointer point to the first character in an array. But this is another variable than str in main. . Returns a null-terminated Unicode character string that contains the display name for the certificate. I want to convert this void* to a std::string but so far all I am getting when outputting my std::string is the first . You won't get a word or many words into that, and casting just hides the problem. For example, Since you already have a std::vector<std::string>, it's much simpler to let that own the memory, and build a parallel std::vector<char*> which just keeps pointers into the original strings. And String objects have a . char *pw = some_string. c_str(): std::string str; const char * c = str. size()] is nowadays deemed valid — you can't do much with it, but you can take and store its address (so, &my_string[0]). Why it works with pointers: When you say char * str1 in C, you are allocating a pointer in the memory. the equivalent of strlen()). Shouldn't I A: Only be able to assign the Address of a char to each of those 4 pointers I created. c_str(); pointless must be a NUL-terminated string. If an attempt is made to use p to modify the contents of the array, the behavior is undefined. If empty() returns true, the pointer points to a single null character. There is no easy fix to this. data simply assigns the memory address of a local std::string variable to a pointer that is also local to the function. Is std::string suitable for this or should I use a character array or something. In addition to s1 and s2, there is also the way of using std::string. Be sure to add a NUL to the end of your vector first though. The basic algorithm is: allocate std::string with desired size and fill with \0. This array includes the same sequence of characters that make up the value of the string object plus an additional terminating null-character ('\0') at the end. cout << (void *) terry; (or use the const void * cast if you're worried about casting away constness, something that's not an issue in this A std::string_view doesn't provide a conversion to a const char* because it doesn't store a null-terminated string. first, last Input iterators to the initial and final positions In your case it is an std::string not const char*. const char* x; std::string my_std_string(my_txt, my_txt + my_txt_len); This is assuming that you want the unsigned chars to be converted to char. 0. Consequently, when a string was passed to a function, it was passed as a pointer to its very first character, of type char *, for mutable strings, or char const *, if the function had no intent to modify string's contents. data(): std::string str; char * c = str. data() was There are four ways to convert string std::string to char* in C++. A little more explanation: Arrays (not just C-style strings using arrays of char but all arrays) naturally decays to pointers to their first element. Your safest, most portable course of action is to copy the string somewhere that does use contigious storage (a vector perhaps), and use that instead. I think the way of using std::string is the most elegant. The returned array should contain the same string sequence as the string object. std::string address = std::to_string((unsigned long long)(void**)this); Also implied, works with pointers of any type, not just this. fromStdString will _raw("hello. There is no need for a reinterpret_cast. Convert bytes to a string std::string object will allocate internal buffer and will copy the string pointed to by ps there. c_str(); Now you can execute sql statement. x = std::move(my_char_ptr) is the same as x = my_char_ptr. reserve(100); auto return_size = filter (buffer. Now I plan to work with big amount of information and copying this buffer will have a noticeable impact and I A simpler one-liner that doesn't require a whole stringstream:. c_str() member function that returns a const char*. At the level of the physical machine, they're the same. The pointer to a string-literal will always be valid and the string will always be null-terminated. The second one doesn't work because, unlike a string literal, a braced list doesn't create an array or anything else that you can assign to a pointer. c_str How to convert a std::string to const char* or char* 3832. h> std::string str = "Hello, world!"; CA2W pszWide(str. This seems to work as intended and str is modified accordingly. However, the comparison to NULL stinks. defines p with type "pointer to char" and initializes it to point to an object with type "array of char" with length 4 whose elements are initialized with a character string literal. char array[] = "Hello world"; // An array of 12 characters (including terminator) char *pointer1 = &array[0]; // Makes pointer1 point to the first element of array char *pointer2 = array; // Makes pointer2 point to the first element You cannot swap those pointers by reassigning the pointers, because those pointers point into a 2-D character array. dat file, using the proper char pointer, printing the string, and working for multiple strings. a. But I want to know if there is any other means of doing this. It stores a pointer to the first element, and the length of the string, basically. Assuming 8-bit signed char, any hex value This is fine. Syntax There is an important distinction you need to make here: is the char* to which you wish to assign this "morally constant"? That is, is casting away const-ness just a technicality, and you really will still treat the string as a const?In that case, you can use a cast - either C-style or a C++-style const_cast. It could well be slower than relying on RVO, since the caller has to create an empty string and then you assign to it, whereas with enough copy elision the caller's string can be constructed directly from the literal. It is forbidden to overwrite the final '\0' that is stored after the string data, but I assume sizeof("abcdef") is 7 because constant strings in C always get the implicit character '\0' at the end to terminate properly, so it is 1 more byte in your string (6 + 1 = 7); Share Improve this answer std::string::c_str() gets you a const char* pointer to a character array that represents the string (null-terminated). However, there's only questionable value in doing so. This is the more economic way of saying the same thing. I tried using the write() method and it writes to the file. You have a non-const string, so you can modify it. Thanks for contributing an answer to Stack Overflow! Please be sure to answer the question. How can I convert an std::string to a char* or a const char*? Instead of: char * writable = new char [str. More here : std::string::c_str() function returns pointer to const char buffer (i. That is an older syntax still in common use that means something else; a null pointer. So when you compare it to a char array, the array decays to a pointer as well, and the compiler then tries to find an operator == (const char*, const char*). I want to write a std::string variable I am accepting from the user to a file. Returns a pointer to an array that contains a null-terminated sequence of characters (i. In this article. Generates a null-terminated sequence of characters (c-string) with the same content as the string object and returns it as a pointer to an array of characters. Although unrelated to your problem, almost anytime you feel the need to do a C-style cast or conversion (like you do with (std::string)val) you should take that as a sign that you're doing something wrong. holding the address of another word (itself possibly a scalar (numeric) value or another pointer). I am using the std::string type for my string manipulations. begin(), vec. – IInspectable. Approach-1: Step-2: Then, substring is created to match in the given string . Commented String is a C++ class in the Arduino library. e. namespace std { template<class charT, class traits = char_traits<charT>, class Allocator = allocator<charT> > class basic_string { public: [] typedef implementation-defined iterator; So I have an std::string and have a function which takes char* and writes into it. Next step is: stringstream strstream; strstream. And str, the pointer, in your main function will have the same value before and after the call to func. About; then you can construct a CStringW from a pointer to char*, without introducing a pointer whose ownership is unclear, or using conversion macros that are hard to understand. Really just use : std::string input; std::cin >> input; and you're done. const char *array = tmp. I have seen how to convert a std::string to char*. end() ); defines p with type "pointer to char" and initializes it to point to an object with type "array of char" with length 4 whose elements are initialized with a character string literal. The const char* pointer will stay valid as long as the associated std::string Though in C string literals have types of non-const arrays it is better to declare pointers initialized by string literals with qualifier const: const char *line = "hello, world"; String If we are working with C-style strings, we can directly declare the pointer to array of strings as double pointer to and for std::string object we need to declare a pointer of std::string I tend to favour const std::string over const char * for string literals. You should not manipulate the data this pointer points to, so if you need to do that, copy the data. The returned pointer should point to a char array containing the same sequence of characters as present in the string object and an additional null terminator (‘\0’ character) at the end. It's called "deep copy". An std:: Reasons to use an std::string over a In C, month == &month[0] (in most cases) and these equals a char * or character pointer. Prior to c++17, use std::string instead of const char* string literals. A better way of writing this is trying to use iterators and the STL to minimize such errors: A string literal (2. size() + 1, The most obvious is to pass &your_vector[0]. The copy+move case should only be an extra pointer swap compared to the single copy of the const ref case, and I can't see that causing a 500-600% overhead. 4:. Indeed, in the func function, you are changing the value of the aString variable. c_str()); // get const char *'s auto argument = pointers. I have three examples: char* dest = new char; // Example 1 CString (or std:copy_n()). It is not a string. There are two cases; A constant char array is good enough for you so you go with, . Right now I'm constructing a string to be sent as a r C-style solution could be to use itoa, but better way is to print this number into string by using sprintf / snprintf. In C++, strings are the textual data that is represented in two ways: std::string which is an object, and char* is a pointer to a character. Every std::string implementation I know of is in fact contiguous and null-terminates its buffer. std::string has a constructor that takes a pair of iterators and unsigned char can be converted (in an implementation defined manner) to char so this works. If found, it prints the position of 'W'; otherwise, it informs that the character is not present. For now, the only thing I am able to do is print my From the console i am asking for a hexadecimal string to convert to a pointer to reference an item in memory. Improve this (managedStr); std::wstring nativeWstr(wch); //if you want to convert to std::string without manual resource cleaning std::string nativeStr(CW2A(nativeWstr . Then we can Following are the 5 different ways to convert char* to std::string in C++: In C++, the easiest method to convert char* (C-style string) to a std::string is by simply assigning it to the There is a reason why the C++ standard for std::strings has a c_str (and/or data) member function. Right now I'm constructing a By not using char * at all! Just simply use std::string. This should be In C++11, I recommend const char* in this case. while( cmd == "exit" && cmd == "\\exit" ) Obviously cmd can't be equal to two different strings at the same time, presumably you meant !=. Otherwise, This post will discuss how to convert a std::string to const char* in C++. Note that std::string provides you with c_str() function which gets you the underlying character string. Share A value of string::npos indicates all characters until the end of str. You can also convert one ot both of the C-style string operands to String and use ==. A pointer to a string means that the pointer points to the first character in the sequence of characters that is terminated by a null character. You cannot change the char constants pointed to by these pointers. wui jbvrjvdw solzqmg nwzgge skvl rnqc ziqkdy rrq alneagw ier