What is the use of the c_str() function?

Question

I understand c_str converts a string, that may or may not be null-terminated, to a null-terminated string.

Is this true? Can you give some examples?

Dwaindwaine

Answer 1

c_str returns a const char* that points to a null-terminated string (i.e., a C-style string). It is useful when you want to pass the "contents"¹ of an std::string to a function that expects to work with a C-style string.

For example, consider this code:

std::string string("Hello, World!");
std::size_t pos1 = string.find_first_of('w');

std::size_t pos2 = static_cast<std::size_t>(std::strchr(string.c_str(), 'w') - string.c_str());

if (pos1 == pos2) {
    std::printf("Both ways give the same result.\n");
}

See it in action.

Notes:

¹ This is not entirely true because an std::string (unlike a C string) can contain the \0 character. If it does, the code that receives the return value of c_str() will be fooled into thinking that the string is shorter than it really is, since it will interpret \0 as the end of the string.

Superhighway

Very interesting point you made in Notes: what i would like to know if std::string is already contained \0 then c_str also append \0 at the end of string?? – Dwaindwaine

@AmitSinghTomar: Yes, so you will have two null bytes -- one which is legitimately part of the string and one which is supposed to be the null terminator. But the c-style function that receives the pointer doesn't know this. – Superhighway

Note: a number of C-API will ask for two arguments (char const*, size_t), the second being the size, of course. – Parkinson

This is not working: string str("0.85"); newVolume = _tstof((TCHAR*)str.c_str()); how can i convert TCHAR* argv[] input 0.85 into that? – Doralia

@YumYumYum you cannot use std::string together with TCHAR stuff, the whole point of TCHAR is to automatically switch between char/wchar while std::string works only in terms of char. You will have to use an appropriate abstraction, or just simply don't use TCHAR at all and always do a Unicode build. – Superhighway

One possible "appropriate abstraction" is std::basic_string<TCHAR>. std::string is actually a pseudonym for std::basic_string<char>. – Iorgos

@Superhighway a stupid question, does using string with c_str cause any performance plenty? i.e if I want to make a desktop application if i used std::string with c_str will the performance be the same as using TCHAR/LPCSTR – Susannesusceptibility

Does c_str() create a copy of the original string? – Operon

Answer 2

In C++, you define your strings as

std::string MyString;

instead of

char MyString[20];.

While writing C++ code, you encounter some C functions which require C string as parameter.
Like below:

void IAmACFunction(int abc, float bcd, const char * cstring);

Now there is a problem. You are working with C++ and you are using std::string string variables. But this C function is asking for a C string. How do you convert your std::string to a standard C string?

Like this:

std::string MyString;
// ...
MyString = "Hello world!";
// ...
IAmACFunction(5, 2.45f, MyString.c_str());

This is what c_str() is for.

Note that, for std::wstring strings, c_str() returns a const w_char *.

Rilke

Answer 3

Most old C++ and C functions, when dealing with strings, use const char*.

With STL and std::string, string.c_str() is introduced to be able to convert from std::string to const char*.

That means that if you promise not to change the buffer, you'll be able to use read-only string contents. PROMISE = const char*

Milzie

@There's no such thing as stl::string. In fact, there isn't even such a thing as "STL" other than in a museum somewhere. – Idiophone

@Kerrek SB: STL is (was?) used as a synonym for the C++ standard library. – Lodged

Here is an example from 2020. Or maybe it is due to old books? – Lodged

Here is another example (2010) - "I'm using the STL string" – Lodged

Answer 4

In C/C++ programming there are two types of strings: the C strings and the standard strings. With the <string> header, we can use the standard strings. On the other hand, the C strings are just an array of normal chars. So, in order to convert a standard string to a C string, we use the c_str() function.

For example

// A string to a C-style string conversion //

const char *cstr1 = str1.c_str();
cout<<"Operation: *cstr1 = str1.c_str()"<<endl;
cout<<"The C-style string c_str1 is: "<<cstr1<<endl;
cout<<"\nOperation: strlen(cstr1)"<<endl;
cout<<"The length of C-style string str1 = "<<strlen(cstr1)<<endl;

And the output will be,

Operation: *cstr1 = str1.c_str()
The C-style string c_str1 is: Testing the c_str

Operation: strlen(cstr1)
The length of C-style string str1 = 17

Egomania

Answer 5

c_str() converts a C++ string into a C-style string which is essentially a null terminated array of bytes. You use it when you want to pass a C++ string into a function that expects a C-style string (e.g., a lot of the Win32 API, POSIX style functions, etc.).

Jollenta

I wouldn't say "converts". Rather, the function "provides access" to a suitable read-only character array -- most likely an array which has always been there in the implementation of std::string to begin with. – Idiophone

Answer 6

It's used to make std::string interoperable with C code that requires a null terminated char*.

Cryptonymous

Answer 7

You will use this when you encode/decode some string object you transfer between two programs.

Let’s say you use Base64 to encode some array in Python, and then you want to decode that into C++. Once you have the string you decode from Base64-decoded in C++. In order to get it back to an array of float, all you need to do here is:

float arr[1024];
memcpy(arr, ur_string.c_str(), sizeof(float) * 1024);

This is pretty common use, I suppose.

Gigi

That kind of code is not portable (endianness and float format among other) – Tuinenga