What is use of c_str function?

I understand is c_str convert a string that may or may not be null-terminated to a null-terminated string.

Is this true? Can you give some examples?

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.

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 *.

Most OLD c++ and c functions, when deal 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*

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

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).

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

Oh must add my own pick here, you will use this when you encode/decode some string obj you transfer between two programs.

Lets say you use base64encode some array in python, and then you want to decode that into c++. Once you have the string you decode from base64decode in c++. In order to get it back to 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.

const char* c_str() const; Returns a pointer to an array that contains a null-terminated sequence of characters(i.e., a C - string) representing the current value of the string object.

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.

std::string str = "hello";
std::cout << str;          // hello
printf("%s", str);         // ,²/☺
printf("%s", str.c_str()); // hello