.c_str()

In C, the .c_str() method returns a pointer to a null-terminated array of characters, equivalent to the contents of the string on which it is called.

The c_str() method is commonly used when a C-style string is needed, such as when passing a std::string to functions that expect a const char*. It’s especially useful for interoperability with legacy C libraries or APIs like printf(), fopen(), or other system-level functions.

Note: The .c_str() method runs in constant time O(1), as it returns a pointer to the internal character array of the std::string.

Syntax

string.c_str()

Parameters:

The .c_str() method does not take any parameters.

Return value:

Returns a const char*, which is a pointer to a null-terminated character array representing the contents of the string.

Example

This example shows the size of a std::string object and a pointer returned by .c_str(), along with the actual string length using length() and strlen():

#include <string>
#include <iostream>
#include <cstring> // for strlen

int main() {
  std::string str = "Foobar";
  const char* arr = str.c_str();

  std::cout << "Size of 'arr' (pointer): " << sizeof(arr) << " bytes" << std::endl;
  std::cout << "Size of 'str' (std::string object): " << sizeof(str) << " bytes" << std::endl;

  std::cout << "Length of string using str.length(): " << str.length() << std::endl;
  std::cout << "Length of string using strlen(arr): " << strlen(arr) << std::endl;

  return 0;
}

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The output of this code is:

Size of 'arr' (pointer): 8 bytes
Size of 'str' (std::string object): 32 bytes
Length of string using str.length(): 6
Length of string using strlen(arr): 6

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Note: sizeof gives the size of the variable’s type, not the number of characters.

Codebyte Example

In this codebyte example, we have a function printPointerCharArrToStdOut that expects a pointer to a character array. The .c_str() method is used to convert a std::string to a C-style string, allowing it to be passed to the function. The function then outputs each character of the array individually:

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