std::string c_str() method

since C++20

// Const version only
constexpr const CharT* c_str() const noexcept;

since C++11

// Const version only
const CharT* c_str() const noexcept;

until C++11

// Const version only
const CharT* c_str() const;

Returns a pointer to a null-terminated character array with data equivalent to those stored in the string.

The pointer is such that the range [ c_str(); c_str() + size() ] is valid and the values in it correspond to the values stored in the string with an additional null character after the last position.

Invalidation

The pointer obtained from c_str() may be invalidated by:

• Passing a non-const reference to the string to any standard library function, or
• Calling non-const member functions on the string, excluding operator[], at(), front(), back(), begin(), rbegin(), end() and rend() (since C++11).

Undefined Behavior

Writing to the character array accessed through c_str is undefined behavior.

Parameters

(none)

Return value

A pointer to the underlying character storage.

since C++11

c_str()[i] == operator[](i) for every i in [ 0, size() ).

until C++11

c_str() + i == std::addressof(operator[](i)) for every i in [ 0, size() ].

Complexity

Constant - O(1).

Notes

The pointer obtained from c_str() may only be treated as a pointer to a null-terminated character string if the string object does not contain other null characters.

c_str() and data() perform the same function. (since C++11)

Example

Main.cpp

#include <algorithm>
#include <cassert>
#include <cstring>
#include <string>

extern "C" {
void c_func(const char* c_str) { printf("c_func called with '%s'\n", c_str); }
}

int main() {
  std::string const s("Emplary");
  const char* p = s.c_str();
  assert(s.size() == std::strlen(p));
  assert(std::equal(s.begin(), s.end(), p));
  assert(std::equal(p, p + s.size(), s.begin()));
  assert('\0' == *(p + s.size()));

  c_func(s.c_str());
}
}

Output

c_func called with 'Emplary'