std::basic_string<CharT,Traits,Allocator>::operator[] - cppreference.com

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CharT& operator[]( size_type pos );	(1)	(constexpr since C++20)
const CharT& operator[]( size_type pos ) const;	(2)	(constexpr since C++20)

Returns a reference to the character at specified location pos if pos < size(), or if pos == size():

The behavior is undefined.	(until C++11)
Returns a reference to `CharT()`, if the object referred by the returned reference is modified to any value other than `CharT()`, the behavior is undefined.	(since C++11)

2) Returns a reference to CharT().

If pos > size() is true, the behavior is undefined.

(until C++26)

If pos > size() is true:

If the implementation is hardened, a contract violation occurs.
If the implementation is not hardened, the behavior is undefined.

(since C++26)

Parameters

pos

position of the character to return

Return value

1) *(begin() + pos) if pos < size(), or a reference to CharT() if pos == size()(since C++11).

2) *(begin() + pos) if pos < size(), or a reference to CharT() if pos == size().

Complexity

Constant.

Example

#include <iostream>
#include <string>
 
int main()
{
    const std::string e("Exemplar");
    for (unsigned i = e.length() - 1; i != 0; i /= 2)
        std::cout << e[i];
    std::cout << '\n';
    
    const char* c = &e[0];
    std::cout << c << '\n'; // print as a C string
    
    // Change the last character of s into a 'y'
    std::string s("Exemplar ");
    s[s.size() - 1] = 'y'; // equivalent to s.back() = 'y';
    std::cout << s << '\n';
}

Output:

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 259	C++98	overload (1) could return const lvalue `data()[pos]`, which was ill-formed	changed to return `*(begin() + pos)`
LWG 2475	C++11	if `pos == size()`, the behavior of modifying the object referred by the returned reference was undefined	well-defined if modified to `CharT()`