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std::basic_string<CharT,Traits,Allocator>::operator[]

(1)
reference operator[]( size_type pos );
(until C++20)
constexpr reference operator[]( size_type pos );
(since C++20)
(2)
const_reference operator[]( size_type pos ) const;
(until C++20)
constexpr const_reference operator[]( size_type pos ) const;
(since C++20)

Returns a reference to the character at specified location pos if pos < size(), or a reference to CharT() if pos == size(). No bounds checking is performed.

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

For overload (1), if pos == size(), the behavior is undefined if the object referred by the returned reference is modified to any value other than CharT() (since C++11).

Parameters

pos - position of the character to return

Return value

*(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:

rmx
Exemplar
Exemplary

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 data()[pos] was returned if pos < size()
(its type is const CharT&, which conflicts with reference)
changed to
*(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()

See also

accesses the specified character with bounds checking
(public member function)
(DR*)
accesses the first character
(public member function)
(DR*)
accesses the last character
(public member function)
(C++17)
accesses the specified character
(public member function of std::basic_string_view<CharT,Traits>)

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