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

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< cpp‎ | string‎ | basic string
 
C++
 
Strings library
 
std::basic_string
 
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():

1)

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. Moreover, if the contract-violation handler returns under “observe” evaluation semantic, the behavior is undefined.
  • If the implementation is not hardened, the behavior is undefined.
(since C++26)

Contents

[edit] Parameters

pos - position of the character to return

[edit] 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().

[edit] Complexity

Constant.

[edit] Example

Run this code
#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

[edit] 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()

[edit] See also

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