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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Strings library

Classes

basic_string

basic_string_view (C++17)

char_traits

 

std::basic_string

Member functions basic_string::basic_string basic_string::~basic_string basic_string::operator= basic_string::assign basic_string::assign_range (C++23)   basic_string::get_allocator Element access basic_string::at basic_string::operator[] basic_string::front (DR*) basic_string::back (DR*) basic_string::data basic_string::c_str basic_string::operator  basic_string_view (C++17) Iterators basic_string::beginbasic_string::cbegin (C++11) basic_string::endbasic_string::cend (C++11) basic_string::rbeginbasic_string::crbegin (C++11) basic_string::rendbasic_string::crend (C++11) Search basic_string::find basic_string::rfind basic_string::find_first_of basic_string::find_first_not_of basic_string::find_last_of basic_string::find_last_not_of

Modifiers basic_string::clear basic_string::insert basic_string::insert_range (C++23) basic_string::erase basic_string::push_back basic_string::pop_back (DR*) basic_string::append basic_string::append_range (C++23) basic_string::operator+= basic_string::replace basic_string::replace_with_range (C++23) basic_string::copy basic_string::resize basic_string::resize_and_overwrite (C++23)  basic_string::swap Capacity basic_string::empty basic_string::sizebasic_string::length basic_string::max_size basic_string::reserve basic_string::capacity basic_string::shrink_to_fit (DR*) Operations basic_string::compare basic_string::starts_with (C++20) basic_string::ends_with (C++20) basic_string::contains (C++23) basic_string::substr

Constants basic_string::npos Non-member functions operator+ swap(std::basic_string) erase(std::basic_string)erase_if(std::basic_string) (C++20)(C++20) I/O operator<<operator>> getline Comparison operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20) Numeric conversions stoistolstoll (C++11)(C++11)(C++11) stoulstoull (C++11)(C++11)  stofstodstold (C++11)(C++11)(C++11) to_string (C++11) to_wstring (C++11) Literals operator""s (C++14) Helper classes hash<std::basic_string> (C++11) Deduction guides (C++17)

 

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

See More

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)

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

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

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()

See also

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