std::basic_string<CharT,Traits,Allocator>::at

 

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)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

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Filesystem library (C++17)

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Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Strings library

Null-terminated strings

Byte strings

Multibyte strings

Wide strings

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) 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*) 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

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CharT& at( size_type pos );	(1)	(constexpr since C++20)
const CharT& at( size_type pos ) const;	(2)	(constexpr since C++20)

Returns a reference to the character at specified location pos. Bounds checking is performed, exception of type std::out_of_range will be thrown on invalid access.

Parameters

pos

-

position of the character to return

Return value

Reference to the requested character.

Exceptions

Throws std::out_of_range if pos >= size().

If an exception is thrown for any reason, these functions have no effect (strong exception safety guarantee).

Complexity

Constant.

Example

Run this code

#include <iostream>
#include <stdexcept>
#include <string>
 
int main()
{
    std::string s("message"); // for capacity
 
    s = "abc";
    s.at(2) = 'x'; // OK
    std::cout << s << '\n';
 
    std::cout << "string size = " << s.size() << '\n';
    std::cout << "string capacity = " << s.capacity() << '\n';
 
    try
    {
        // This will throw since the requested offset is greater than the current size.
        s.at(3) = 'x';
    }
    catch (std::out_of_range const& exc)
    {
        std::cout << exc.what() << '\n';
    }
}

Possible output:

abx
string size = 3
string capacity = 7
basic_string::at

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 847	C++98	there was no exception safety guarantee	added strong exception safety guarantee
LWG 2207	C++98	the behavior was undefined if pos >= size()	always throws an exception in this case

See also

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