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

	(1)
reference at( size_type pos );		(until C++20)
constexpr reference at( size_type pos );		(since C++20)
	(2)
const_reference at( size_type pos ) const;		(until C++20)
constexpr const_reference at( size_type pos ) const;		(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, this function has no effect (strong exception safety guarantee).

Complexity

Constant.

Example

#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

operator[]	accesses the specified character (public member function)
at (C++17)	accesses the specified character with bounds checking (public member function of `std::basic_string_view<CharT,Traits>`)