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Showing content from https://en.cppreference.com/w/cpp/language/../container/../string/basic_string/at.html below:

std::basic_string<CharT,Traits,Allocator>::at - cppreference.com

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.

[edit] Parameters pos - position of the character to return [edit] Return value

Reference to the requested character.

[edit] 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).

[edit] Complexity

Constant.

[edit] 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

[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 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() is true always throws an exception in this case [edit] See also accesses the specified character
(public member function) [edit] accesses the specified character with bounds checking
(public member function of std::basic_string_view<CharT,Traits>) [edit]

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