# std::map<Key,T,Compare,Allocator>::at

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< cpp‎ | container‎ | map

T& at( const Key& key ); | (1) |
const T& at( const Key& key ) const; | (2) |
template< class K > T& at( const K& x ); | (3) | (since C++26)
template< class K > const T& at( const K& x ) const; | (4) | (since C++26)

Returns a reference to the mapped value of the element with specified key. If no such element exists, an exception of type std::out_of_range is thrown.

1,2) The key is equivalent to key.
3,4) The key compares equivalent to the value x. The reference to the mapped value is obtained as if by expression this->find(x)->second.
The expression this->find(x) must be well-formed and have well-defined behavior, otherwise the behavior is undefined.
These overloads participate in overload resolution only if the qualified-id Compare::is_transparent is valid and denotes a type. It allows calling this function without constructing an instance of `Key`.

### Parameters

key | - | the key of the element to find
x | - | a value of any type that can be transparently compared with a key

### Return value

A reference to the mapped value of the requested element.

### Exceptions

1,2) std::out_of_range if the container does not have an element with the specified key.
3,4) std::out_of_range if the container does not have the specified element, that is, if find(x) == end() is true.

### Complexity

Logarithmic in the size of the container.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_associative_heterogeneous_insertion` | `202311L` | (C++26) | Heterogeneous overloads for the remaining member functions in ordered and unordered associative containers. (3,4)

### Example

Run this code

    #include <cassert>
    #include <iostream>
    #include <map>

    struct LightKey { int o; };
    struct HeavyKey { int o[1000]; };

    // The container must use std::less<> (or other transparent Comparator) to
    // access overloads (3,4). This includes standard overloads, such as
    // comparison between std::string and std::string_view.
    bool operator<(const HeavyKey& x, const LightKey& y) { return x.o[0] < y.o; }
    bool operator<(const LightKey& x, const HeavyKey& y) { return x.o < y.o[0]; }
    bool operator<(const HeavyKey& x, const HeavyKey& y) { return x.o[0] < y.o[0]; }

    int main()
    {
        std::map<int, char> map{{1, 'a'}, {2, 'b'}};
        assert(map.at(1) == 'a');
        assert(map.at(2) == 'b');
        try
        {
            map.at(13);
        }
        catch(const std::out_of_range& ex)
        {
            std::cout << "1) out_of_range::what(): " << ex.what() << '\n';
        }

    #ifdef __cpp_lib_associative_heterogeneous_insertion
        // Transparent comparison demo.
        std::map<HeavyKey, char, std::less<>> map2{{{1}, 'a'}, {{2}, 'b'}};
        assert(map2.at(LightKey{1}) == 'a');
        assert(map2.at(LightKey{2}) == 'b');
        try
        {
            map2.at(LightKey{13});
        }
        catch(const std::out_of_range& ex)
        {
            std::cout << "2) out_of_range::what(): " << ex.what() << '\n';
        }
    #endif
    }

Possible output:

    1) out_of_range::what(): map::at:  key not found
    2) out_of_range::what(): map::at:  key not found

### 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 464 | C++98 | `map` did not have this member function | added
LWG 703 | C++98 | the complexity requirement was missing | added
LWG 2007 | C++98 | the return value referred to the requested element | refers to its mapped value

### See also

operator[] | access or insert specified element (public member function)
find | finds element with specific key (public member function)

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