# deduction guides for `std::map`

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< cpp‎ | container‎ | map

Defined in header `<map>` |  |
template <class InputIt, class Comp = std::less <iter_key_t<InputIt>>, class Alloc = std::allocator <iter_to_alloc_t<InputIt>>> map(InputIt, InputIt, Comp = Comp(), Alloc = Alloc()) -> map<iter_key_t<InputIt>, iter_val_t<InputIt>, Comp, Alloc>; | (1) | (since C++17)
template<class Key, class T, class Comp = std::less <Key>, class Alloc = std::allocator < std::pair <const Key, T>>> map( std::initializer_list < std::pair <Key, T>>, Comp = Comp(), Alloc = Alloc()) -> map<Key, T, Comp, Alloc>; | (2) | (since C++17)
template<class InputIt, class Alloc> map(InputIt, InputIt, Alloc) -> map<iter_key_t<InputIt>, iter_val_t<InputIt>, std::less <iter_key_t<InputIt>>, Alloc>; | (3) | (since C++17)
template<class Key, class T, class Allocator> map( std::initializer_list < std::pair <Key, T>>, Allocator) -> map<Key, T, std::less <Key>, Allocator>; | (4) | (since C++17)

where the type aliases iter_key_t, iter_val_t, iter_to_alloc_t are defined as if as follows

template<class InputIt> using iter_key_t = std::remove_const_t < typename std::iterator_traits <InputIt>::value_type::first_type>; |  | (exposition only)
template<class InputIt> using iter_val_t = typename std::iterator_traits <InputIt>::value_type::second_type; |  | (exposition only)
template<class InputIt> using iter_to_alloc_t = std::pair < std::add_const_t <typename std::iterator_traits <InputIt>::value_type::first_type>, typename std::iterator_traits <InputIt>::value_type::second_type > |  | (exposition only)

This deduction guide is provided for map to allow deduction from an iterator range (overloads (1,3)) and std::initializer_list (overloads (2,4)). These overloads only participate in overload resolution if `InputIt` satisfies LegacyInputIterator, `Alloc` satisfies Allocator, and `Comp` does not satisfy Allocator.

Note: the extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum integral types do not qualify as input iterators. Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum the member type `Alloc::value_type` must exist and the expression std::declval <Alloc&>().allocate( std::size_t {}) must be well-formed when treated as an unevaluated operand.

### Example

Run this code

    #include <map>
    int main() {
    // std::map m1 = {{"foo", 1}, {"bar", 2}}; // Error: braced-init-list has no type;
                                               // cannot deduce pair<Key, T> from
                                               // {"foo", 1} or {"bar", 2}

       std::map m1 = {std::pair{"foo", 2}, {"bar", 3}}; // guide #2
       std::map m2(m1.begin(), m1.end()); // guide #1
    }

### 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 3025 | C++17 | initializer-list guides take std::pair <const Key, T> | use std::pair <Key, T>

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