std::map<Key,T,Compare,Allocator>::find

 

C++

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Freestanding and hosted

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Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

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Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

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Technical specifications

Symbols index

External libraries

 

Containers library

node-handle (C++17)

Sequence

array (C++11)

vector

vector<bool>

inplace_vector (C++26)

hive (C++26)

deque

forward_list (C++11)

list

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set

multiset

map

multimap

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unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

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flat_set (C++23)

flat_multiset (C++23)

flat_map (C++23)

flat_multimap (C++23)

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span (C++20)

mdspan (C++23)

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Non-member function table

 

std::map

Member functions

map::map map::~map               map::operator= map::get_allocator

Element access map::at map::operator[] Iterators map::beginmap::cbegin (C++11) map::endmap::cend (C++11) map::rbeginmap::crbegin (C++11)      map::rendmap::crend (C++11) Capacity map::size map::max_size map::empty Observers map::key_comp map::value_comp Modifiers map::clear map::insert map::erase map::swap map::extract (C++17) map::merge (C++17) map::insert_range (C++23) map::insert_or_assign (C++17) map::emplace (C++11) map::emplace_hint (C++11) map::try_emplace (C++17) Lookup map::count map::find map::contains (C++20) map::equal_range map::lower_bound map::upper_bound

Non-member functions

operator==operator<=> (C++20) std::swap(std::map) erase_if(std::map) (C++20)   operator!=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)

Deduction guides (C++17)

 

iterator find( const Key& key );	(1)	(constexpr since C++26)
const_iterator find( const Key& key ) const;	(2)	(constexpr since C++26)
template< class K > iterator find( const K& x );	(3)	(since C++14) (constexpr since C++26)
template< class K > const_iterator find( const K& x ) const;	(4)	(since C++14) (constexpr since C++26)

1,2) Finds an element with key equivalent to key.

3,4) Finds an element with key that compares equivalent to x.

This overload participates in overload resolution only if Compare is transparent. It allows calling this function without constructing an instance of Key.

Parameters

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

Return value

An iterator to the requested element. If no such element is found, past-the-end (see end()) iterator is returned.

Complexity

Logarithmic in the size of the container.

Notes

Feature-test macro	Value	Std	Feature
__cpp_lib_generic_associative_lookup	201304L	(C++14)	Heterogeneous comparison lookup in associative containers; overloads (3,4)

Example

Run this code

#include <iostream>
#include <map>
 
struct LightKey
{
    int x;
};
 
struct FatKey
{
    int x;
    int data[1000]; // a heavy blob
};
 
// As detailed above, the container must use std::less<> (or other transparent
// Comparator) to access these overloads. This includes standard overloads,
// such as comparison between std::string and std::string_view.
bool operator<(const FatKey& fk, const LightKey& lk) { return fk.x < lk.x; }
bool operator<(const LightKey& lk, const FatKey& fk) { return lk.x < fk.x; }
bool operator<(const FatKey& fk1, const FatKey& fk2) { return fk1.x < fk2.x; }
 
int main()
{
    // Simple comparison demo.
    std::map<int, char> example{{1, 'a'}, {2, 'b'}};
 
    if (auto search = example.find(2); search != example.end())
        std::cout << "Found " << search->first << ' ' << search->second << '\n';
    else
        std::cout << "Not found\n";
 
    // Transparent comparison demo.
    std::map<FatKey, char, std::less<>> example2{{{1, {}}, 'a'}, {{2, {}}, 'b'}};
 
    LightKey lk = {2};
    if (auto search = example2.find(lk); search != example2.end())
        std::cout << "Found " << search->first.x << ' ' << search->second << '\n';
    else
        std::cout << "Not found\n";
 
    // Obtaining const iterators.
    // Compiler decides whether to return iterator of (non) const type by way of
    // accessing map; to prevent intentional modification, one of the simplest
    // options is to access the map via a constant reference.
    const auto& example2ref = example2;
    if (auto search = example2ref.find(lk); search != example2.end())
    {
        std::cout << "Found " << search->first.x << ' ' << search->second << '\n';
    //  search->second = 'c'; // error: assignment of member
                              // 'std::pair<const FatKey, char>::second'
                              // in read-only object
    }
}

Output:

Found 2 b
Found 2 b
Found 2 b

See also

at	access specified element with bounds checking (public member function)
operator[]	access or insert specified element (public member function)
count	returns the number of elements matching specific key (public member function)
equal_range	returns range of elements matching a specific key (public member function)