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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Containers library

Sequence

array (C++11)

vector

deque

forward_list (C++11)

list

Associative

set

multiset

map

multimap

Unordered associative

unordered_set (C++11)

unordered_multiset (C++11)

unordered_map (C++11)

unordered_multimap (C++11)

Adaptors

stack

queue

priority_queue

flat_set (C++23)

flat_multiset (C++23)

flat_map (C++23)

flat_multimap (C++23)

Views

span (C++20)

mdspan (C++23)

Iterator invalidation

Member function table

Non-member function table

 

std::multimap

Member functions

multimap::multimap

multimap::~multimap

multimap::operator=

multimap::get_allocator

Iterators

multimap::beginmultimap::cbegin (C++11)

multimap::endmultimap::cend (C++11)

multimap::rbeginmultimap::crbegin (C++11)

multimap::rendmultimap::crend (C++11)

Capacity

multimap::empty

multimap::size

multimap::max_size

Modifiers

multimap::clear

multimap::insert

multimap::insert_range (C++23)

multimap::emplace (C++11)

multimap::emplace_hint (C++11)

multimap::erase

multimap::swap

multimap::extract (C++17)

multimap::merge (C++17)

Lookup

multimap::count

multimap::find

multimap::contains (C++20)

multimap::equal_range

multimap::lower_bound

multimap::upper_bound

Observers

multimap::key_comp

multimap::value_comp

Non-member functions

std::swap

erase_if (C++20)

operator==operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

Deduction guides(C++17)

 

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

1,2) Finds an element with key equivalent to key. If there are several elements with key in the container, any of them may be returned.

3,4) Finds an element with key that compares equivalent to the value x. This overload participates 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	-	key value of the element to search for
x	-	a value of any type that can be transparently compared with a key

Return value

Iterator to an element with key equivalent to key. 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	Comment
__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 FatKey   { int x; int data[1000]; };
struct LightKey { int x; };
// Note: as detailed above, the container must use std::less<> (or other 
//   transparent Comparator) to access these overloads.
// This includes standard overloads, such as 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::multimap<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::multimap<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 modification on purpose, one of easiest choices is to access map by
    // const 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

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)