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

From cppreference.com
< cpp‎ | container‎ | map
 
 
Containers library
Sequence
(C++11)
Associative
Unordered associative
Adaptors
Views
(C++20)
 
 
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.
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

Contents

[edit] Parameters

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

[edit] 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.

[edit] Complexity

Logarithmic in the size of the container.

[edit] Example

#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::map<int,char> example = {{1,'a'},{2,'b'}};
 
    auto search = example.find(2);
    if (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};
    auto search2 = example2.find(lk);
    if (search2 != example2.end()) {
        std::cout << "Found " << search2->first.x << " " << search2->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;
    auto search3 = example2ref.find(lk);
    if (search3 != example2.end()) {
        std::cout << "Found " << search3->first.x << ' ' << search3->second << '\n';
    //  search3->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

[edit] See also

returns the number of elements matching specific key
(public member function) [edit]
returns range of elements matching a specific key
(public member function) [edit]