std::map find() 方法

自 C++14 起

// (1) Non const version
iterator find( const Key& key );

// (2) Const version
const_iterator find( const Key& key ) const;

// (3) Non const version
template< class K > 
iterator find( const K& x );

// (4) Const version
template< class K > 
const_iterator find( const K& x ) const;

C++98 起

// (1) Non const version
iterator find( const Key& key );

// (2) Const version
const_iterator find( const Key& key ) const;

• (1-2) 查找键等同于 key 的元素。
• (3-4) 查找键与值 x 比较等效的元素。此重载仅在限定 ID Compare::is_transparent 有效且表示一个类型时才参与重载解析。它允许在不构造 Key 实例的情况下调用此函数。

参数

• key - 要计数的元素的键值
• x - 可以与键透明比较的任何类型的值

返回值

指向键与键等效的元素的迭代器。如果未找到此类元素，则返回末尾（参见 end()）迭代器。

复杂度

对容器大小呈对数关系 - O(log size())。

异常

(无)

备注

特性测试宏：__cpp_lib_generic_associative_lookup（用于重载 (3) 和 (4)）。

示例

Main.cpp

#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
    }
}

输出

Found 2 b
Found 2 b
Found 2 b