Difference between revisions of "cpp/container/map/find"

#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
    }
}

#include <string>
#include <iostream>
#include <map>
 
int main()
{
    typedef std::map<std::string,int>  mapT;
 
    mapT my_map;
    my_map["first"]=  11;
    my_map["second"]= 23;
 
    mapT::iterator  it= my_map.find("first");
    if( it != my_map.end() ) std::cout << "A: " << it->second << "\n";
 
    it= my_map.find("third");
    if( it != my_map.end() ) std::cout << "B: " << it->second << "\n";
 
    // Accessing a non-existing element creates it
    if( my_map["third"] == 42 ) std::cout << "Oha!\n";
 
    it= my_map.find("third");
    if( it != my_map.end() ) std::cout << "C: " << it->second << "\n";
 
    return 0;
}