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

#include <cassert>
#include <iostream>
#include <map>
struct LightKey { int o; };
struct HeavyKey { int o[1000]; };
// Der Container muss std::less<> (oder einen anderen transparenten Comparator) verwenden,
// um die Überladungen (3,4) zu nutzen. Dies beinhaltet Standardüberladungen, wie
// den Vergleich zwischen std::string und std::string_view.
bool operator<(const HeavyKey& x, const LightKey& y) { return x.o[0] < y.o; }
bool operator<(const LightKey& x, const HeavyKey& y) { return x.o < y.o[0]; }
bool operator<(const HeavyKey& x, const HeavyKey& y) { return x.o[0] < y.o[0]; }
int main()
{
    std::map<int, char> map{{1, 'a'}, {2, 'b'}};
    assert(map.at(1) == 'a');
    assert(map.at(2) == 'b');
    try
    {
        map.at(13);
    }
    catch(const std::out_of_range& ex)
    {
        std::cout << "1) out_of_range::what(): " << ex.what() << '\n';
    }
#ifdef __cpp_lib_associative_heterogeneous_insertion
    // Demonstration transparenter Vergleiche.
    std::map<HeavyKey, char, std::less<>> map2{{{1}, 'a'}, {{2}, 'b'}};
    assert(map2.at(LightKey{1}) == 'a');
    assert(map2.at(LightKey{2}) == 'b');
    try
    {
        map2.at(LightKey{13});
    }
    catch(const std::out_of_range& ex)
    {
        std::cout << "2) out_of_range::what(): " << ex.what() << '\n';
    }
#endif
}