Modernes C++ programmieren

// https://godbolt.org/z/TdEnhKxGK
#include <map>

struct Number {
    int value_ = 0;
    explicit Number(int v) : value_{v} {}
};

bool operator<(const Number& left, const Number& right) {
    return left.value_ < right.value_;
}

int main() {
    std::map<Number,int> numbers{};                  // okay
    numbers.insert( std::make_pair(Number{4},100) ); // needs operator<
    numbers[Number{5}] = 200;                        // here as well
}