lst-0086-book.cpp
// https://godbolt.org/z/Yhzqo6dxT
#include <set>
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::set<Number> numbers{}; // okay
numbers.insert( Number{3} ); // operator< is needed here
}