I'm looking for a way to sort std::multimap's entries in ascending order by keys, but if the keys match, in descending order by values.

Is it possible to implement with a custom Compare predicate?

CodePudding user response：

Map's Compare predicate only takes keys as arguments. Unfortunately you cannot use values to sort the entries within the same bucket with use of the predicate only.

Important - it's still possible to implement such scenario it with other means. Here is the answer how to do it if you are ok to use emplace_hint.

The keys are sorted in ascending order by default with use of std::less. In order to implement a custom predicate, you can use a lambda (or any other form of the binary predicate):

const auto lessPredicate = [](const MyClass& lhs, const MyClass& rhs) {
    return lhs.value < rhs.value;
};
std::multimap<MyClass, std::string, decltype(lessPredicate)> my_map { lessPredicate };