a.h

#include <memory>
#include <unordered_map>


/// --------------
class ImplBase;

class Class {
    std::unique_ptr<ImplBase> impl;

public:
    size_t foo() const;
    bool operator ==(const Class &other);
};

namespace std {
    template <>
    struct hash<Class> {
        size_t operator ()(const Class &cls) const {
            return cls.foo();
        }
    };
}


using MyMap = std::unordered_map<Class, size_t>;
void foo() {
    MyMap map = MyMap();
}

main.cpp

#include "a.h"