简单示例

#include <unordered_map>

class A {
public:
    std::string s;

    static size_t hashFun(const A & a) {
        return std::hash<std::string>()(a.s);
    }

    bool operator==(const A & a) const {
        // do something
        return true;
    }
};


int main() {
    std::unordered_map<A, std::string, decltype(&A::hashFun)> a(10, A::hashFun);
    return system("pause");
}