std:hash with access to private members of a class

I would like to hash a class that has two private members e.g.:

foo.h

class Foo {
    private:
        std::string a;
        std::string b;

    public:
        Foo (std::string a, std::string b);
        bool operator==(const Foo& other) const;
        bool operator!=(const Foo& other) const;
        std::size_t operator()(const Foo& ) const;
};

namespace std {
    template <> struct hash<Foo> {
        std::size_t operator()(const Foo& cp) const;
    };
}

foo.cpp

Foo::Foo (std::string _a, std::string _b) {
    this->a = _a;
    this->b = _b;
}

bool Foo::operator== (const Foo& other) const {
    return this->a == other.a && this->b == other.b;
}

bool Foo::operator!= (const Foo& other) const {
    return !operator==(other);
}

std::size_t std::hash<Foo>::operator()(Foo const& foo) const {
    std::string f = foo.a; // << This wont compile!
    return 1;
}

In C++ how is hashing of Foo typically done when that final hash function doesn't have access to the private member of foo.

Feel free to include boost or abseil in your answer.