Passkey Idiom

2018-06-08 ☼ coding ☼ c++

Pretty simple but an example helps:

class Foo {
public:
    class Permission { 
        Permission()= default; 
        template<typename T> friend class Bar; 
    };
    void kinda_private(int i, Permission) {
        // can only be called by Bar<T>s
    }
};

Lets you make single methods “kinda public” but can only be called by things that are friends of the Permission class.