C++ How to stop private member variables constructor being called. It supposed to reference another object

Question

I'm new to C++ and not an experienced programmer.

I have a class which contains a member variable of another class I have created. I want this variable to be passed through by the user in the constructor. However, the variable calls its constructor the second it is initialised. How do I prevent this from happening

class Cypher {

private:
    // This definition calls the constructor, I dont want this, i want an
    // it to be an instance or a copy of the variable sent through in this classes constructor
    KeyCreator key;

public:
    Cypher(KeyCreator key) {
        key = key;
    }
}

Any help is much appreciated, I really have tried searching but can't find anything. I must be using incorrect terms.