C ++：类成员的前向声明（变量）

Question

Is it possible to forward declare a class member variable? I want to do something like the following (But it doesn't work):

class myClass;
bool myClass::myvar;

void main()
{
    myClass* aaa;
    ...
    aaa->myvar = false;
}

In this example, myvar is a boolean member of myClass.

Answer 1

No, you can't. Imagine the class is scattered across different translation units this way.
What members will it have then? And what will be the object layout?

That's intractable.

Answer 2

I would recommend to use set/get methods in this case. Then you don't expose your internal data to the outside world.

In the header of myClass:

class myClass
{
    public: setMyVar(const bool& value);
    public: inline const bool& getMyVar() const;

    private: bool myVar;
};

In your implementation:

class myClass;

void main()
{
    myClass* aaa;
    ...
    aaa->setMyVar(false);
}