invalid use of nonstatic member

[nielsp]

Hi!

Why does'nt the following work?

#include <iostream>
 
class X
{
public:
    X()
     : m(0)
    { }
 
    virtual ~X()
    { }
 
protected:
    int m;
};
 
class Y : public X
{
private:
    class Z 
    {
        void do_something() 
        {
            std::cout << m << std::endl;
        }
    };
};

I get:

error: invalid use of nonstatic data member ‘X::m’ 

Thanks!

Niels

[Banfa]

m is a non static member of X which is the inherited by Y. You try to us it in Z but Z is a separate class, it's only link with Y is that it is a nested definition.

That means that when Z::do_something is running it is not in the presence of a this pointer that points to an instance of Y. It is in the presence of a this pointer pointing to the instance of Z. So there is no conext present through which to access m.

Like the error message says you have tried to use m in an invalid context.

[nielsp]

Is there a way to access m although?

Niels

[Banfa]

Only if you have a reference to, pointer to or instance of Y (or X).

m can't be accessed in Z because the existence of an object of type Z does not imply the existence of an object of type Y.

There is no m to access in Z::do_something

[weaknessforcats]

You will need to write an accessor function for Y to return m and then call that accessor in the Z member function.