Access non static data member outside class

Question

Is it possible to access non static data members outside their class? Say you have an example like the following. I know it does not make much sense as an example, but I just want to understand how to access a non static data member. If the following is compiled it generates an error:

 C.h|70|error: invalid use of non-static data member ‘C::age’|

//C.h

class C{
  public:
    int age;
};
 int getAge();

//C.cpp

C::C()
{
    age  = 0;
}
int getAge(){
   return (C::age);
}

Answer 1

Non-static members are instance dependent. They are initialized when a valid instance is initialized.

The problem with your example is that, it tries to access a non-static member through the class interface without first initializing a concrete instance. This is not valid.

You can either make it static:

class C{
public:
    static int age;
};

which requires you to also define age before using at runtime by: int C::age = 0. Note that value of C::age can be changed at runtime if you use this method.

Or, you can make it const static and directly initialize it like:

class C{
public:
    const static int age = 0;
};

In this case, value of C::age is const.

Both of which will let you get it without an instance: C::age.