invalid use of non-static data member

Question

For a code like this:

class foo {   protected:     int a;   public:     class bar {       public:         int getA() {return a;}   // ERROR     };     foo()       : a (p->param) }; 

I get this error:

 invalid use of non-static data member 'foo::a' 

currently the variable a is initialized in the constructor of foo.

if I make it static, then it says:

 error: 'int foo::a' is a static data member; it can only be initialized at its definition 

However I want to pass a value to a in the constructor. What is the solution then?

Answer 1

In C++, unlike (say) Java, an instance of a nested class doesn't intrinsically belong to any instance of the enclosing class. So bar::getA doesn't have any specific instance of foo whose a it can be returning. I'm guessing that what you want is something like:

    class bar {       private:         foo * const owner;       public:         bar(foo & owner) : owner(&owner) { }         int getA() {return owner->a;}     }; 

But even for this you may have to make some changes, because in versions of C++ before C++11, unlike (again, say) Java, a nested class has no special access to its enclosing class, so it can't see the protected member a. This will depend on your compiler version. (Hat-tip to Ken Wayne VanderLinde for pointing out that C++11 has changed this.)