Question

I have a Non-integral Constant declaration in a class.

I keep getting the following:

ComponentClass.h:14: error: template declaration of const typename ComponentClass<T> ::position NULLPOSITION
ComponentClass.h:14: error: position was not declared in this scope
ComponentClass.h:14: error: expected ; before numeric constant

Please find below my code.

ComponentClass.h

#ifndef _ComponentClass_H
#define _ComponentClass_H

template< class T>
class ComponentClass
{
public:

       typedef ComponentClass* position;
       ComponentClass();
};

template<class T>
const typename ComponentClass<T>::position NULLPOSITION=(position)0;

template<class T>
ComponentClass<T>::ComponentClass(){}
#endif

Answer 1

The reason you’re seeing this error is that position is not in scope in the expression (position)0. You can omit the cast altogether (i.e, 0). If you wanted to include it, you would need to use typename ComponentClass<T>::position as you did in the definition of NULLPOSITION.

You appear to be defining a static member variable without first declaring it in the class like so:

static const position NULLPOSITION;

Then you can define it outside the class as you do now. In order to avoid redundant definitions, however, the typical solution is as follows:

// ComponentBase.h
class ComponentBase {
public:
    typedef ComponentBase* position;
    static const position NULLPOSITION;
};

// ComponentClass.h
template<class T>
class ComponentClass : public ComponentBase { ... };

// ComponentBase.cpp
const ComponentBase::position ComponentBase::NULLPOSITION = 0;

That is, don’t make NULLPOSITION a member of every instantiation of ComponentClass, but instead let all ComponentClass instantiations share a single definition.