C++ Template Basics - Function Accept Sub Class or Super

Question

Say you have a sub class B which inherits from super class A. You want a function that can accept either A or B.

template <typename T>
void someFunc(T* pObj, bool someOtherArg)
{
    pObj->AnInheritMethod();
    if (pObj->IsASub())
    {
       pObj->ANonInhertMethod();
    }
}

When I compile this (Visual Studio 6) I get:

error C2065: 'pObj' : undeclared identifier

Am I way off base here?

Answer 1

You don't need templates for that, that is a free behaviour courtesy of polymorphism.

Edit: also if you write something like:

if (pObj->IsASub())

then there's maybe something wrong in your design. The method is supposed to work for any type in the derivation chain.

Answer 2

You don't need a function template for this; the following will do just fine:

void someFunc(A* pObj)
{
    pObj->AnInheritMethod();
    if (B* pObjAsB = dynamic_cast<B*>(pObj))
    {
        pObjAsB->ANonInheritMethod();
    }
}

Or, if you prefer to use your IsASub() member function instead of dynamic_cast:

void someFunc(A* pObj)
{
    pObj->AnInheritMethod();
    if (pObj->IsASub())
    {
        B* pObjAsB = static_cast<B*>(pObj);
        pObjAsB->ANonInheritMethod();
    }
}

Aside from the missing return type, I don't see anything obviously wrong with the code in your example; I don't have Visual C++ 6 installed to check.

Answer 3

You're asking a question that is completely unrelated to the code and error that you included.

In order to have a function take an A or a class derived from A, all it needs to do is take a pointer or reference to A, e.g.

someFunc(A* pObj, bool someOtherArg);

or

someFunc(A& obj, bool someOtherArg);

It will work by virtue of inheritance. That's kind of the whole point of deriving classes from each other. The way you have written it with templates, it will work with any class that defines the three methods you use, whether or not it derives from A.

Now the error you posted is unrelated to this question but is bizarre. There's nothing wrong with the code you posted, but Visual Studio 6 is an ancient compiler; it's twelve years old and does not fully support modern ISO standard C++. This error may be an artifact of a sub-standard templating implementation in the compiler.