Question

I’m converting a project from Visual Studio 6 to Visual Studio 2010. The project heavily uses STL. There is a custom class that subclasses list to add more methods to it. Following is the code and compiler error that I’m getting.

    #include <list> 
    namespace mySpace 
    { 
    template <class T> 
    class MyList : public std::list<T> 
    { 
        ...
    };

    template <class T>
    MyList<T>::reference MyList<T>::find(const_reference p_constreferenceItem) const
    {
     return std::find(this->begin(), this->end(), p_constreferenceItem);
    }
   } // namespace mySpace

Error:

Error   2   error C2143: syntax error : missing ';' before 'MySpace::MyList<T>::find'   c:\myproject\mylist.h   300 1
Error   3   error C4430: missing type specifier - int assumed. Note: C++ does not support default-int   c:\myproject\mylist.h   300 1   
Error   4   error C2888: 'MyList<T>::reference reference' : symbol cannot be defined within namespace 'MySpace' c:\myproject\mylist.h   300 1

Any ideas what is causing this error?

Answer 1

It seems like you’re just missing a typename in front of MyList<T>::reference, since it’s a dependent type (one that depends on the type of T) and the compiler doesn’t know that MyList<T>::reference is a type and assumes by default that it’s a variable name:

template <class T>
typename MyList<T>::reference MyList<T>::find(
                              const_reference p_constreferenceItem) const

You don’t need this for the const_reference though, as the compiler is in MyList<T>‘s scope as soon as he’s seen the :: (that of the method name) and now knows that const_reference names a type. In the same way you don’t need to use typename when you’re already inside the MyList<T>‘s scope, e.g. when declaring or inline-defining the method inside the class definition.

I’m not sure why VC6 allowed this, though. But we all know that VC6 and templates aren’t that big friends, not to speak of the loose specification of C++ templates and their behaviour at that time.

disclaimer: Sorry to the language lawyers if the terminology used here is a bit informal.