C++ templated function and forward declarations

Question

I'm working on some code that compiles and links (and even has released commercial products) on Windows using MSVC. It doesn't compile with GCC though, I get the following errors:

.../CBaseValue.h: In member function 'bool CBaseValue::InstanceOf()':
.../CBaseValue.h:90:18: error: invalid use of incomplete type 'struct CValueType'
.../CBaseValue.h:11:7: error: forward declaration of 'struct CValueType'

CBaseValue.h

class CValueType;

class CBaseValue {
public:

...

    template <typename _Type>
    bool InstanceOf() {
        CValueType* pType = GetType();
        if(pType == NULL) {
            return false;
        }
        else {
            return pType->IsDerivedFrom<_Type>();
        }
    }

...

}

CValueType.h

class CValueType : public CBaseValue  {
public:

...

    template <typename _Type>
    bool IsDerivedFrom() {
        return IsDerivedFrom(_Type::TYPEDATA);
    }

...

}

I understand why this is a problem. The base class (CBaseValue) has a templated function that uses a derived class (in this case CValueType).

It looks like MSVC isn't exactly obeying the C++ spec here and I've just been bitten by it. But the MSVC behavior of using the forward declaration until code calling the templated function is actually compiled is also more desirable right now. Does anybody know of a work-around where I can get this code working with GCC without having to rewrite a lot of base code?

From my own research it looks like passing '-fno-implicit-templates' to g++ would help but then I'd need to explicitly define the called template types. There are a lot of them so if I can avoid that I'd prefer it. If the general consensus is that this is my best option... so be it!

And in case anybody is wondering, I'm porting the code over to the Mac which is why we're now using GCC.

Answer 1

This is ill-formed by the Standard, but no diagnostic is required. MSVC is fine not diagnosing this particular case (even when instantiation happens!).

More specifically, the (C++03) Standard rules at 14.6/7

If a type used in a non-dependent name is incomplete at the point at which a template is defined but is complete at the point at which an instantiation is done, and if the completeness of that type affects whether or not the program is well-formed or affects the semantics of the program, the program is ill-formed; no diagnostic is required.

So the solution is to just make the type dependent, but arrange it that during instantiation, that type is designated. For example, you can do that by rewriting your template like this

template<typename T, typename> // just ignore second param!
struct make_dependent { typedef T type; };

template <typename Type> // eww, don't use "_Type" in user code
bool InstanceOf() {
    typename make_dependent<CValueType, Type>::type* pType = GetType();
    // ...
        return pType->template IsDerivedFrom<Type>();
    // ...
}

Answer 2

It seems that the CBaseValue::InstanceOf() function is useless to anyone not including CValueType.h.

So wait to provide the definition until all needed types are available. (EDIT: This is exactly what is suggested by Charles Bailey's comment that he posted while I was typing -- I guess we think alike.)

CBaseValue.h

class CValueType;

class CBaseValue {
public:

...

    template <typename _Type>
    bool InstanceOf();

...

}

CValueType.h

class CValueType : public CBaseValue  {
public:

...

    template <typename T>
    bool IsDerivedFrom() {
        return IsDerivedFrom(T::TYPEDATA);
    }

...

}


template <typename T>
inline bool CBaseValue::InstanceOf() {
        CValueType* pType = GetType();
        if(pType == NULL) {
            return false;
        }
        else {
            return pType->IsDerivedFrom<T>();
        }
    }

They seem very tightly coupled though, so maybe having just one header file for both classes, or one public header file that includes the individual headers in the correct order, would be better.