Question

I have a bunch of templated code that compiles fine under g++, but now when I try to build under windows with Visual C++ 2010 I get a bunch of errors.

I have a collection of template functions for getting and setting values in C++ objects from Lua code. For example, I have this template:

//        Class       Return type Getter function
template <typename T, typename U, U (T::*Getter)() const>
int luaU_get(lua_State* L)
{
    T* obj = luaW_check<T>(L, 1); // Gets userdata from stack and checks if it's of type T
    luaU_push(L, (obj->*Getter)()); // Runs the getter function specified in the template, and pushes the
    return 1;
}

(The complete file can be found here)

Which is instantiated here:

static luaL_reg TextArea_MT[] =
{
    //                             Class     Return type   Getter function
    { "GetCharacterSize", luaU_get<TextArea, unsigned int, &TextArea::GetCharacterSize> },
    { NULL, NULL }
};

The signature for that getter is as follows:

unsigned int GetCharacterSize() const;

I’m getting a bunch of errors like this:

2>C:\Users\Alex\Documents\Visual Studio 2010\Projects\game\dev\src\game\lua\LuaTextArea.cpp(103): error C2440: 'specialization' : cannot convert from 'unsigned int (__thiscall ag::ui::TextArea::* )(void) const' to 'unsigned int *(__thiscall ag::ui::TextArea::* const )(void) const'
2>          Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast
2>C:\Users\Alex\Documents\Visual Studio 2010\Projects\game\dev\src\game\lua\LuaTextArea.cpp(103): error C2973: 'luaU_get' : invalid template argument 'unsigned int (__thiscall ag::ui::TextArea::* )(void) const'
2>          C:\Users\Alex\Documents\Visual Studio 2010\Projects\game\dev\src\extern\LuaWrapper\LuaWrapperUtil.hpp(147) : see declaration of 'luaU_get'
2>C:\Users\Alex\Documents\Visual Studio 2010\Projects\game\dev\src\game\lua\LuaTextArea.cpp(103): error C2440: 'specialization' : cannot convert from 'unsigned int (__thiscall ag::ui::TextArea::* )(void) const' to 'unsigned int *ag::ui::TextArea::* const '
2>          There is no context in which this conversion is possible
2>C:\Users\Alex\Documents\Visual Studio 2010\Projects\game\dev\src\game\lua\LuaTextArea.cpp(103): error C2973: 'luaU_get' : invalid template argument 'unsigned int (__thiscall ag::ui::TextArea::* )(void) const'
2>          C:\Users\Alex\Documents\Visual Studio 2010\Projects\game\dev\src\extern\LuaWrapper\LuaWrapperUtil.hpp(131) : see declaration of 'luaU_get'
2>C:\Users\Alex\Documents\Visual Studio 2010\Projects\game\dev\src\game\lua\LuaTextArea.cpp(103): error C2440: 'specialization' : cannot convert from 'unsigned int (__thiscall ag::ui::TextArea::* )(void) const' to 'unsigned int ag::ui::TextArea::* const '
2>          There is no context in which this conversion is possible
2>C:\Users\Alex\Documents\Visual Studio 2010\Projects\game\dev\src\game\lua\LuaTextArea.cpp(103): error C2973: 'luaU_get' : invalid template argument 'unsigned int (__thiscall ag::ui::TextArea::* )(void) const'
2>          C:\Users\Alex\Documents\Visual Studio 2010\Projects\game\dev\src\extern\LuaWrapper\LuaWrapperUtil.hpp(123) : see declaration of 'luaU_get'
2>C:\Users\Alex\Documents\Visual Studio 2010\Projects\game\dev\src\game\lua\LuaTextArea.cpp(103): error C2440: 'initializing' : cannot convert from 'overloaded-function' to 'lua_CFunction'
2>          None of the functions with this name in scope match the target type

Answer 1

This is a compiler bug in VC++. The following code is valid:

#include <iostream>

struct TextArea
{
    unsigned GetCharacterSize() const { return 0; }
};

template<typename T, typename U, U (T::*)() const>
int foo()
{
    return 1;
}

template<typename T, typename U, U* (T::*)() const>
int foo()
{
    return 2;
}

int main()
{
    std::cout << foo<TextArea, unsigned, &TextArea::GetCharacterSize>() << '\n';
}

And compiles with GCC 4.3.4, GCC 4.5.1, and Comeau 4.3.10.1 Beta2 (no link), but yields the following error with VC++ 2010 SP1:

error C2668: 'foo' : ambiguous call to overloaded function

---

EDIT: As for a workaround, it’s ugly, but the only thing I can think of offhand is to use an extra layer of indirection so that there is no overloading involved:

#include <iostream>

struct WithPointer
{
    unsigned* GetCharacterSize() const { return nullptr; }
};

struct WithoutPointer
{
    unsigned GetCharacterSize() const { return 0u; }
};

template<bool UsePointerImplB>
struct kludge
{
    template<typename T, typename U, U (T::*Getter)() const>
    static int foo() { return 1; }
};

template<>
struct kludge<true>
{
    template<typename T, typename U, U* (T::*Getter)() const>
    static int foo() { return 2; }
};

int main()
{
    std::cout
        << kludge<false>::foo<WithoutPointer, unsigned, &WithoutPointer::GetCharacterSize>() << '\n'
        << kludge<true>::foo<WithPointer, unsigned, &WithPointer::GetCharacterSize>() << '\n';
}

Effectively this is no different than just giving each overload a different name…