Member function pointer wrapper using variadic template (gcc, clang)

Question

Why does the following code not compile under either gcc or clang:

class Foo {
public:
    void bar(int) {}
};

template< class T, typename ...Args, void(T::*Member)(Args...) >
void member_dispatch(Args&&... args, void* userdata)
{
    T* obj = static_cast<T*>(userdata);
    (obj->*Member)(std::forward<Args>(args)...);
}

int main()
{
    Foo foo;
    member_dispatch<Foo, int, &Foo::bar>(1, &foo);
    return 0;
}

See e.g. here.

This question can possibly be merged with this one, though here I get unclear compilation errors from gcc and clang (instead of VS).

Why can't you simply solve your problem with standard std::function and std::bind (or lambdas)? Or like almost all standard library functions and classes that needs callable objects, by using a single typename template argument for the function type? — Some programmer dude
– Some programmer dude, Commented Mar 24, 2018 at 12:15
This is for interfacing with C libraries that use callbacks in the form of void callback(<some params>, void* userdata) — zse
– zse, Commented Mar 24, 2018 at 12:45

llllllllll · Accepted Answer · 2018-03-24 14:21:08Z

3

When you explicitly specify the types, a parameter pack is greedy. &Foo::bar will be parsed as part of typename ...Args, which causes the error.

The correct way to write this is to put it in the function parameter list, instead of a non-type template parameter.

template< class T, typename ...Args>
void member_dispatch(Args&&... args, void(T::*Member)(Args...), void* userdata)
{
    T* obj = static_cast<T*>(userdata);
    (obj->*Member)(std::forward<Args>(args)...);
}

int main()
{
    Foo foo;
    member_dispatch<Foo, int>(1, &Foo::bar, &foo);
    return 0;
}

A Better Way:

It would be better to take advantage of C++'s template argument deduction. But here you doesn't put the parameter pact at the end of a function parameter list, which is a non-deduced context. So I suggest you re-order it, so that you don't need to specify the template argument:

template<class T, class K, typename ...Args>
void member_dispatch(K T::*ptr, void* userdata, Args&&... args)
{
    T* obj = static_cast<T*>(userdata);
    (obj->*ptr)(std::forward<Args>(args)...);
}

int main()
{
    Foo foo;
    member_dispatch(&Foo::bar, &foo, 1);
    return 0;
}

edited Mar 24, 2018 at 14:21

answered Mar 24, 2018 at 12:32

llllllllll

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5 Comments

StoryTeller - Unslander Monica Over a year ago

No wait. Empty parameter pack. You need the int. I think it'd be better to suggest a better alternative to the OP. In terms of ordering the types and parameters.

llllllllll Over a year ago

@StoryTeller I missed something. My last comment is wrong. But still non-deduced context, because pact is not the last in function parameter list

StoryTeller - Unslander Monica Over a year ago

Yeah, that's what my comment is going for too. Ultimately, it'd be better to rearrange things to allow for as much deduction as possible. Utilize C++ to it's full power and all those niceties.

zse Over a year ago

For my use case I can't pass the member function as a parameter, it needs to be compiled in as a non-type template parameter somehow

zse Over a year ago

Anyway, I guess the answer itself appears correct, thanks. According to cppreference "In a function template, the template parameter pack may appear earlier in the list provided that all following parameters can be deduced from the function arguments" and I missed that condition

Red.Wave · Accepted Answer · 2018-03-24 15:51:23Z

0

Try using a class template instead of a function template. There has also been a std syntax proposal "template << auto x >> " for none-type template parameters. Proper implementation will simplify your library syntax.

answered Mar 24, 2018 at 15:51

Red.Wave

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