将接受任意数量和类型的参数的函数作为类模板参数传递

Question

I am aware that functions can be template arguments. However GCC, Clang and MSVC (compile versions on rextester ) do not compile compile when the template is variadic, as shown below:

void Func( int ){}

template<void (*)(int)>
struct Foo{};

template struct Foo<Func>; // Compiles

template<typename>
struct Bar;

template<typename ...Args>
struct Bar<void(*)(Args...)>
{
};

template struct Bar<Func>; // Does NOT compile (why???)

int main()
{
}

MSVC produces the most verbose output and possible explanation (rightly or wrongly) as to why the code does not compile.

source_file.cpp(20): error C2923: 'Bar': 'Func' is not a valid template type argument for parameter 'T'
source_file.cpp(1): note: see declaration of 'Func'
source_file.cpp(20): error C2990: 'Bar': non-class template has already been declared as a class template
source_file.cpp(13): note: see declaration of 'Bar'
source_file.cpp(20): error C2946: explicit instantiation; 'Bar' is not a template-class specialization  

What is the appropriate syntax for passing functions that accept any number of arguments themselves as class template arguments.

Answer 1

Func is not a type, but a function,

you might want:

template struct Bar<decltype(&Func)>;

or maybe

template<typename F, F f> struct Bar;

template <typename ...Args, void(*f)(Args...)>
struct Bar<void(*)(Args...), f>
{
};

and Bar<decltype(&Func), &Func> .

Which can be simplified to (since C++17):

template <auto> struct Bar;

template <typename ...Args, void(*f)(Args...)>
struct Bar<f>
{
};

and Bar<&Func> .