接受可变参数 lambda 时 std::function 的签名

Question

Suppose I have a lambda and a class; I want to assign the lambda to the class as a static variable. How could I do that?

auto func = [](void* p, auto&&... args) { /* do things */};
class A {
  static std::function<void(void*, ???)> f_;
};

I am not very sure what to put in the question marks. I tried

class A {
  template<class...Arg>
  static std::function<void(void*, Arg...)> f_;
};

But when I assign like

A::f_ = func;

Then it is complaining about invalid use of incomplete type 'class std::function<void(void*, Arg ...)>'

Please help!

Answer 1

std::function is a function wrapper; it is not a function template wrapper. It has to have a specific set of arguments. You can do this for example for argument list int, double, float :

inline static std::function<void(void*, int, float, double)> f_ = func;

---

I want to assign the lambda to the class as a static variable.

Then, perhaps it would be better to not wrap it in a std::function :

inline static decltype(func) f_ = func;

---

But when I assign like

A::f_ = func;

Then it is complaining about ...

This is because you didn't pass the template arguments to the template variable. This would work:

A::f_<int, double, float> = func;