invalid use of void expression in context of c++ std::function

Question

In below code snippet while calling call back function "Invalid use of void expression" error is flashed by the compiler.

#include <iostream>
#include <functional>
using namespace std;
template<class type>
class State {
public:
State(type type1,const std::function<void (type type1 )> Callback)
 {

 }

};

template <class type>
void Callback(type type1 )
{
  //Based on type validation will be done here
}

 int main()
 {
  State<int> obj(10,Callback(10));
  return 0;
}

Just want to know what is the wrong here so that same can be addressed .

Answer 1

It seems that you want to pass the Callback<int> function itself, not its return value (which there is none), to the constructor of obj. So do just that:

State<int> obj(10, Callback<int>);

Your current code actually calls Callback(10) first and then tries to take its void "return value" to pass it to the constructor of obj. Passing void is not allowed in C++, which is why the compiler is complaining. (Callback(10) is the "void expresson" here.)