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Showing content from https://cplusplus.com/is_function below:

class template

<type_traits>

std::is_function

template <class T> struct is_function;

Is function

Trait class that identifies whether T is a function type.

It inherits from integral_constant as being either true_type or false_type.

Template parameters

T: A type.

Member types Inherited from integral_constant:

Member constants Inherited from integral_constant:
member constant definition value either true or false
Member functions Inherited from integral_constant:

operator bool: Returns value (public member function)

Example

// is_function example
#include <iostream>
#include <type_traits>

int a(int i){return i;}                           // function
int(*b)(int)=a;                                   // pointer to function
struct C { int operator()(int i){return i;} } c;  // function-like class

int main() {
  std::cout << std::boolalpha;
  std::cout << "is_function:" << std::endl;
  std::cout << "decltype(a): " << std::is_function<decltype(a)>::value << std::endl;
  std::cout << "decltype(b): " << std::is_function<decltype(b)>::value << std::endl;
  std::cout << "decltype(c): " << std::is_function<decltype(c)>::value << std::endl;
  std::cout << "C: " << std::is_function<C>::value << std::endl;
  std::cout << "int(int): " << std::is_function<int(int)>::value << std::endl;
  std::cout << "int(*)(int): " << std::is_function<int(*)(int)>::value << std::endl;
  return 0;
}

Output:

is_function:
decltype(a): true
decltype(b): false
decltype(c): false
C: false
int(int): true
int(*)(int): false