mycomparison是对象，函数还是函数指针？

Question

I was going through following code:

template <typename String>
void test_decimals()
{
  SensibleLessThan<String> mycomparison;
  String lhs = "1.212";
  String rhs = "1.234";
  CHECK_EQUAL(mycomparison(lhs, rhs), true); // CHECK EQUAL is macro

}

I do not understand the meaning of the following constructs:

• SensibleLessThan<String> mycomparison;

• mycomparison(lhs, rhs)

Is mycomparison an object, a function or a function pointer?

Answer 1

SensibleLessThan<String> is a type. mycomparison is an object of that type. Now it appears that the type overloads operator() , which allows it to be called as though it were a function. Objects of such types are usually known as function objects or functors . Such objects are, like functions, considered callable .

For a simple example, here's an adder struct that overloads operator() . We can create an object of the adder type and then use that object as though it were a function.

#include <iostream>

struct adder
{
  int operator()(int a, int b) { return a + b; }
};

int main()
{
  adder my_adder;
  std::cout << my_adder(5, 6) << std::endl;
}

Here it is in action . In fact, a similar type already exists in the C++ standard library: std::plus .