Equality comparison

Synopsis

#include <origin/generic.hpp>

template<typename T>
concept bool Equality_comparable() {
  return requires (T a, T b) {
    { a == b } -> Optional;
    { a != b } -> Optional;
  };
}

Documentation

An equality comparable type can be used with == to determine when its objects are equal (i.e., have the same value), and can be used with != to determine when its objects differ.

Semantics

• == shall be an equivalence relation (reflexive, symmetric, and transitive).

• If a == b is true, a is equal to b. In other words, when a == b is true, a and b can be substituted in equality preserving expressions.

• The expression a != b is equal to !(a == b).

Example

// Assume that g is a regular function.
template<typename T>
int g(T);

template<Equality_comparable T>
void f(T a, T b)
{
  if (a == b) {
    // Because a and b are equal, we can use 
    // g(a) or g(b) and get the same results.
  }
}