CODEWITHSUNDEEP
c++c++20c++-concepts
Mathieu Dutour Sikiric
Mathieu Dutour Sikiric

Reputation: 624

How to implement properties in C++ concepts

The C++ concepts are a relatively new idea in C++. But we have few available explanation as to how it works.

struct contain {
public:
  using Tin = int;
  using Tout = int;
  Tout sqr(Tin x)
  {
    return x * x;
  }
  contain(int _x) : x(_x)
  {
  }
  int get_x() const
  {
    return x;
  }
private:
  int x;
};


int cube(contain u)
{
  int x = u.get_x();
  return x * x * x;
}

That is we want to have a concept that tests that contain contains Tin, Tout and a member function Tout sqr(Tin). Would it also be possible to test if there is a non-member function cube(contain)?

Upvotes: 0

Views: 165

Answers (1)

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170064

That is we want to have a concept that tests that contain contains Tin, Tout and a member function Tout sqr(Tin). Would it also be possible to test if there is a non-member function cube(contain)?

It kinda depends on what you mean by that. You can of course write a concept for something that behaves "enough like" contain:

#include <concepts>

template<class C>
concept ContainLike = requires(C c, typename C::Tin in) // typename only needed for Clang, until it implements P0634
{
    { c.sqr(in) } -> std::same_as<typename C::Tout>;
    cube(c);
};

See it live

The concept checks for a type C, that when given two hypothetical objects of type C and C::Tin, a list of explicit constraints holds:

  1. c.sqr(in) is well-formed and returns C::Tout.
  2. cube(c) is well-formed (can be found by ADL in our case). The type of the expression isn't constrained here.

Note that the test for C::Tout and C::Tin is implicit here. If C doesn't contain those types, it will be a substitution failure, which will make the concept not be satisfied (in a SFINAE friendly way).

Another thing worth mentioning is that this doesn't check C contains a sqr method whose parameter is a Tin. Rather it checks that a Tin may be passed as an argument (which may entail conversions to the actual parameter type). This is the more natural use of concepts: duck typing.

Upvotes: 2

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