Use C++20 Concepts to force a class to implement a set of methods

Question

I am wondering if there is a way in C++ (especially with C++20) to write some kind of interface for classes/structs.

For example in Java interface is a completely "abstract class" that is used to group related methods with empty bodies:

interface Animal
{
  public void animalSound();
  public void run();
}

In C++ you could use a pure virtual method declarations to achieve the same behavior.

class Animal
{
public:
  virtual void animalSound() = 0;
  virtual void run() = 0;
};

But with virtual methods you have runtime costs, and I am not interested in inheritance. So this runtime cost should not be necessary. I just want compile time checks for my "Animal" classes/structs.

With C++20's Concepts I am sure that it is achievable to build a construct that you can apply to a class to guarantee that a certain set of methods is provided.

What I was trying to do looked a bit like this.

template concept Animal =
requires()
{
  (Animal_{}); // default constructable

  (Animal_{}.animalSound());
  (Animal_{}.run());
};

But I am not sure that this is very c++ish to do.

(By the way is there a way to require the return type of method to be of a specific type?)

And I am not sure how to attach this to a class/struct.

My first thought was to use a static_assert inside the class/struct:

class Cow
{
private: // compile time type checking
  static_assert(std::is_matching_concept);

public:
  void animalSound() const noexcept {}
  void run() const noexcept {}
};

Where std::is_matching_concept is a placeholder for a constraint that I can not find.

I am looking for best practice feedback and suggestions to solve my problem.

EDIT - Add use case

// given the following code

template concept Vector =
requires()
{
  (Vector_{}); // default constructable

  (Vector_{}.X())->Float_;
  (Vector_{}.Y())->Float_;
};

[[nodiscard]] constexpr auto Pow2(const auto x) noexcept
{
  return x * x;
}

[[nodiscard]] constexpr auto LengthPow2(Vector auto vec) noexcept // the use of Vector
{
  return Pow2(vec.X()) + Pow2(vec.Y());
}

// Now I want to implement a Vector
// and I want compile time checking, that I have no missed any properties

struct VectorImpl1
{
// EDITED: as @QuentinUK mentioned the static_assert should be in a public scope
//         "If in the private part of a class the concepts
//         can pass for private members which isn't what you'd want."
public:
  // EDITED: as @DavisHerring mentioned this is the way to go
  static_assert(Vector);

public:
  constexpr VectorImpl1() noexcept = default;

  constexpr VectorImpl1(float x, float y) noexcept : x_(x), y_(y) {}

private:
  float x_{};
  float y_{};

public:
  [[nodiscard]] constexpr float X() const noexcept
  { return x_; }

  [[nodiscard]] constexpr float Y() const noexcept
  { return y_; }
};

struct VectorImpl2
{
public:
  static_assert(Vector);

public:
  constexpr VectorImpl2() noexcept = default;

  constexpr VectorImpl2(float rad, float length) noexcept : rad_(rad), length_(length) {}

private:
  float rad_{};
  float length_{};

public:
  [[nodiscard]] constexpr float X() const noexcept
  { return CalcX(rad_, length_); } 

  [[nodiscard]] constexpr float Y() const noexcept
  { return CalcY(rad_, length_); }
};

user_5021064 · Answer

You can, the question is why you want to do this. If your type doesn't do what it's suppose to - you'll get a compilation error, right?

If you want to get a compilation error in the same header for some reason, you can do something like:

template 
using void_t = void;    // available since c++17 in std

template 
using cow_test = void_t<
    decltype(std::declval().moo(0)),
    decltype(std::declval().chew(0))
>;

class cow {
  public:
    void moo(int);
    void chew(int);
};

using test_cow = cow_test;

class cat {
  public:
    void meaw(int);
    void chew(int);
};

using test_cat = cow_test;

This will fail in test_cat with:

r #1) C++
x86-64 gcc 10.1
Compiler options...
1

x86-64 gcc 10.1 - 364ms
#1 with x86-64 gcc 10.1
: In substitution of 'template using cow_test = void_t().moo(0)), decltype (declval().chew(0))> [with T = cat]':

:26:30:   required from here

:8:33: error: 'class cat' has no member named 'moo'

    8 |      decltype(std::declval().moo(0)),

      |               ~~~~~~~~~~~~~~~~~~^~~

godbolt

I have a suspicion, though, that what you want is to act upon this information: basically - if my class can moo() - moo, otherwise meaw.

This can be achieved, before concepts, by using detection idiom, I suggest watching this: two part talk by Walter Brown or read this blog by Simon Brand

Use C++20 Concepts to force a class to implement a set of methods

1 Answers