C++ Concepts : CRTP

Question

It's probably just that the concept is bad, but I can't see why. And didn't find any example with constructor. Or maybe it has nothing to do with the constructor...

template < typename T >
concept bool C_Object() {
  return requires {

    T();
  };
}

template < C_Object Object>
class  DefaultManager {

  //  Content;
};

template < C_Object Derived >
class  Component {

  // Content
};

struct Test : public Component<Test> {

  int data;

  Test() = default;
};

int main() {

  Test test;
  return 0;
}

Give the error :

test2.cpp:21:36: error: template constraint failure
 struct Test : public Component<Test> {
                                    ^
test2.cpp:21:36: note:   constraints not satisfied
test2.cpp:2:14: note: within ‘template<class T> concept bool C_Object() [with T = Test]’
 concept bool C_Object() {
              ^~~~~~~~
test2.cpp:2:14: note: the required expression ‘T()’ would be ill-formed

That sound like a : "Hey my code is broken, please fix it", sorry.

Anyway thanks

Have a great day

You know that concepts didn't make it to the standard?

SergeyA
– SergeyA

2016-11-28 20:42:43 +00:00
Commented Nov 28, 2016 at 20:42 — SergeyA
– SergeyA, Commented Nov 28, 2016 at 20:42
@SergeyA yes probably c++20

Mathieu Van Nevel
– Mathieu Van Nevel

2016-11-28 20:44:36 +00:00
Commented Nov 28, 2016 at 20:44 — Mathieu Van Nevel
– Mathieu Van Nevel, Commented Nov 28, 2016 at 20:44

Casey · Accepted Answer · 2016-11-28 21:03:51Z

14

The problem is here:

struct Test : public Component<Test> {

Whenever you so much as name a specialization of a constrained class template, the given parameters are validated against the constraints. In this particular case, that means that C_Object<Test> is checked for satisfaction, but since Test is incomplete - the compiler hasn't parsed its definition yet - C_Object is not satisfied.

This is the "concepts" version of the classical problem with CRTP bases: you must delay inspection of the derived class until its definition is complete.

answered Nov 28, 2016 at 21:03

Casey

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5 Comments

Mathieu Van Nevel Over a year ago

I even didn't know about CRTP. Too much for this night I think. If you're crazy enough I love an explanation :) But anyway I'm really lucky that you was there. PS : I'll close the question when I'll read about CRTP and stuffs (Tomorrow).

Mathieu Van Nevel Over a year ago

Even on StackOverflow : stackoverflow.com/questions/19886041/… I didn't find why my code is not working, the CRTP seems to be okay (works perfect without concepts). I must delay inspection of the derived class as you said, but can't find any information about that.

Luc Danton Over a year ago

@MathieuVanNevel the usual way I achieve the delaying you mention is by making sure to make the types re-dependent, informally speaking (i.e. I'm really using an actually dependent type).

Mathieu Van Nevel Over a year ago

That seems to be tricky with the constructor, but I'll try something. Thanks for the code, I was thinking it was not necessary. There is a lot of meta programming tricky like this?

Luc Danton Over a year ago

@MathieuVanNevel as a heads-up that particular technique cannot work for things that can't be templates, such as the copy/move constructors and assignment operators as well as the destructor. it works for constructor templates though.

Vinci · Accepted Answer · 2020-12-01 06:28:36Z

5

I found the most readable option to be an "impl" method with a constrained return type. Let's take the example in Luc Danton's comment and transfer it to C++20:

template <typename Var>
concept Fooable = requires(Var var) {
  {var.foo()};
};

template <typename Derived>
struct mixin {
  void bar() { impl().foo(); }
  Fooable auto& impl() { return static_cast<Derived&>(*this); } // Constrain return here!
};

// no constraint violation
struct ex1 : mixin<ex1> {
  void foo() {}
};

// no violation either
struct ex2 : mixin<ex2> {};

int main() {
  // mixin<ex1>::bar constraints respected
  ex1{}.bar();

  // mixin<ex2>::bar constraints violation
  ex2{}.bar();
}

answered Dec 1, 2020 at 6:28

Vinci

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