如何使用 C++20 概念检查模板本身的一些约束

Question

Sorry for the ambiguous title, I couldn't find the right words to phrase it. I'm trying to force a class to define some functions/operators to be correct. for example ForwardIterator is required to have some operators otherwise I should get a compiler error. I have asked the question here and I was given a method with inheritance, it works perfectly but C++20 concepts seem to be more intuitive and give better errors. The initial idea is:

template <typename T> concept is_ForwardIterator= requires(T x, T y)
{
    x == y;
};
template <typename T>
requires  is_ForwardIterator<T>
struct ForwardIterator {
};

Then whenever I want to implement a forward iterator I inherit from ForwardIterator :

struct MyCustomIterator
   : public ForwardIterator <MyCustomIterator> 
{
  bool operator ==(const MyCustomIterator& other) const;
};

But the compiler keeps complaining that MyCustomIterator doesn't satisfy the requirement:

error: constraints not satisfied for class template 'ForwardIterator' [with T = MyCustomIterator]
note: because 'MyCustomIterator' does not satisfy 'is_ForwardIterator'
template <typename T> requires is_ForwardIterator<T> struct ForwardIterator {
                               ^
note: because 'x == y' would be invalid: invalid operands to binary expression ('MyCustomIterator' and 'MyCustomIterator')
        x == y;

Answer 1

When you try to use the CRTP, during the compilation of the base class template, the derived class is incomplete . As such, asking most questions about its behavior is impossible. This includes most of the reasons to use a concept.

What you're trying to do cannot reasonably be done that way. You may need a traits class or something, depending on what your goal is.

Answer 2

The problem is that where the constraint on ForwardIterator is specified, the class T (that is, MyCustomIterator ) is incomplete - it exists as a class name only.

You need to delay checking of the constraint until after MyCustomIterator is complete. One way to do this is a constraint on the destructor of ForwardIterator :

template <typename T>
struct ForwardIterator {
    ~ForwardIterator() requires is_ForwardIterator<T> {}
};

The only problem here is that this will not be checked until there is a reason to instantiate the destructor ~ForwardIterator , ie if you do not actually instantiate a MyCustomIterator object it will not be checked. But this should not be a problem in practice.

Answer 3

In CRTP, derived class in incomplete, so checking its property is not possible.

One way to check interface with traits/concept is simply static_assert after the class:

template <typename T> concept is_ForwardIterator= requires(T x, T y)
{
    x == y;
    // ...
};

struct MyCustomIterator
{
    bool operator ==(const MyCustomIterator& other) const;
    // ...
};
static_assert(is_ForwardIterator<MyCustomIterator>);