c 20 concepts in derived template class-CodePudding

for my test about CRTP I created this base class:

template<typename Derived>
struct Base
{
    void print() const { std::cout << "print\n"; }
};

And this class:

struct A: public Base<A>
{
    void printSub() { std::cout << "printSub from A\n"; }
};

And I can use these classes without problem:

A a1{};
a1.print();
a1.printSub();

My problem is: how is possible to use concept/requires for template class?

I want add something like this:

template<typename Derived> requires requires (Derived t) {
    t.printSub();
}
struct Base
{
    void print() { std::cout << "print\n"; }
};

To accept only Derived class with printSub method. But I receive this error:

template constraint failure...

I did some tests but I did not find solutions (I also found this thread but it can't help me).

Where am I wrong? Thanks for any help

CodePudding user response：

You cannot meaningfully constrain the derived class template parameters of a CRTP base class for the same reason that you can't do this:

template<typename Derived>
class Base
{
  using alias = Derived::SomeAlias;
};

Derived isn't complete yet, and using a constraint usually requires completeness.