why are type aliases not resolved during crtp expansion?

[Matt Langston]

I wanted my crtp base class to use a type alias defined in my derived class, but it does not work:

template <typename T>

class Base {

public:

  using int_t = typename T::int_t; // error: no type named 'int_t' in 'Derived'

};

class Derived : public Base<Derived> {

public:

  using int_t = int;

};

This seemed like a very crtp thing do - delegate to the derived class.

I worked around the issue by passing the dependent type as an additional template parameter, but I don’t like having to do this since it breaks my encapsulation:

template <typename T, typename U>

class Base {

public:

    using int_t = U;

};

class Derived : public Base<Derived, int> {

public:

    using int_t = int;

};

Why doesn’t my first example work?

Warmest regards, Matt

[Richard Smith]

The CRTP base class is instantiated before the definition of the derived class is instantiated, so Derived::int_t does not yet exist. The CRTP pattern normally works around this by putting all uses of the derived class into member functions, whose instantiations are delayed until they themselves are used (typically after the definition of the derived class is complete).

 

Warmest regards, Matt

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[Karsten Ahnert]

I think I understand why it is currently not possible to use this kind of CRTP base class. But I wonder wether it is possible to implement such a feature in the C++ language? It seems that Derived needs to be parsed before Base<Derived> can be instantiated. I am not sure if this is possible in general. Any ideas?

Thanks, Karsten

[Thiago Macieira]

On segunda-feira, 12 de setembro de 2016 22:13:18 PDT 'Karsten Ahnert' via ISO

C++ Standard - Discussion wrote:
> I think I understand why it is currently not possible to use this kind of
> CRTP base class. But I wonder wether it is possible to implement such a
> feature in the C++ language? It seems that Derived needs to be parsed
> before Base<Derived> can be instantiated. I am not sure if this is possible
> in general. Any ideas?

No, it's not. For Derived to be parsed, the compiler needs to parse its base
class, which is Base<Derived>.

--
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
Software Architect - Intel Open Source Technology Center