Using nested class of template parameter in CRTP

Question

Suppose I define a template T that uses a nested class of the template parameter P, as follows:

template<class P> class T
{
public:
    T(P& p) : p(p) {}
    P& p;
    typename P::Nested& get_nested()    { return p.nested; }
};

If I declare a class A that includes a nested class named Nested, I can define a variable of type T<A> with no problem:

class A
{
public:
    class Nested
    {
    public:
        int i;
    };
    Nested nested;
};

void test2a()
{
    A a;
    a.nested.i = 1;
    T<A> t_a(a);
    t_a.get_nested().i = 2;
}

Now, I want to declare a class B that, in the same way, includes a nested class named Nested and that inherits from T<B>, as follows:

class B : public T<B>
{
public:
    class Nested
    {
    public:
        int i;
    };
    Nested nested;
};

Compilation of the above code fails with error: "Nested is not a member of B"

I think I understand what's happening: at the time the template is entered, class B is incompletely defined because of inheritance.

However, I am wondering if there is any way to do such a thing...

Thanks for help.

ecatmur · Accepted Answer · 2015-11-20 14:22:27Z

8

You need to defer the resolution of the return type of get_nested until it is called.

One way to do this is to make the return type dependent on a template parameter:

  template<typename unused = void>
    typename std::conditional<false, unused, P>::type::Nested&
    get_nested()    { return p.nested; }

Another way (since C++14) is to use return type deduction:

  auto& get_nested()    { return p.nested; }

answered Nov 20, 2015 at 14:22

ecatmur

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4 Comments

dragonxlwang Over a year ago

Thanks for the solution and it worked great! I don't really your explanation that "defer the resolution of the return type of get_nested until it is called". Do you mean it's resolved at running time? But I thought all template including get_nested is expanded at running time?

ecatmur Over a year ago

No, this all happens at compile time - but compile time is split into "phases". What's going on here is that template instantiations occur later in compilation, after the whole file is read.

dragonxlwang Over a year ago

looks like that the linked "Phases of translation" didn't length about template/ dependent name lookup. But I'm guessing that in order to make the example work, nested template are looked up after the whole file is read (and therefore the class definitions are completed)? Was this due to sfinae (Because you put into std::conditional trait)

ecatmur Over a year ago

Yes, the return type is being looked up after the whole file is read and so the class definitions are complete. No, it's not dependent on SFINAE - although because of SFINAE, if something goes wrong the error will be that get_nested cannot be resolved, rather than that P::Nested cannot be resolved.

Petr · Accepted Answer · 2015-11-20 14:27:42Z

4

I was able to have your example compile with simply

template<class P> class T
{
public:
    T(P& p) : p(p) {}
    P& p;
    auto& get_nested()    { return p.nested; }
};

Another approach, exploiting the same trick as @ecatmur, but a bit simpler:

template<class R = P>
typename R::Nested& get_nested()    { return p.nested; }

Similarly, here the compiler has to postpone evaluation of P::Nested until you call get_nested().

edited Nov 20, 2015 at 14:27

answered Nov 20, 2015 at 14:22

Petr

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7 Comments

shrike Over a year ago

Thank you very much for these tricks, both Petr and ecatmur. However, I am using MSVC 2010 compiler which wont compile any of them. I am seriously thinking about upgrading to C++14 compliant compiler... now. Anyway, thanks a lot for your answers.

Petr Over a year ago

@SharpDressedMan, what's the exact problem and exact error message with my second solution? It seems to be even C++03 for me...

shrike Over a year ago

Error is <b>C4519: default template arguments are only allowed on a class template</b>.

shrike Over a year ago

I can change this error to a warning using "#pragma warning (1 : 4519)" but compilation fails a bit later on line "t_a.get_nested().i = 2;", with error C2783: "could not deduce template argument for 'R'". Error fixed using "t_a.get_nested<A>().i", but not that great. This works, anyway, thanks.

Petr Over a year ago

@SharpDressedMan, try to make get_nested a functor then. I will probably add such a solution when I get to a computer (I'm on tablet now)

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