CRTP fails to inherit nested types and usings when derived type is itself a template?

Asked Oct 25 '20 at 20:27

Active Oct 25 '20 at 20:36

Viewed 134 times

I have the following code which works fine:

template<typename T>
struct Foo
{
    using Bar = int;
};

struct Baz : public Foo<Baz>
{
    Bar bar;
};

Now I want to change Baz to accept a template parameter itself.

template<typename T>
struct Foo
{
    using Bar = int;
};


template<typename T>
struct Baz : public Foo<Baz<T>>
{
    Bar bar;
};

This fails to compile in every compiler with error: unknown type name 'Bar'. In fact, even if I put using Foo<Baz>::Bar; above that line, it still fails. (Yet the using itself compiles file!) Why?

asked Oct 25 '20 at 20:27

Paul Accisano

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Uh yeah I fail to see how the dupe has anything to do with what I asked.. – Paul Accisano Oct 25 '20 at 20:33
`typename Foo>::Bar bar;` – Pixelchemist Oct 25 '20 at 20:35
The dupe is better now, but if you don't want to do a `using`, you can simply do `typename Foo::Bar bar;`. – cigien Oct 25 '20 at 20:35
Okay cool, that does compile, but my question wasn't how to solve this problem, my question was why? What is it about the inclusion of a template that makes this code not compile? – Paul Accisano Oct 25 '20 at 20:36
2

@PaulAccisano - Then read the second duplicate I edited in. Types or variables make little difference here, neither does the syntax (`this->`) affect the explanation. – StoryTeller - Unslander Monica Oct 25 '20 at 20:37
Ah, the second dupe does explain much better. Thanks! – Paul Accisano Oct 25 '20 at 20:38
See: https://stackoverflow.com/questions/1567730/inheritance-and-templates-in-c-why-are-inherited-members-invisible https://stackoverflow.com/questions/1643035/propagating-typedef-from-based-to-derived-class-for-template – Pixelchemist Oct 25 '20 at 20:38

CRTP fails to inherit nested types and usings when derived type is itself a template?

0 Answers0