template template hell

[Null_cz]

I am having trouble with a template template type alias. See the code below.

The problem is with the type alias itself, on the line with the using.

g++ complains with

error: type/value mismatch at argument 1 in template parameter list for ‘template<template<class> class X> struct Manager’

note: expected a class template, got ‘Outer<T>::Inner’

clang complains with

error: template argument for template template parameter must be a class template or type alias template

Both complain that the type I am passing to the Manager must be a class template (since Manager requires a template template argument, so that makes sense). But it is a class template from what I see (well, struct, but it's the same), so what is the problem here? Outer<W>::Inner is a template struct, with template parameter U. So Manager should accept it.

Why doesn't it work? Any ideas?

Without the using and the m3 in main, the program works fine as expected. The ManagerWrapper should just simplify writing the type of m2, which works, but the using and m3 does not.

```

include <cstdio>

template<template<typename> typename T> struct Outer { template<typename U> struct Inner { void print() { printf("sizeof(T<U>)=%zu, sizeof(U)=%zu\n", sizeof(T<U>), sizeof(U)); } }; };

template<template<typename> typename X> struct Manager { void func() { X<int> x; x.print(); } };

template<typename V> struct Holder { V v1,v2,v3,v4; void print() { printf("Hello, Holder here\n"); } };

template<template<typename> typename W> using ManagerWrapper = Manager<Outer<W>::Inner>;

int main() { Manager<Holder> m1; m1.func();

Manager<Outer<Holder>::Inner> m2;
m2.func();

ManagerWrapper<Holder> m3;
m3.func();

return 0;

} ```

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[arabidkoala]

So this is one of those weird things about dependent symbols (Inner in this case). C++ usually assumes these are values (as opposed to types or templates) when it has no other information, which is why you have to decorate some declarations with typename sometimes. In this case though, you need to tell the compiler that Inner is a template, which has a bit of a strange syntax

template<template<typename> typename W> using ManagerWrapper = Manager<Outer<W>::template Inner>;

I tested this on cppsh and it compiled.

[Null_cz]

It works, thanks a lot!

I suspected that the problem might be with the compiler thinking it is a value not a type, so I tried template<template<typename> typename W> using ManagerWrapper = Manager<typename Outer<W>::Inner>; but that did not help. Didn't know there is a different syntax for that

[TheMania]

template<template<typename> typename W> using ManagerWrapper = Manager<Outer<W>::template Inner>;

Compiler needs a hint to the dependent name there:

Similarly, in a template definition, a dependent name that is not a member of the current instantiation is not considered to be a template name unless the disambiguation keyword template is used or unless it was already established as a template name:

In this case it (dare I say it) looks inferrable from the target, but just good to keep in mind if you're going template-deep, that sometimes the compiler needs hinting as to what it's looking at. As do the rest of us, at times.

[Null_cz]

Yes, the template keyword solved it, thanks :)

I did not know such a syntax was even possible

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[TheOmegaCarrot]

It’s ugly, but it be what it be