Aliasing the member template typedef with using makes compiler think it is not a class template

Question

Eventually I need to filter a typelist (brigand::list - defined in brigand meta-programming library). But to start:

I have a settings class that holds a type list:

   template<typename TComponentList>
  class Settings {

    // this is an integral_constant holding bool
    template<typename TList, typename T>
    using ContainsFilter = brigand::found<TList, std::is_same<T, brigand::_1>>;

    // is the type present in component list?
       template<typename T>
    using IsComponentFilter = ContainsFilter<TComponentList, T>;
}

The brigand library offers a filter "function", but to pass my filter there i need to wrap it using brigand::bind. Here's an example of that:

using ls = brigand::list<TagA, SignatureA, ComponentB>;

// binds the template parameter - passes it to my filter
using wrapped = brigand::bind<TestSettings::IsComponentFilter, brigand::_1>;

// a list holding only ComponentB
using filtered = brigand::filter<ls, wrapped>;

The last step - which i cannot overcome, is to put the filtering inside a class, depending on a template param:

   template <class TSettings>
    struct FilteringClass
    {
        // problem line
        using wrapped = brigand::bind<TSettings::IsComponentFilter, brigand::_1>;

        // template<typename TSignature>
        // using SignatureComponents = brigand::filter<TSignature, wrapped>;
    };

When compiling the problem line i first get an error that i need to use typename keyword before TSettings:

to refer to a type member of a template parameter, use 'typename TSettings::

That sounds exactly like what i want! I guess it is needed to explicitly tell compiler to treat TSettings as a type? However when i add it, i get this error from inside brigand's bind function:

error: type/value mismatch at argument 1 in template parameter list for 'template<template<class ...> class<template-parameter-1-1>, class ...> struct brigand::bind'
     using wrapped = brigand::bind<TSettings, brigand::_1>;
 note:   expected a class template, got '(TSettings:: IsComponentFilter < <expression error>)'

I'm guessing Jarod has the answer. But we would have a better hint if you could show an example usage of `FilteringClass` (with whatever you provide as a template argument also declared). — aschepler, Jul 25 '18 at 23:07

score 1 · Accepted Answer · answered Jul 25 '18 at 23:10

So, i found what i was missing, albeit almost randomly To add that another alias correctly i needed to add ::template, like that:

template<typename T>
using wrapped = brigand::bind<typename TSettings::template IsComponentFilter, brigand::_1>;

I guess without it the fact that original alias was templated was somehow lost to the compiler.

The meaning of the keyword is nicely explained in this answer.

Aliasing the member template typedef with using makes compiler think it is not a class template

1 Answers1