Is it possible to capture a type template into a template argument?

user_2340002 0
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Is it possible to capture a template from a template argument i.e. have a nested template specifier with a template argument that holds the template type?

template< typename T, typename Label = std::string>
class foo {
    // ...
};

template <
            template < typename C, typename T > typename C<T>,
            // ...
            typename Label = std::string
         >
class bar {
    // ...
    C< foo< T, Label > > A;
};

For instance, I'd like to pass a generic STL container (std::vector< int >) as template argument, but declare a member of the same meta-type (std::vector) but with different value type (foo< int >) i.e. std::vector< foo< int > >. It may seem convoluted, but it would be helpful to not hardcode the type of STL container.

For context, I'm aiming at a generic container adapter/wrapper (in the line of std::stack or std::queue) that provides some higher-level functionality.

containers templates stl c++ generics
2 Answers

Yes, you can just use template specialization:

#include <string>

template<typename T, typename Label = std::string>
class foo {};

template <class T, typename Label = std::string>
class bar;

template <template<class...> class C, typename T, typename Label>
class bar<C<T>, Label> {
  C<foo<T, Label>> A;
};

Demo.

user_116387180

The other answer's approach can be generalized as a reusable template rebinder:

template<typename T>
struct rebinder;

template<template<typename...> typename T, typename... Args>
struct rebinder<T<Args...>> {
  template<typename... Us>
  using rebind = T<Us...>;
};

template<typename T, typename... Us>
using rebound = rebinder<T>::template rebind<Us...>;

// example:
#include <vector>

template<typename T>
struct MyStruct {
    rebound<T, float> vec;
};

int main() {
    MyStruct<std::vector<int>> x;
    static_assert(std::is_same_v<std::vector<float>, decltype(x.vec)>);
}

see on godbolt

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