Two template classes use each other as template argument

» cpp » Two template classes use each other as template argument

By : benekk

Date : January 12 2021, 09:11 PM

wish helps you Two template classes can use each other as template argument, but if I use anything defined in one class scope, it cannot be compiled. , gcc's error message is much more informative:

code :

t.C:8:5: error: need ‘typename’ before ‘A::pointer’ because ‘A’ is a dependent scope
    8 |     A::pointer pa;
      |     ^
      |     typename

template<class B> struct A {
    typedef A *pointer;
};
template<class A> struct B {
    //typedef A::pointer APtr;
    //using APtr = A::pointer;
    //APtr pa;
    typename A::pointer pa;
};

struct AA;
struct BB;
struct AA : public A<BB> {};
struct BB : public B<AA> {};

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Why the template with default template arguments can't be used as template with less template argument in Template Templ

By : user3277441

Date : March 29 2020, 07:55 AM

it helps some times From the standard (see 14.3.3 paragraph 1 - [temp.arg.template):

use of class template requires template argument list, nested classes

By : Milan Lama Sindhuli

Date : March 29 2020, 07:55 AM

hop of those help? I'm trying to implement by own generic/template ArrayList and I've run into a snag. I know the error comes from not having somewhere in the argument list but for me I can't figure it out here, I get a different error if I do. For brevity I've removed functions I'm unable to debug until this one is debugged first. , Try this:

code :

template <class T> 
typename ArrayList<T>::Node* ArrayList<T>::Node::getNext()
{
    return this->next;
}

template <class T> 
auto ArrayList<T>::Node::getNext() -> Node*
{
    return this->next;
}

How to declare a template function taking a template template parameter with a known template argument type?

By : Nivag1288

Date : March 29 2020, 07:55 AM

around this issue I have the following two functions: , In C++11, you can use a variadic template template parameter:

code :

template <template<typename...> class T>
void SpitLeaves(std::string & sdata, T<Leaf> const & leaves)

template <template<typename...> class T, typename... TX>
void SpitLeaves(std::string & sdata, T<Leaf, TX...> const & leaves)

Why does template argument deduction/substitution fail with std::tuple and template derived classes?

By : Thanawach Nilnarong

Date : March 29 2020, 07:55 AM

help you fix your problem Deduction doesn't work this way. It pulls in before any conversion or whatever. Here the compiler expects a Base from which to deduce T and you are trying to pass a DerivedN. They are completely different beasts from the point of view of the type system and the function is discarded when trying to find a good match for the call.
Look at the error, it's quite clear.

code :

#include<type_traits>

// ...

template<template<typename> class C1, template<typename> class C2, typename T1, typename T2>
std::enable_if_t<(std::is_base_of<Base<T1>, C1<T1>>::value and std::is_base_of<Base<T2>, C2<T2>>::value)>
function(std::tuple<C1<T1>&, C2<T2>&> arg)
{
    std::cout << "Hello\n";
}

// ...

How can a recover the non-typename template argument from each type in a variadic pack of template classes?

By : Aman

Date : March 29 2020, 07:55 AM

Does that help
For clarification what I am trying to accomplish is recovering the template arguments from one template class to use them in another. Below are three template classes: MyObject, MyTuple, MyPack. MyTuple takes MyObject objects as its template parameters. I want to recover the MyObject template parameters to use as the template argument for a MyPack object.

code :

template <typename>
struct getMyPack;

template <template <typename...> class C,
          template <int> class ... Cs, int ... Is>
struct getMyPack<C<Cs<Is>...>>
 { using type = MyPack<Is...>; };

#include <type_traits>

template <int>
struct MyObject1
 { };

template <int>
struct MyObject2
 { };

template <int...>
struct MyPack
 { };

template <typename...>
struct MyTuple
 { };

template <typename>
struct getMyPack;

template <template <typename...> class C,
          template <int> class ... Cs, int ... Is>
struct getMyPack<C<Cs<Is>...>>
 { using type = MyPack<Is...>; };


int main ()
 {
   using T0 = MyTuple<MyObject1<1>, MyObject2<2>, MyObject1<3>>;
   using T1 = MyPack<1, 2, 3>;
   using T2 = typename getMyPack<T0>::type;

   static_assert( std::is_same<T1, T2>::value, "!" );
 }