Given the following class template
template <typename T, typename VT, typename PF, typename ... Sub>
class InheritableAccess;
template <typename T, typename VT, typename PF, typename F, typename ... Sub>
class InheritableAccess<T, VT, PF, F, Sub...> : public InheritableAccess<T, typename F::Type, F, Sub...>{};
template <typename T, typename PF, typename ... Sub>
class InheritableAccess<T, T, PF, Sub...>{};
Trying to instantiate it
internal::InheritableAccess<inheritable, sub1::Type, sub1, sub2> t;
Produces the following compilation error:
Test.h(32,1): error C2504: 'internal::InheritableAccess<T,SubItem2,F>': base class undefined
1> with
1> [
1> T=inheritable,
1> F=sub2
1> ]
1>test-project2.cpp(21): message : see reference to class template instantiation 'internal::InheritableAccess<inheritable,SubItem1,sub1,sub2>' being compiled
Why is that?
Runnable code example below:
Test.h
#pragma once
template <typename T, typename A, int8_t I = 0>
struct SubClass
{
typedef T Type;
typedef A AttributeType;
static const int8_t id = I;
};
namespace internal {
template <typename T, typename VT, typename PF, typename ... Sub>
class InheritableAccess;
template <typename T, typename VT, typename PF, typename F, typename ... Sub>
class InheritableAccess<T, VT, PF, F, Sub...> : public InheritableAccess<T, typename F::Type, F, Sub...>
{
};
template <typename T, typename PF, typename ... Sub>
class InheritableAccess<T, T, PF, Sub...>
{
};
}
template <typename Super, typename ... Sub>
class Inheritable;
Test.cpp
#include <iostream>
#include "Test.h"
class Item {
};
class SubItem1 : Item {};
class SubItem2 : Item {};
class SubItem3 : Item {};
class ItemAttr {};
int main()
{
using sub1 = SubClass<SubItem1, ItemAttr, 1>;
using sub2 = SubClass<SubItem2, ItemAttr, 2>;
using sub3 = SubClass<SubItem3, ItemAttr, 3>;
using inheritable = Inheritable<Item, sub1, sub2, sub3>;
internal::InheritableAccess<inheritable, sub1::Type, sub1, sub2> t;
return 0;
}
>Solution :
The primary internal::InheritableAccess template doesn’t have a definition.
Add:
namespace internal {
template <typename T, typename VT, typename PF, typename ... Sub>
class InheritableAccess{};
// ^^