CODEWITHSUNDEEP
templatesc++14partial-specialization
Garo
Garo

Reputation: 111

Why is the partial specialisation of a template template argument not working?

I am considering Class1, Class2, Class3.

Class2 and Class3 have a partial specialisation with int, so they are identical in their definition.

On the other hand, Class1 has a specialisation for Class3<T> and for a general template template with only one argument, i.e. Unary<T>. So Class1 has no specialisation for Class2<T>.

It happens that Class1 <Class3<T>>::type is actually Class3<T>. Indeed I explicitly wrote the specialisation. However, the compiler says Class1 <Class2<T>>::type it is not defined. But I defined the specialisation for a the template template case, Class1<Unary<T>>. Why is the compiler not recognising it? How can I make the compiler choose the most specialised case (Class1 <Class3<T>>::type), if it exists, and, if does not, the template template case (Class1<Unary<T>>)? Thank you

template<typename...T>
class Class1;

template<typename...T>
class Class2;

template<typename...T>
class Class3;

template<>
class Class2<int>
{};

template<>
class Class3<int>
{};

template<typename T>
class Class1<Class3<T>>
{
 public:
  using type=Class3<T>;
};

template<template<class> class Unary, typename T>
class Class1<Unary<T>>
{
 public:
  using type=Unary<T>;
};

Upvotes: 2

Views: 43

Answers (1)

bartop
bartop

Reputation: 10315

Before C++17 such template:

template<template<class> class Unary, typename T>
class Class1<Unary<T>>

should not mach with variadic templates. Same for templates with defaulted arguments (check this with std::vector for example) - Template template parameter and default values. It was a defect in standard and fix came in C++17

Upvotes: 1

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