How to define the constructor of an inner template class of another template class?

Question

I have an inner template class of another template class:

// hpp file
template< class T1 > class C1
{
   // ...
public:
   // ...
   C1();
   template< class T2 > C2
   {
      // ...
      C2();
   };
};

When I declare the inner class constructor I get some errors:

//cpp file
template<> C1< MyType >::C1()
{
   // ...
}

template<> template< class T2 > C1< MyType >::C2::C2() // error: invalid use of template-name ‘class C1<MyType>::C2’ without an argument list    
{
   // ...
}

I have also tried :

template<> template< class T2 > C1< MyType >::C2<T2>::C2() // error: invalid use of incomplete type ‘class C1<MyType>::C2<T2>’
{
   // ...
}

Incomplete type, but constructor has no type...

I am a little stuck here. How to declare it?

First off [You cannot split a template class in a .hpp file and .cpp file](http://stackoverflow.com/questions/1724036/splitting-templated-c-classes-into-hpp-cpp-files-is-it-possible) — NathanOliver, Oct 27 '15 at 15:29
No, I am not trying to specialize the inner class as well, is that the problem? Why? :) — sop, Oct 27 '15 at 15:32
@NathanOliver I know that you can split it if there is a specialization... So that is why i shall specialize also the inner class? — sop, Oct 27 '15 at 15:34
Yes, I am trying to do a specialization for MyType, but I am not specialising the inner class; is that the problem? — sop, Oct 27 '15 at 15:35
You (all) are right, I shall define the inner class' members in the header... — sop, Oct 27 '15 at 15:42

score 2 · Accepted Answer · answered Oct 27 '15 at 15:29

2

Perform the following:

template<typename T1>
template<typename T2>
C1<T1>::C2<T2>::C2()
{
}

answered Oct 27 '15 at 15:29

James Adkison

9,412
2
29
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That is it, but why is it not working for a specialization? – sop Oct 27 '15 at 15:31
@sop What are you trying to specialize? – James Adkison Oct 27 '15 at 15:35
1

@sop You should verify this but if you specialize the outer class I'm pretty sure you need to specialize the inner class as well. – James Adkison Oct 27 '15 at 15:46

score 1 · Answer 2 · answered Oct 28 '15 at 03:29

1

You can't specialize the outer class by defining an inner template class's method. If you want to specialize both the inner class and outer class, you can:

template<>
template<> 
C1<MyType>::C2<char>::C2()
{
   // ...
}

LIVE

If you want to keep the inner class generic, you should specialize the outer class first:

template<> 
class C1<MyType>
{
   // ...
public:
   // ...
   C1();
   template< class T2 > class C2
   {
      public:
      // ...
      C2();
   };
};

and then define the constructor of C2 like,

template<class T2> 
C1<MyType>::C2<T2>::C2()
{
   // ...
}

LIVE

answered Oct 28 '15 at 03:29

songyuanyao

169,198
16
310
405

Well I have another question then: what does `template< class T = MyClass > class C1 {};` means? Isn't it a specialization? – sop Oct 28 '15 at 08:01
1

@sop It's another definition of `C1`, so you'll get a redefinition error. `class T = MyClass` means `MyClass` is the default template parameter, so you can use it like `C1<> c1`, but it's not specialization. – songyuanyao Oct 28 '15 at 08:12

How to define the constructor of an inner template class of another template class?

2 Answers2