template<> keyword - unexpected hides base typedefs

[Grizlyk]

Hello.

Why were base class "typedefs" hidden by template<> and explicit usage
of them does not work too?

Try open only one of the lines in the example below
//using Tparent::Tptr;
//typedef Tparent::Tptr Tptr;

Consider example:

template< class A, class B> class Z;
template< class A, class B> class Ext1;


template< class A, class B>
class Base
{
public:
typedef Z<A,B> Tptr;
};


template< class A >
class Derived: public Base<A, Ext1<int,int> >
{
public:
typedef Base<A, Ext1<int,int> > Tparent;

using Tparent::Tptr;
typedef Tparent::Tptr Tptr;

inline void test(const Tptr obj)throw();
};


class Base2
{
public:
typedef Z<int,int> Tptr;
};


class Derived2: public Base2
{
public:
typedef Base2 Tparent;

using Tparent::Tptr;
//typedef Tparent::Tptr Tptr;

inline void test(const Tptr obj)throw();
};


/*

5.cpp:18: error: ISO C++ forbids declaration of 'Tptr' with no type
5.cpp:18: error: cannot declare member 'Base<A, Ext1<int, int> >::Tptr'

within 'Derived<A>'
5.cpp:18: error: expected ';' before 'Tptr'
5.cpp:20: error: expected ',' or '...' before 'obj'
5.cpp:20: error: ISO C++ forbids declaration of 'Tptr' with no type

*/

 

[Ondra Holub]

Grizlyk napsal:

Hello.

Why were base class "typedefs" hidden by template<> and explicit usage
of them does not work too?

Try open only one of the lines in the example below
//using Tparent::Tptr;
//typedef Tparent::Tptr Tptr;

Consider example:

template< class A, class B> class Z;
template< class A, class B> class Ext1;


template< class A, class B>
class Base
{
public:
typedef Z<A,B> Tptr;
};


template< class A >
class Derived: public Base<A, Ext1<int,int> >
{
public:
typedef Base<A, Ext1<int,int> > Tparent;

using Tparent::Tptr;
typedef Tparent::Tptr Tptr;

inline void test(const Tptr obj)throw();
};


class Base2
{
public:
typedef Z<int,int> Tptr;
};


class Derived2: public Base2
{
public:
typedef Base2 Tparent;

using Tparent::Tptr;
//typedef Tparent::Tptr Tptr;

inline void test(const Tptr obj)throw();
};


/*

5.cpp:18: error: ISO C++ forbids declaration of 'Tptr' with no type
5.cpp:18: error: cannot declare member 'Base<A, Ext1<int, int> >::Tptr'

within 'Derived<A>'
5.cpp:18: error: expected ';' before 'Tptr'
5.cpp:20: error: expected ',' or '...' before 'obj'
5.cpp:20: error: ISO C++ forbids declaration of 'Tptr' with no type

*/

There is missing keyword typedef:

template< class A, class B> class Z;
template< class A, class B> class Ext1;

template< class A, class B>
class Base
{
public:
typedef Z<A,B> Tptr;

};

template< class A >
class Derived: public Base<A, Ext1<int,int> >
{
public:
typedef Base<A, Ext1<int,int> > Tparent;

using Tparent::Tptr;
typedef typename Tparent::Tptr Tptr2;

inline void test(const Tptr2 obj)throw();

};

class Base2
{
public:
typedef Z<int,int> Tptr;

};

class Derived2: public Base2
{
public:
typedef Base2 Tparent;

using Tparent::Tptr;
//typedef Tparent::Tptr Tptr;

inline void test(const Tptr obj)throw();

};

int main()
{
}

 

[Grizlyk]

There is missing keyword typedef:

Where exactly?

I want to say, that compiler can not compile even well defined (known)
type from base class.

 

[Ondra Holub]

Grizlyk napsal:

Where exactly?

I want to say, that compiler can not compile even well defined (known)
type from base class.

Oh, sorry. I made typo - there is missing typename (not typedef) in
typedef typename Tparent::Tptr Tptr2

Here is a touched part of code:

template< class A >
class Derived: public Base<A, Ext1<int,int> >
{
public:
typedef Base<A, Ext1<int,int> > Tparent;

using Tparent::Tptr;
typedef typename Tparent::Tptr Tptr2;

inline void test(const Tptr2 obj)throw();
};

 

[Grizlyk]

Ondra Holub write:

Oh, sorry. I made typo - there is missing typename (not typedef) in
typedef typename Tparent::Tptr Tptr2

Here is a touched part of code:

template< class A >
class Derived: public Base<A, Ext1<int,int> >
{
public:
typedef Base<A, Ext1<int,int> > Tparent;

using Tparent::Tptr;
typedef typename Tparent::Tptr Tptr2;

inline void test(const Tptr2 obj)throw();
};

Yes, it is work. Thanks.
But it is very not clear to me, why I can declare with "using" method
of base class, but can not do type.