Template question...

[barcaroller]

I have two template classes, where one uses the first for a list type.

template <typename T> class B
{
};


template <typename T> class A
{
typedef list< B<T> >::iterator MyIterator_t; // <--- error here
}


Compiler error: type std::list, std::allocator > > is not derived
from type A


Does anyone know what the error means and how to fix it?

 

[Juha Nieminen]

I have two template classes, where one uses the first for a list type.

template <typename T> class B
{
};


template <typename T> class A
{
typedef list< B<T> >::iterator MyIterator_t; // <--- error here
}


Compiler error: type std::list, std::allocator > > is not derived
from type A


Does anyone know what the error means and how to fix it?

It's fixed by adding the keyword 'typename' after the 'typedef'.

The reason is that the type definition uses the dependent name T, and
thus list< B<T> >::iterator being a type name has to be specified
explicitly.

 

[Juha Nieminen]

typedef class list< B<T> >::iterator MyIterator_t;

I think 'typename' is the more appropriate keyword.

 

[Alf P. Steinbach]

* barcaroller:

I have two template classes, where one uses the first for a list type.

template <typename T> class B
{
};

template <typename T> class A
{
typedef list< B<T> >::iterator MyIterator_t; // <--- error here
}


Compiler error: type std::list, std::allocator > > is not derived
from type A


Does anyone know what the error means and how to fix it?

The following compiles fine:

#include <list>

using namespace std;

template <typename T> class B
{
};

template <typename T> class A
{
typedef typename list< B<T> >::iterator MyIterator_t;
};


int main()
{
A<int>();
}

I added a semicolon and a word to your code.

Check the FAQ item on dependent names (assuming there is such a FAQ item, but
check -- it's nearly always a good idea to consult the FAQ ).


Cheers & hth.,

- Alf (helping others but would rather work, however, none in Norway want me)

 

[red floyd]

* barcaroller:








The following compiles fine:

     #include <list>

     using namespace std;

     template <typename T> class B
     {
     };

     template <typename T> class A
     {
         typedef typename list< B<T> >::iterator  MyIterator_t;
     };

     int main()
     {
         A<int>();
     }

I added a semicolon and a word to your code.

Check the FAQ item on dependent names (assuming there is such a FAQ item, but
check  --  it's nearly always a good idea to consult the FAQ ).

That's 35.19 and 35.20, and it doesn't discuss typename in this
context, alas. 35.19 really needs updating, or alternatively, a new
item should be added to section 35.

 

[barcaroller]

I have two template classes, where one uses the first for a list type.

template <typename T> class B
{
};


template <typename T> class A
{
typedef list< B<T> >::iterator MyIterator_t; // <--- error here
}


Compiler error: type std::list, std::allocator > > is not derived
from type A


Does anyone know what the error means and how to fix it?

Thank you all for your (correct) answers.