Template parameter depending on another one

[Arne Claus]

Hi
I got a headache on this problem, maybe someone can help me here
I want to create a class which can be handled like this

manager<int, list> // possible variant
manager<int, vector> // another possible variant
manager<float, vector> // and so on

I cannot just create / pass a vector<int>. I need the first information
(e.g. int) inside the manager class, because I wrap this type inside
another template (see below)
The problem is - how do I define such a template? My first Idea was
"leave it to the compiler"

template <class tContent, class tContainer> class manager {

void add(anotherBase<tContent>);

protected:
tContainer<anotherBase<tContent> > m_container;
};

which does not work, because "tContainer is not a template".
So my question - is this possible at all? I did not find anything on
that topic in "The C++ Programming Language", and that's a IMHO a bad
sign ...
Maybe someone knows another way of doing such a thing (derivation would
be possible, but ugly in this case).

Arne

 

[Fraser Ross]

I think you are not using the keyword typename when using a dependent type
name. If you still have a problem then post again.

Fraser.

 

[Dan Cernat]

Hi
I got a headache on this problem, maybe someone can help me here
I want to create a class which can be handled like this

manager<int, list> // possible variant
manager<int, vector> // another possible variant
manager<float, vector> // and so on

I couldn't find a way using templates, so I switched to macros. The
code below compiles on VC 7.1

// manager.h

#include <vector>
#include <list>

#define DECLARE_MANAGER_CLASS(Content, Namespace, Container) \
class manager_##Content##_Namespace##_##Container \
{ \
public: \
void add(Content value) \
{ \
m_container.push_back(value); \
} \
\
protected: \
Namespace::Container <Content> m_container; \
}

#define MANAGER_CLASS(Content, Namespace, Container) \
manager_##Content##_Namespace##_##Container

// we declare the classes we need
DECLARE_MANAGER_CLASS(int, std, vector);
DECLARE_MANAGER_CLASS(float, std, list);


// main.cpp

#include "manager.h"

int main()
{
MANAGER_CLASS(int, std, vector) x;
MANAGER_CLASS(float, std, list) y;

return 0;
}

I don't like the idea of maintaining identical parameter lists in two
places, but at least, if they are mismatched, you'll get a compilation
error.

dan

 

[Victor Bazarov]

I got a headache on this problem, maybe someone can help me here
I want to create a class which can be handled like this

manager<int, list> // possible variant
manager<int, vector> // another possible variant
manager<float, vector> // and so on

I cannot just create / pass a vector<int>. I need the first information
(e.g. int) inside the manager class, because I wrap this type inside
another template (see below)
The problem is - how do I define such a template? My first Idea was
"leave it to the compiler"

template <class tContent, class tContainer> class manager {

void add(anotherBase<tContent>);

protected:
tContainer<anotherBase<tContent> > m_container;
};

which does not work, because "tContainer is not a template".

So, what prevents you from declaring 'tContainer' a template?

So my question - is this possible at all? I did not find anything on
that topic in "The C++ Programming Language", and that's a IMHO a bad
sign ...
Maybe someone knows another way of doing such a thing (derivation would
be possible, but ugly in this case).

template<class tContent, template<class> tContainer> class manager {
....
};

Now, you won't be able to use 'vector' or 'list' directly because they
can have more than one template argument. You can, however, do a trick
with a special template template wrapper:

template<class T> struct use_vector { typedef std::vector<T> t; };
template<class T> struct use_list { typedef std::list<T> t; };

and now, when you declare your manager, do:

manager<int, use_vector::t> miv;
manager<double, use_list::t> mdl;

V

 

[Arne Claus]

So, what prevents you from declaring 'tContainer' a template?

template<class tContent, template<class> tContainer> class manager {

Ah - I already tried (which did not work)

template<class tContent, template<class T> tContainer<T>> class manager

but your attempt somehow makes more sense

Now, you won't be able to use 'vector' or 'list' directly because they
can have more than one template argument.

That's no problem, as I don't actually use vector or list but own
classes (wrappers on e.g. vectors) with exactly one parameter - so that
works fine. But nice to know what to do in such a case.

Thank you.
Arne