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How to get template parameter, when i match a template?

I am matching a template, and specializing based of a template, rather
than a single class.

The codes are like,
template<template<typename T,typename Alloc = std::allocator<T>
>class C>
class pix{
};

template<>
class pix<vector>{
};
The matching works perfectly.
Now how will i get the template parameters in the specialized class?
I want to do things like,
template<>
class pix<std::vector>{
private:
std::vector<T>* v_;
};
From where will I get T & Alloc fro which it is a match?

Thanks
abir
Jun 30 '08 #1
  • viewed: 1746
Share:
4 Replies
abir wrote:
I am matching a template, and specializing based of a template, rather
than a single class.

The codes are like,
template<template<typename T,typename Alloc = std::allocator<T>
>>class C>
class pix{
};

template<>
class pix<vector>{
};
The matching works perfectly.
Now how will i get the template parameters in the specialized class?
I want to do things like,
template<>
class pix<std::vector>{
private:
std::vector<T>* v_;
};
Why a pointer to a vector. That looks fishy.
From where will I get T & Alloc fro which it is a match?
You won't get them from anywhere. You have to supply them.

BTW: this is not related to the class being specialized. If you consider the
unspecialized pix, you will find that you cannot get at T and Alloc either
because they are just placeholders for something you still need to specify.
Best

Kai-Uwe Bux
Jun 30 '08 #2
On Jun 30, 4:52 pm, Kai-Uwe Bux <jkherci...@gmx.netwrote:
abir wrote:
I am matching a template, and specializing based of a template, rather
than a single class.
The codes are like,
template<template<typename T,typename Alloc = std::allocator<T>
>class C>
class pix{
};
template<>
class pix<vector>{
};
The matching works perfectly.
Now how will i get the template parameters in the specialized class?
I want to do things like,
template<>
class pix<std::vector>{
private:
std::vector<T>* v_;
};

Why a pointer to a vector. That looks fishy.
From where will I get T & Alloc fro which it is a match?

You won't get them from anywhere. You have to supply them.

BTW: this is not related to the class being specialized. If you consider the
unspecialized pix, you will find that you cannot get at T and Alloc either
because they are just placeholders for something you still need to specify.

Best

Kai-Uwe Bux
It can be reference to a vector instead of pointer. I want to have an
external iterator kind of thing, which i name pix, which is portable,
i.e can be stored as long as the element at the location exists,
irrespective of whether container resizes or not. (Of course they can
be made immune to pop_front,push_front, insert, erase etc also)
The pointer to a vector is not at all fishy in this case, almost all
(not all) iterators has a pointer to the container.
I want something like,
typedef vector<int V;
V v; v.push_back() a few elements ...
pix<Vp(v);
++p; *p; get element
v.push_back() a few more.
*p to get element.

so for a container C , i wanted pix as,
template<typename C>
class pix{
C& c_;
C::size_type index_;
};
when C is a vector, but another implementation for another C. Only
problem is C itself is a template.

Why the template template parameters are "just placeholders" when the
instance is a full class?
Any other way to do it?

Thanks
abir
Jun 30 '08 #3
abir wrote:
On Jun 30, 4:52 pm, Kai-Uwe Bux <jkherci...@gmx.netwrote:
>abir wrote:
I am matching a template, and specializing based of a template, rather
than a single class.
The codes are like,
template<template<typename T,typename Alloc = std::allocator<T>
class C>
class pix{
};
template<>
class pix<vector>{
};
The matching works perfectly.
Now how will i get the template parameters in the specialized class?
I want to do things like,
template<>
class pix<std::vector>{
private:
std::vector<T>* v_;
};

Why a pointer to a vector. That looks fishy.
From where will I get T & Alloc fro which it is a match?

You won't get them from anywhere. You have to supply them.

BTW: this is not related to the class being specialized. If you consider
the unspecialized pix, you will find that you cannot get at T and Alloc
either because they are just placeholders for something you still need to
specify.

Best

Kai-Uwe Bux

It can be reference to a vector instead of pointer. I want to have an
external iterator kind of thing, which i name pix, which is portable,
i.e can be stored as long as the element at the location exists,
irrespective of whether container resizes or not. (Of course they can
be made immune to pop_front,push_front, insert, erase etc also)
The pointer to a vector is not at all fishy in this case, almost all
(not all) iterators has a pointer to the container.
I want something like,
typedef vector<int V;
V v; v.push_back() a few elements ...
pix<Vp(v);
++p; *p; get element
v.push_back() a few more.
*p to get element.

so for a container C , i wanted pix as,
template<typename C>
class pix{
C& c_;
C::size_type index_;
};
when C is a vector, but another implementation for another C. Only
problem is C itself is a template.
I see. In this case, I think, you want a pointer and not a reference. That
way, you won't run into trouble implementing an assignment operator for
pix.

Why the template template parameters are "just placeholders" when the
instance is a full class?
This is too vague. The instance of what is a class?

Anyway, when you have a template template parameter, what you pass as a
parameter is a template. In you case, you pass std::vector. Note that you
do not pass an instantiation of that template, i.e., you do not pass
something like std::vector<int>. You just pass std::vector. The pix class
would be able to use the parameter C to do stuff like C<int>.
Any other way to do it?
You don't want to use template template parameters. Use an ordinary typename
parameter:

template < typename Sequence >
class pix;

and then specialize for vectors:

template < typename T, typename A >
class pix< std::vector< T, A {
...
};

for deques:

template < typename T, typename A >
class pix< std::deque< T, A {
...
};

and for lists:

template < typename T, typename A >
class pix< std::list< T, A {
...
};

(not run by a compiler)
Best

Kai-Uwe Bux
Jun 30 '08 #4
On Jun 30, 5:31 pm, Kai-Uwe Bux <jkherci...@gmx.netwrote:
abir wrote:
On Jun 30, 4:52 pm, Kai-Uwe Bux <jkherci...@gmx.netwrote:
abir wrote:
I am matching a template, and specializing based of a template, rather
than a single class.
The codes are like,
template<template<typename T,typename Alloc = std::allocator<T>
class C>
class pix{
};
template<>
class pix<vector>{
};
The matching works perfectly.
Now how will i get the template parameters in the specialized class?
I want to do things like,
template<>
class pix<std::vector>{
private:
std::vector<T>* v_;
};
Why a pointer to a vector. That looks fishy.
From where will I get T & Alloc fro which it is a match?
You won't get them from anywhere. You have to supply them.
BTW: this is not related to the class being specialized. If you consider
the unspecialized pix, you will find that you cannot get at T and Alloc
either because they are just placeholders for something you still need to
specify.
Best
Kai-Uwe Bux
It can be reference to a vector instead of pointer. I want to have an
external iterator kind of thing, which i name pix, which is portable,
i.e can be stored as long as the element at the location exists,
irrespective of whether container resizes or not. (Of course they can
be made immune to pop_front,push_front, insert, erase etc also)
The pointer to a vector is not at all fishy in this case, almost all
(not all) iterators has a pointer to the container.
I want something like,
typedef vector<int V;
V v; v.push_back() a few elements ...
pix<Vp(v);
++p; *p; get element
v.push_back() a few more.
*p to get element.
so for a container C , i wanted pix as,
template<typename C>
class pix{
C& c_;
C::size_type index_;
};
when C is a vector, but another implementation for another C. Only
problem is C itself is a template.

I see. In this case, I think, you want a pointer and not a reference. That
way, you won't run into trouble implementing an assignment operator for
pix.
Yes, that is the only reason I had to use pointer instead of
reference.
Why the template template parameters are "just placeholders" when the
instance is a full class?

This is too vague. The instance of what is a class?

Anyway, when you have a template template parameter, what you pass as a
parameter is a template. In you case, you pass std::vector. Note that you
do not pass an instantiation of that template, i.e., you do not pass
something like std::vector<int>. You just pass std::vector. The pix class
would be able to use the parameter C to do stuff like C<int>.
Any other way to do it?

You don't want to use template template parameters. Use an ordinary typename
parameter:

template < typename Sequence >
class pix;

and then specialize for vectors:

template < typename T, typename A >
class pix< std::vector< T, A {
...
};

for deques:

template < typename T, typename A >
class pix< std::deque< T, A {
...
};

and for lists:

template < typename T, typename A >
class pix< std::list< T, A {
...
};

(not run by a compiler)
Got it. It runs perfectly & works according to my wishes!
Thanks a lot.
BTW, is there any already existing code which can refer to a slice of
sequence which doesn't get invalidated due to memory capacity related
issues? (I know blitz array boost numerics etc have slicing , but the
containers are non re sizable there ). I am very much in need for
that.
Best

Kai-Uwe Bux
abir
Jun 30 '08 #5

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