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template template partial specialisation advice needed

 
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matt
Guest
PostPosted: Sun Jan 23, 2005 3:02 am    Post subject: template template partial specialisation advice needed Reply with quote


Hi All,

Quote:
From spending an hour reading previous posts to this group, I have
found these types of questions continually arise. Here is another that

I have been wrestling with that appears not to have been specifically
discussed. I have been having problems with the following simpler
scenario (ie in a short form):

template <typename A>
class B {
....
};

template <template
class E {
typedef typename D::some_type some_type;
....
some_type doIt();
....
};

--- compiles fine up to here ---

template <typename D>
E<B, D>::some_type
E<B, D>::doIt() { <== errors appear for this line
....
}

gcc version 3.4.2 20041017 (Red Hat 3.4.2-6.fc3) complains:
expected init declarator before 'E'
expected ; before 'E'


In other words, I would like to specialise E::doIt() for the case where
B is the supplied model of C. Somehow I thought this would be as easy
as a template partial specialization... :-)


Thanks, Matt


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Efrat Regev
Guest
PostPosted: Sun Jan 23, 2005 10:16 am    Post subject: Re: template template partial specialisation advice needed Reply with quote


"matt" <dmatt001 (AT) yahoo (DOT) com> wrote

Quote:
Hi All,

In other words, I would like to specialise E::doIt() for the case where
B is the supplied model of C. Somehow I thought this would be as easy
as a template partial specialization... Smile

From looking at your code, it seems indeed that your problem does not
lie with template templates. Here is a simplification of
your code (I've eliminated template templates):

// For simplicity, B is now not a template
class B{};

// For simplicity, C is not, itself, a template
template<typename C, typename D>
class E
{
typedef typename D::some_type some_type;

some_type doIt();
};


/*****************/Problems!
**************************
// Following is problematic (won't compile)
template<typename D>
E<B, D>::some_type // especially, this line
E<B, D>::doIt()
{
}
/*****************/Problems!
**************************

Now your modified code contains no template templates, but still won't
compile. As far as I undersand, the code I surrounded by asterikses has two
problems:
1. The compiler sees a method of a specialization of E which is never
declared.
2. It has no idea what is E<B, D>::some_type

Here's how to fix this (your original version):

template<typename A>
class B{};

template<template
class E
{
typedef typename D::some_type some_type;

some_type doIt();
};


// Specialize the entire class
template<typename D>
class E<B, D>
{
typedef typename D::some_type some_type;

some_type doIt();
};


template<typename D>
// notice change in next line
typename E<B, D>::some_type
E<B, D>::doIt()
{
}


____________________________________________________

(Note that while the code above compiles, it is problematic. E.g., the
line "typedef typename D::some_type some_type;" appears twice. This can be
solved by various means, for example, by inheriting both the primary E and
specialized E from the same base class).

Possibly, you might want to read "C++ Templates: the Complete Guide" by
Vandevoorde and Jossutis.



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Carl Barron
Guest
PostPosted: Sun Jan 23, 2005 10:18 am    Post subject: Re: template template partial specialisation advice needed Reply with quote


In article <1106438996.985790.273250 (AT) c13g2000cwb (DOT) googlegroups.com>,
matt <dmatt001 (AT) yahoo (DOT) com> wrote:

Quote:
Hi All,

From spending an hour reading previous posts to this group, I have
found these types of questions continually arise. Here is another that
I have been wrestling with that appears not to have been specifically
discussed. I have been having problems with the following simpler
scenario (ie in a short form):

template <typename A
class B {
...
};

template