Template partial specializations like <A, B> specialized for <A, int>- possible?

[year1943]

For
template <typename T> class My ;
I can define partial spec-ns somewhat like
template <typename T> class My<T*> ; or
template <typename T> class My<Another<T> > ;
And full spec-n, say
template <> class My<int> ;

What if I have a template class
template <typename T, typename U> class My ;
and want to have spec-ns like
template <typename T, typename> class My<T, int> ;
when spec-n is not "full", but one of template-parameters is "not
used"?

Is it possible? If it is, what is right syntax. For my tries GCC says
something like

60-tpl_partial.cpp:49: error: template parameters not used in partial
specialization:
60-tpl_partial.cpp:49: error: '<template-parameter-1-2>'

If it is not possible, what are reasons to prohibit that?
Thanx.

 

[Gianni Mariani]

For
template <typename T> class My ;
I can define partial spec-ns somewhat like
template <typename T> class My<T*> ; or
template <typename T> class My<Another<T> > ;
And full spec-n, say
template <> class My<int> ;

What if I have a template class
template <typename T, typename U> class My ;
and want to have spec-ns like
template <typename T, typename> class My<T, int> ;
when spec-n is not "full", but one of template-parameters is "not
used"?

Is it possible? If it is, what is right syntax.

You mean this ?

template <typename T, typename U> class My ;
template <typename T> class My<T, int> ;
template <typename T, typename U, typename V>
class My<std::map<T,U>, V> ;

 

[year1943]

You mean this ?
template <typename T, typename U> class My ;
template <typename T> class My<T, int> ;
template <typename T, typename U, typename V>
class My<std::map<T,U>, V> ;

Thanks, seems it's OK. Perhaps I messed and thought number or tpl-args
should match...
Initially I was interested in spec-n of not whole class, but just a
member-function of it. Still havn't found right way. Here sample code:

#include <iostream>

template <typename A, typename B>
class Test2 {
public:

Test2(A a, B b)
: _a(a), _b(b)
{
return ;
}

void print(void) ;

private:
A _a ;
B _b ;
} ;

template <typename A, typename B>
void Test2<A, B>:rint(void) {
std::cout << "Generic<A,B>: " << _a << ", " << _b << std::endl ;
return ;
}

// template <typename A, typename>
template <typename A>
void Test2<A, int>:rint(void) {
std::cout << "Spec<A, int>: " << _a << ", " << _b << std::endl ;
return ;
}

int main() {
Test2<int, const char*> x1(37, "const char* sample") ;
Test2<double, int> x3(370.37768, 2009) ;
x1.print() ;
x3.print() ;
return 0 ;
}

GCC tells:
60-tpl_partial.cpp:32: error: invalid use of undefined type 'class
Test2<A, int>'
60-tpl_partial.cpp:6: error: declaration of 'class Test2<A, int>'
60-tpl_partial.cpp:32: error: template definition of non-template
'void Test2<A, int>:rint()'

for both variants
template <typename A, typename>
template <typename A>
What yet is missed?

Thanx.

 

[Gianni Mariani]

Thanks, seems it's OK. Perhaps I messed and thought number or tpl-args
should match...
Initially I was interested in spec-n of not whole class, but just a
member-function of it. Still havn't found right way. Here sample code:

....

You can only partially specialize the entire class, not components of a
class. This is how it can be done.

#include <iostream>

template <typename A, typename B>
class Test2_Base {
public:

Test2_Base(A a, B b)
: _a(a), _b(b)
{
return ;
}

void print(void) ;

protected:
A _a ;
B _b ;
} ;


template <typename A, typename B>
class Test2 : public Test2_Base<A,B> {
public:

Test2(A a, B b)
: Test2_Base<A,B>( a, b )
{
return ;
}
} ;

template <typename A>
class Test2<A, int> : public Test2_Base<A,int> {
public:

Test2(A a, int b)
: Test2_Base<A,int>( a, b )
{
return ;
}

void print(void) ;

} ;

template <typename A, typename B>
void Test2_Base<A, B>:rint(void) {
std::cout << "Generic<A,B>: " << _a << ", " << _b << std::endl ;
return ;
}

// template <typename A, typename>
template <typename A>
void Test2<A, int>:rint(void) {
std::cout << "Spec<A, int>: " << this->_a << ", " << this->_b <<
std::endl ;

return ;
}

int main() {
Test2<int, const char*> x1(37, "const char* sample") ;
Test2<double, int> x3(370.37768, 2009) ;
x1.print() ;
x3.print() ;
return 0 ;
}

.... however, depending on what you're really trying to do, it may be
better to use a traits class specialization.

 

[year1943]

You can only partially specialize the entire class, not components of a
class. This is how it can be done. ..............
... however, depending on what you're really trying to do, it may be
better to use a traits class specialization.

I've got the rule and the idea (and traits too), thank you very much.

The last (not practical) question may be _why_ is it so...
For
template <typename T>
MyClass<T*>::method()
partial spec-n of member w/o spec-n of whole class can be problematic
and vogue, so as for generic<some_type*> T is some_type* while for
member spec-n<T*> T is just some_type...
But for
template <typename T1, typename T2>
MyClass<T1, concrete_type>::method()
are there some difficulties that let us not to use separate member
template partial specializations?..

 

[Gianni Mariani]

I've got the rule and the idea (and traits too), thank you very much.

The last (not practical) question may be _why_ is it so...
For
template <typename T>
MyClass<T*>::method()
partial spec-n of member w/o spec-n of whole class can be problematic
and vogue, so as for generic<some_type*> T is some_type* while for
member spec-n<T*> T is just some_type...
But for
template <typename T1, typename T2>
MyClass<T1, concrete_type>::method()
are there some difficulties that let us not to use separate member
template partial specializations?..

I don't know other than "it is". Maybe someone else can shed light on
the issue.

 

[year1943]

I don't know other than "it is". Maybe someone else can shed light on
the issue.

Thanks a lot, anyway.
I found quite similar topic, yesterday it had no answers, now it got
some - in neibour c++.moderated:
http://groups.google.ru/group/comp....a0fccb5cb38/ee035ce2081b3379#ee035ce2081b3379
One decision looks strange as for me, two others are close to
"traits" (realized as "helpers").