Partial Implementation of templates

[Greg]

Hi,

I would like to specify behavior of a class member relatively to
template implemetation. It works in usual cases but it seems to fail
with to templates when one of the two is specified...

Exemple:

template <class T, int Num>
class A
{
void foo ();
}

// The generic implementation is:
template <class T, int Num>
void
A<T,Num>
::foo ()
{
// generic code
}

// I would like to add a specific code when Num equals 2 fior
instance:
template <class T>
void
A<T,2>
::foo()
{
// an other code...
}

// But g++ 3.2.3 (on Redhat Fedora 5) does not like it
Is there any solution, knowing that a complete non-genric code is
possible (i.e. defined a void A<int,2>::foo() directly)?

Any help is appreciated.
Greg

 

[Ondra Holub]

Hi,

I would like to specify behavior of a class member relatively to
template implemetation. It works in usual cases but it seems to fail
with to templates when one of the two is specified...

Exemple:

template <class T, int Num>
class A
{
void foo ();

}

// The generic implementation is:
template <class T, int Num>
void
A<T,Num>
::foo ()
{
// generic code

}

// I would like to add a specific code when Num equals 2 fior
instance:
template <class T>
void
A<T,2>
::foo()
{
// an other code...

}

// But g++ 3.2.3 (on Redhat Fedora 5) does not like it
Is there any solution, knowing that a complete non-genric code is
possible (i.e. defined a void A<int,2>::foo() directly)?

Any help is appreciated.
Greg

Partial specialization works only for classes/structs:

template <class T, int Num>
class A
{
void foo ()
{
// generic code

}

}

template <class T>
class A<T, 2>
{
void foo ()
{
// specialized code

}

}

 

[Frank Birbacher]

Hi!

Partial specialization works only for classes/structs:

And for non-member functions and template functions of non-template
classes. Right?

They just don't work for partially specializing a templated member
function of a class template. Right?

Is there a link to read up on this issue?

Frank

 

[Ondra Holub]

Hi!



And for non-member functions and template functions of non-template
classes. Right?

I think it works only for classes (and structs, what is the same with
different default access type). You can as workaround create class
with static method and use partial specialization of this class.

AFAIK partial specialization of functions is considered in prepared
standard, but I am not sure.

 

[Frank Birbacher]

Hi!

AFAIK partial specialization of functions is considered in prepared
standard, but I am not sure.

No. The following already works (not with MSVC prior to 7.1, though):

//generic
template<typename T>
void process(T t) {}

//partial specialization
template<typename T>
void process(T* const t) {}

//full specialization
template<>
void process(void* const t) { throw "what is it?"; }

int main()
{
int i = 8;
process(i);
process(&i);
process( static_cast<void*>(&i) );
}

Frank

 

[joe]

This also seems to work in VC8 anyway and has the advantage of not
having to reimplement the whole of class A:

#include <iostream>

template <class T, int Num>
class A
{
public:
void foo()
{
foo_impl<Num>();
}

private:

template <int N>
void foo_impl()
{
std::cout << "Default Foo" << std::endl;
}
template <>
void foo_impl<2>()
{
std::cout << "Special Foo" << std::endl;
}
};


int main()
{
A<int, 0> a0;
A<int, 2> a2;

a0.foo();
a2.foo();
}

Hope that helps in some way.

joe

 

[Ondra Holub]

Hi!



No. The following already works (not with MSVC prior to 7.1, though):

//generic
template<typename T>
void process(T t) {}

//partial specialization
template<typename T>
void process(T* const t) {}

//full specialization
template<>
void process(void* const t) { throw "what is it?"; }

int main()
{
int i = 8;
process(i);
process(&i);
process( static_cast<void*>(&i) );

}

Frank

Yes. it works. But it is full specialization, not partial
specialization. Try following:

template<typename T, int N>
void Func()
{
}

template<typename T>
void Func<T, 10>()
{
}

On gcc you'll get error: partial specialization `Func<T, 10>' of
function template, on Visual C++ .net 2003 you'll get error C2768:
'Func' : illegal use of explicit template arguments (the same on
Visual C++ 2005).

 

[hurcan solter]

Yes. it works. But it is full specialization, not partial
specialization. Try following:

No it's just a overloaded template function.the last one is a full
specialization though

 

[Frank Birbacher]

Hi!


Is the above really a "full specialization"?

Yes. it works. But it is full specialization, not partial
specialization. Try following:

template<typename T, int N>
void Func()
{
}

template<typename T>
void Func<T, 10>()
{
}

-.- ok, fails. But why? I don't understand this restriction, especially
because classes can be partially specialized.

Frank

 

[Ondra Holub]

Hi!



Is the above really a "full specialization"?

Well, you're right, I missed this one in the middle

-.- ok, fails. But why? I don't understand this restriction, especially
because classes can be partially specialized.

Frank

I do not understand it too, but this is current situation :-( I
usually use workaround with static class member which works, although
it looks not as good as simple function:

template<typename T, int N>
class X
{
static void Func() { }
};

template<typename T>
class X<T, 10>
{
void Func<T, 10>() { }
};

Maybe we'll have to wait for a new standard... And then it will depend
on supplier of our compiler