Template specialization

[John]

Hi,

Suppose I have a template clasee

template <class T> A
{
//data ....
T a;

//functions ....
func1(T b);
func2(T b);
func3(T b);

};

In one particular case, say int, func3 is different, so I want to use
template specialization for that case. Do I have to redefine all the
data/functions for int

template <> A<int>
{
//data ....
int a;

//functions ....
func1(int b);
func2(int b);
func3(int b);
}

even though almost everything is identical or can I only specialize the
functions that are different

template <> A<int>
{
func3(int b);
}

and still get the original generic functions/data for everything else?
My understanding is that I have to redefine everything in the
specialization, but I wanted to verify that.

Thanks,
John

 

[Neelesh]

Hi,

Suppose I have a template clasee

template <class T> A

{
   //data ....
   T a;

   //functions ....
   func1(T b);
   func2(T b);
   func3(T b);

};

In one particular case, say int, func3 is different, so I want to use
template specialization for that case.  Do I have to redefine all the
data/functions for int
Yes.
template <> A<int>

{
   //data ....
   int a;

   //functions ....
   func1(int b);
   func2(int b);
   func3(int b);

}

even though almost everything is identical or can I only specialize the
functions that are different

template <> A<int>
{
   func3(int b);

}

and still get the original generic functions/data for everything else?
My understanding is that I have to redefine everything in the
specialization, but I wanted to verify that.

Just to quote the standard: 14.7.3(1):
[Example:
template<class T> class stream;
template<> class stream<char> { /* ... */ };
Given these declarations, stream<char> will be used as the definition
of streams of chars; other
streams will be handled by class template specializations instantiated
from the class template.]

 

[John]

In one particular case, say int, func3 is different, so I want to use

template specialization for that case. Do I have to redefine all the
data/functions for int
Yes.

Just to quote the standard: 14.7.3(1):
[Example:
template<class T> class stream;
template<> class stream<char> { /* ... */ };
Given these declarations, stream<char> will be used as the definition
of streams of chars; other
streams will be handled by class template specializations instantiated
from the class template.]

Thanks,
John

 

[Ron AF Greve]

Hi,

I am not a template expert but as far as I know if you specialize the class
it just becomes a different class altoghether and you have indeed specify
everything. However maybe you can do some trick with templates and selecting
a certain template function base on the type. So adding two template
functions one for one type and one for the other (lookup SFINAE).


Regards, Ron AF Greve

http://informationsuperhighway.eu

 

[Jerry Coffin]

In one particular case, say int, func3 is different, so I want to
use template specialization for that case. Do I have to redefine
all the data/functions for int [ ... ] even though almost
everything is identical or can I only specialize the functions that
are different

You get that capability via inheritance rather than specialization.
In this case, you get what you want by combining the two, something
like this:

template <class T>
class A_base {
T a;
public:
void func1(T b);
void func2(T b);
};

template <class T>
class A : public A_base<T> {
public:
void func3(T b);
};

template <>
class A<int> : public A_base<int> {
public:
void func3(int b);
};

 

[tni]

You can do something like:

#include <boost/type_traits.hpp>

template<class T>
class Test {
public:
int f(T i);
};

template<class T>
int Test<T>::f(T i) {
if(boost::is_same<T, int>::value) {
return i * i;
}
else {
return i + i;
}
}


The if/else can be evaluated at compile time and will normally be
optimized away.

http://www.boost.org/doc/libs/1_39_0/libs/type_traits/doc/html/index.html
http://www.boost.org/doc/libs/1_39_.../html/boost_typetraits/reference/is_same.html

If you can't use Boost, you can also delegate to a function template or
do something like:

template<class T>
struct is_int {
static const bool v = false;
};

template<>
struct is_int<int> {
static const bool v = true;
};

template<class T>
int Test<T>::f(T i) {
if(is_int<T>::v) {
....