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-   -   Concerning Partial Template Specialization (http://www.velocityreviews.com/forums/t622374-concerning-partial-template-specialization.html)

Ioannis Gyftos 06-26-2008 01:14 PM

Concerning Partial Template Specialization
 
Hello,

First the code :)


///////////////////////////////////////////////////////////////////////////////////
// in another header file


namespace LJC{

struct DomainInterface {
... // pure virtual functions
};

struct ConnectionTypeInterface {
... // pure virtual functions
};




struct Domain {
struct Local : public DomainInterface
{
...
};
struct Internet : public DomainInterface
{
...
};
};

struct ConnectionType {
struct Stream : public ConnectionTypeInterface
{
...
};
struct Datagram : public ConnectionTypeInterface
{
...
};
};

.....

}; // end of namespace




///////////////////////////////////////////////////////////////////////////////////




template <
class DomainInterface,
class ConnectionTypeInterface
>

class Connection{
public:

Connection(const int port);
Connection(const char* address);

int init();

private:
DomainInterface Domain;
ConnectionTypeInterface ConnectionType;
};





// This works.
template <>
inline Connection<
LJC::Domain::Internet,
LJC::ConnectionType::Datagram
>::Connection(const char* a)

{
}





// This works.
template <
class DomainInterface,
class ConnectionTypeInterface
>

inline Connection<DomainInterface,
ConnectionTypeInterface>::Connection(const char* a)
{
}





// This does not work with the error message:
// invalid use of incomplete type ‘class
Connection<LJC::Domain::Internet, ConnectionTypeInterface>’
// declaration of ‘class Connection<LJC::Domain::Internet,
ConnectionTypeInterface>’
template <
class ConnectionTypeInterface
>

inline Connection<
LJC::Domain::Internet,
ConnectionTypeInterface
>::Connection(const char* a)

{
}




// This works
template <
class DomainInterface,
class ConnectionTypeInterface
>

int Connection<DomainInterface, ConnectionTypeInterface>::init()
{
}


///////////////////////////////////////////////////////////////////////////////////



To give some background on what I am trying to do:
I have recently created some TCP, UDP and UnixSocket libraries with
very similar interface for different projects. After getting inspired
by Alexandrescu's book, I was trying to combine those in a single
templated 'Connection' class, which would use such templates to define
protocols and policies (like server/client, blocking/non-blocking
function calls). The general flow of either of these combination is
similar, so I figured I would train myself into creating something
like this.

The specific problem I was trying to solve is the constructor call. A
class instantiated with Domain::Local should be constructed with a
const char* (socket path), but a class instantiated with
Domain::Internet should use an integer (port). To solve this, I though
I would create two constructors, one with char* and one with int. I
would implement a partial template specialization for <Internet,int>
and one for <Local,char*>, so if a user would call it with correct
combination it would work, but if he tried to call a wrong one it
would throw a compiler undefined reference error.

I looked around the web, some books etc, but I could not understand
why am I getting this error. I can use the constructor unspecialized
or fully specialized, but not partially. Full specialization is not
really acceptable since I plan to add more template parameters later
(starting of slow). Most examples I found on the net use ints or bools
while partially specializing, I tried with those but I got the same
result (I am saying this because I stumbled across someone mentioning
that I couldn't use a class for specialization, but I am confused
about what he meant).

I also read something else on the net, which I did not fully
understand; the partial specialization failed because the top-level
class is templated for <class DomainInterface, class
ConnectionTypeInterface>, and there is no top-level class
specialization for the partial-specialized case. Following this, I
tried (with several combinations) something like this:

template <
class ConnectionTypeInterface
>

class Connection{
public:

Connection(const int port);
Connection(const char* address);
....
};

But I got redefinition error messages.

Can what I describe above be done? If so, what is my mistake and how
can it be corrected? My C++ intuition so far tells me that it does,
but if it's impossible, I haven't thought of a secondary plan so far,
so I might ask later on :P

Ioannis Gyftos 06-26-2008 01:17 PM

Re: Concerning Partial Template Specialization
 
Apparently, the Google groups have eaten some ">" that end the
template parameter lists when on a single line.

(And by the way, I use gcc 4.3.0, and the manpage didn't mention
anything about not supporting partial specializations, so I think/hope
it's not a compiler error).

Ioannis Gyftos 06-27-2008 07:32 AM

Re: Concerning Partial Template Specialization
 
On Jun 26, 4:17*pm, Ioannis Gyftos <ioannis.gyf...@gmail.com> wrote:
> Apparently, the Google groups have eaten some ">" that end the
> template parameter lists when on a single line.
>
> (And by the way, I use gcc 4.3.0, and the manpage didn't mention
> anything about not supporting partial specializations, so I think/hope
> it's not a compiler error).


Hello again,

Apparently I managed to figure out what the error was. A stumbled
across a topic discussing *nested* template partial specialization
which described better/clearer a situation similar to mine, and I
could make a direct comparison.

My error was that I hadn't partially specialized my base class (a
thing which I tried, but my syntax was wrong - the example I found
helped). to be precise, I had to add:

template < class ConnectionTypeInterface >
class Connection<LJC::Domain::Local, ConnectionTypeInterface>{
...
}

Now I'll use a base class and inherit the common subset of the
functions that do not have to be specialized, as Victor Bazarov
suggested somewhere.

Sorry for the trouble, but I hope this might be helpful to someone in
turn, and not just noise in the group.


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