Concerning Partial Template Specialization

[Ioannis Gyftos]

Hello,

First the code :)


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


namespace LJC{

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

struct ConnectionTypeI nterface {
... // pure virtual functions
};




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

struct ConnectionType {
struct Stream : public ConnectionTypeI nterface
{
...
};
struct Datagram : public ConnectionTypeI nterface
{
...
};
};

.....

}; // end of namespace




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




template <
class DomainInterface ,
class ConnectionTypeI nterface

>

class Connection{
public:

Connection(cons t int port);
Connection(cons t char* address);

int init();

private:
DomainInterface Domain;
ConnectionTypeI nterface ConnectionType;
};





// This works.
template <>
inline Connection<
LJC::Domain::In ternet,
LJC::Connection Type::Datagram

>::Connection(c onst char* a)

{
}





// This works.
template <
class DomainInterface ,
class ConnectionTypeI nterface

>

inline Connection<Doma inInterface,
ConnectionTypeI nterface>::Conn ection(const char* a)
{
}





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

>

inline Connection<
LJC::Domain::In ternet,
ConnectionTypeI nterface

>::Connection(c onst char* a)

{
}




// This works
template <
class DomainInterface ,
class ConnectionTypeI nterface

>

int Connection<Doma inInterface, ConnectionTypeI nterface>::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::Interne t 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
ConnectionTypeI nterface>, 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 ConnectionTypeI nterface

>

class Connection{
public:

Connection(cons t int port);
Connection(cons t 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]

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).