TITLE: templates: recovering a type from a pointer type


PROBLEM: adamnash@cs.stanford.edu (Adam Nash)

[...] 

for semantic reasons, the template assumes that DATA is a pointer type.

[...]

template <class DATA>
void GIDTable::ReadFromStream(LStream *inStream)
{
   DATA data;

   ...

   data = new DATA;
   (*data).ReadFromStream(inStream);

   ...
}


The problem occurs on the second line.  data is of type DATA, so you
really want to allocate a new (*DATA).  IE, if DATA is a CBar *, then data
is a CBar *, so we want it to say: data = new CBar;

How can I do this, only knowing the pointer type?


RESPONSE: fjh@mundook.cs.mu.OZ.AU (Fergus Henderson), 15 Mar 96

You can use a partial template specialization:

	template <class T> struct dereference {};
	template <class T> struct dereference <T*> {
		typedef T type;
	};
		
Then just write

	data = new dereference<Data>::type;

(I note in passing that this would of course be less obfuscated if C++
supported template typedefs...)

Now, the reason that all of this belongs in comp.std.c++ not comp.lang.c++
is that the above code is valid according to the standard, but is highly
unlikely to work with your favourite C++ compiler.  Partial specialization
is a relatively new feature, and few C++ compilers support it yet.