Say I have a template and I want to create a copy of a passed value:

template X(T) {
    T val;
    void set( T v ) {
        val = new T( v );
    }
}

This works fine for class types but not primitive types.  While I can use regular assignment for primitive types, I need a way to determine whether T is a primitive type.  Does D have any feature built-in where I can accomplish this?  I'd prefer to avoid having to create the complex traits template classes needed in C++.


Sean

Sean Kelly <sean@ffwd.cx> wrote in news:c0c7st$i3i$1@digitaldaemon.com:

> Say I have a template and I want to create a copy of a passed value:
> 

You need to specialize the template.

template X(T) {
     T val;
     void set( T v ) {
         val =  v;
     }
}


template X(T : Object) {
     T val;
     void set( T v ) {
         val = new T( v );
     }
}



> 
> This works fine for class types but not primitive types.  While I can use regular assignment for primitive types, I need a way to determine whether T is a primitive type.  Does D have any feature built-in where I can accomplish this?  I'd prefer to avoid having to create the complex traits template classes needed in C++.
> 
> 
> Sean
> 
> 

Patrick Down wrote:
> 
> You need to specialize the template.

Okay so I suppose I can do something like this:

template gen( Ty )
{
    Ty copy( Ty val )
    {
        return val;
    }
}


template gen( Ty : Object )
{
    Ty copy( Ty val )
    {
        return new Ty( val );
    }
}

But it seems that a default copy ctor isn't generated for class types, so the above trick doesn't always work.  And AFAIK using the assignment operator will always result in both class variables referring to the same instance.  ie.

class A {}
A a = new A();
A b = new A();
b = a;

So in the above example, the instance of b is discarded by the assignment so both a and b refer to the same memory location.

Is there any standard way to generate a copy of an arbitrary class?


Sean

> This works fine for class types but not primitive types.
Or more generally, reference types but not value types? Usually (I think) the only reference types you care about are classes, you can do
if((Object)val) {
	// reference semantics
} else {
	// value semantics
}
I don't know about function pointer/delegates as types, and whether they're allowed as templates, that probably wouldn't work.
A more robust solution would be good anyway.
Sam

Sam McCall wrote:
>
> I don't know about function pointer/delegates as types, and whether they're allowed as templates, that probably wouldn't work.
> A more robust solution would be good anyway.

Ideally, containers should be able to hold anything that can be copied, regardless of the semantics.  The template I posted in response to Patrick is the best I've done so far.


Sean