Issues with templates

[Daniel Keep]

Artyom Shalkhakov wrote:
> I'm trying to do byte-order conversions in compile-time rather than run-time, since the latter case is rarely needed (if needed at all).
> 
> /// byte-order conventions (for uint conversion)
> enum BYTEORDER {
> 	ARGB8,
> 	ABGR8,
> 	RGBA8,
> 	BGRA8
> }
> 
> /// color, represented as RGB triple
> struct Col3 {
> 	static Col3 opCall( float r, float g, float b ) {
> 		...
> 	}
> 
>         // byte-order is mostly static, so I tried to do this using templates
> 	static Col3 opCall( BYTEORDER order )( uint rgb ) {
> 		Col3	dst;
> 
> 		// unpack according to byte-order
> 		static if ( order == ARGB8 ) {
> 			...
> 		}
> 		else static if ( order == ABGR8 ) {
> 			...
> 		}
> 		....
> 	}
> 
> 	float	r, g, b;
> }
> 
> compiler says:
> template Col3.opCall( BYTEORDER order ) conflicts with function Col3.opCall at ...
> 
> Why can't I do this? I had to make a parameter for opCall to be able to compile it.
> I'm using Tango 0.99 RC3 / DMD 1.18 on Windows.

Because you have a template and a function with the same symbol name.
Let's remove the struct for a moment to get

static Col3 opCall(float r, float g, float b);
static Col3 opCall(BYTEORDER order)(uint rgb);

What the compiler sees this as is:

static Col3 opCall(float r, float g, float b);
template Col3(BYTEORDER order)
{
    static Col3(uint rgb);
}

That's like creating a class and a function with the same name: you
can't do it.  You also can't overload templated functions, at least not
directly.  You can make it work by turning the other function into a
template and making sure it has a different number of template arguments:

static Col3 opCall()(float r, float g, float b);
static Col3 opCall(BYTEORDER order)(uint rgb);

Note that the above isn't tested.  Also note that, last time I checked,
you can only have one templated member function with any given name, so
it might not work at all.

	-- Daniel