The definition of templates in D

[Artur Skawina]

On 03/18/12 12:34, Derek wrote:
> On Sun, 18 Mar 2012 22:00:06 +1100, Derek <ddparnell at bigpond.com> wrote:
> 
>>
>> The 'adding' is not the point; it could be any functionality. The point I was trying to get across was that it would be useful if the compiler could infer the type parameters of a template instantiation from the types of the data items used in the instantiation reference.
> 
> The best I can come up with so far is ...
> 
> import std.stdio;
> template add(X, Y,Z)
> {
>      auto add(X c, Y a, Z b)
>      {
>          return cast(X)a + cast(X)b;
>      }
> }
> 
> void main()
> {
>      double s;
>      int   t;
>      ulong u;
> 
>      s = 1.23;
>      t = 123;
>      u = 456;
> 
>     writeln(add(t,u,s)); // --> 467
> 
>     writeln(add(s,t,s)); // --> 124.23
> }
> 
> 
> It seems that the templated function's return type is not used when searching for matching templates, so I have to explicitly include something in the function's signature just to use it as the returned data's type.
> 

You can either let the compiler figure out the right common type or give
it explicitly:

auto add(X=void,Y,Z)(Y a, Z b)
{
    static if (is(X==void))
       alias typeof(a+b) RX;
    else
       alias X RX;
    return cast(RX)a + cast(RX)b;
}

Both "add(u,s)" and "add!int(u,s)" will work; note that the first
version returns a 'double', but that's probably what you want, when
not explicitly asking for an 'int'.

artur