Template specialization

[Darrell Gallion via Digitalmars-d-learn]

On Friday, 22 January 2016 at 00:08:56 UTC, Ali Çehreli wrote:
> On 01/21/2016 03:37 PM, Darrell Gallion wrote:
>> How do you create a template that accepts many types.
>> But overrides just one of them?
>> Don't want to write out all of the specializations.
>>
>> Hours of google and I'm sure it's simple...
>>
>> -=Darrell
>
> The straightforward approach is tricky because the ': int' 
> syntax means "is implicitly convertible"; so, even the case 
> where B is 'char' would be bound to the specialization:
>
> void foo(A, B, C)() {
>     pragma(msg, "general");
> }
>
> void foo(A, B : int, C)() {  // <-- 'B : int' specialization
>     pragma(msg, "special");
> }
>
> void main() {
>     foo!(char, string, double)();
>     foo!(short, int, float)();
> }
>
> A better approach is using template constraints:
>
> void foo(A, B, C)()
>         if (!is (B == int)) {  // <-- 'B != int'
>     pragma(msg, "general");
> }
>
> void foo(A, B, C)()
>         if (is (B == int)) { // <-- 'B == int'
>     pragma(msg, "special");
> }
>
> void main() {
>     foo!(char, string, double)();
>     foo!(short, int, float)();
> }
>
> But that has its own problem of needing to reverse the logic 
> for the general case (and other cases), which may become very 
> complicated to write (or to get right) in some cases.
>
> Ali

I must apologize for such a vague question. Frustration had me.


import std.stdio;

void foo(A)()
         if (!is (A == int)) {
     pragma(msg, "int");
}

void foo(A)()
         if (is (A == int[])) {
     pragma(msg, "int[]");
}

void main() {

   foo!(int)();
   foo!(int[])();
}

===========

source\app.d(15): Error: template app.foo cannot deduce function 
from argument types !(int)(), candidates are:
source\app.d(3):        app.foo(A)() if (!is(A == int))
source\app.d(8):        app.foo(A)() if (is(A == int[]))
source\app.d(16): Error: app.foo called with argument types () 
matches both:
source\app.d(3):     app.foo!(int[]).foo()
and:
source\app.d(8):     app.foo!(int[]).foo()