Exporting template function instances to C

[Nicholas Wilson via Digitalmars-d-learn]

On Thursday, 23 March 2017 at 19:46:43 UTC, data pulverizer wrote:
> On Thursday, 23 March 2017 at 17:58:21 UTC, H. S. Teoh wrote:
>> On Thu, Mar 23, 2017 at 05:29:22PM +0000, data pulverizer via
>>> 
>>> Thanks. Is there a less ham-handed way of exporting them 
>>> other than wrapping them in functions as I have?
>>
>> Wrapping them in functions is probably the simplest way to 
>> call them from C.  You *could*, I suppose, use their mangled 
>> names directly, then you wouldn't need a wrapper, but that 
>> would be rather difficult to use on the C end.  On the D side, 
>> there's .mangleof that will tell you what mangled names to 
>> use, but if you're calling from C you don't have that luxury.
>>
>>
>> T
>
> Thanks. Mangling sounds painful and scary, I think I'll stick 
> to wrapping which sounds much less dangerous.

There's nothing scary or dangerous about it. It happens 
automatically to allow overloads and templates so that you get a 
unique symbol foreach version (unless you use extern(C), 
extern(C++) or pragma mangle). C++,Java and any other compiled 
language that has overloads does mangling. Heck, you can even do 
it in C with __attribute__((overloadable)) (at least with clang), 
it just transparently mangles (just as in D)the name as whatever 
C++ would mangle it as.

So instead of doing

T mult(T)(T x, T y)
{
     return x*y;
}

doing something like

template mult(T)
{
     extern(C++) T mult(T x, T y)
     {
         return x*y;
     }
}

in D, and then in C (noting that you have to declare the name and 
signature anyway)

__attribute__((overloadable)) float mult(float,float);
__attribute__((overloadable)) double mult(double, double);

which I think is the least painful way of doing it. I seem to 
remember somewhere in phobos

template Instantiate(alias a)
{
    alias Instantiate = a;
}

to instantiate template, because you reference them from another 
symbol it somehow magically works. Used like

Instantiate!(mult!float); // at module scope