TypeFunction example creatiing a conversion matrix

[Adam D. Ruppe]

On Friday, 2 October 2020 at 03:11:34 UTC, Stefan Koch wrote:
>  - doesn't work for -betterC

This is trivially easy to fix. Wrap the function in a template 
and use an eponymous enum to collapse it to a literal:


----
template makeConvMatrix(T...) { // wrapper added
         string helper()
         {
             string result;
             static foreach(t; T)
             {
                 result ~= "\t" ~ t.stringof;
             }
             result ~= "\n";
             static foreach(t1; T)
             {
                 result ~= t1.stringof;
                 static foreach(t2; T)
                 {
                     result ~=  "\t" ~ (is(t1:t2) ? "yes" : "no");
                 }
                 result ~= "\n";
             }
             return result;
         }

         enum makeConvMatrix = helper(); // eponymous call
}

extern(C) // for betterC
void main()
{
     import core.stdc.stdio;
     static immutable convMatrix = makeConvMatrix!(byte, ubyte, 
short, ushort, int, uint, long, ulong); // no more () there

     printf("%s\n", convMatrix.ptr);
}
------

Generates a reasonably small executable too (this pattern btw is 
what my change to dmd a couple months ago is able to recognize):

$ ls -lh bc
-rwxr-xr-x 1 me users 19K Oct  1 23:19 bc
$ ls -lh bc.o
-rw-r--r-- 1 me users 2.4K Oct  1 23:19 bc.o
$ nm bc.o
0000000000000000 t
0000000000000000 D _D2bc4mainUZ10convMatrixyAa
                  U _GLOBAL_OFFSET_TABLE_
0000000000000000 r _TMP0
0000000000000000 W main
                  U printf


Which is identical to if I delete the template from the source 
entirely and just replace it with a string literal. Nothing of it 
is emitted to the object file, so the linker doesn't even have to 
strip it.

Worth noting that not all cases work out this well. But this one 
actually does.