Re: to!string(double) at compile time

[cal]

On Tuesday, 21 August 2012 at 04:43:09 UTC, Bobby Bingham wrote:
I'm just getting started with D, and was playing around with 
string
mixins.  I've hit a snag, and have reduced it to a minimal test 
case:

    import std.conv;

    string test()
    {
        return to!string(0.0);
    }

    immutable auto testvar = mixin(test());

This gives this result when compiling:

/usr/include/phobos2/std/format.d(1479): Error: snprintf cannot 
be interpreted at compile time, because it has no available 
source code
/usr/include/phobos2/std/conv.d(99):        called from here: 
formatValue(w,src,f)
/usr/include/phobos2/std/conv.d(824):        called from here: 
toStr(value)
/usr/include/phobos2/std/conv.d(268):        called from here: 
toImpl(_param_0)
test.d(6):        called from here: to(0)
test.d(9):        called from here: test()
test.d(9): Error: argument to mixin must be a string, not 
(test())

I guess converting a double to a string can't be done at 
compile time
because it requires calling the C snprintf function?  It 
compiles fine
if I replace the 0.0 with an int literal.  Is there any way 
around
this limitation?

I have had to work around this also. One way is to first multiply 
your float by a large factor (say 10000 depending on what 
precision you want) and then adding a decimal point back in to 
the string. Kinda hacky, but... Here is an example:

int fx = cast(int) (cos(angle)*1000000.);
string fsx = fx.to!string;

string xprefix;
if (fsx[0] == '-')
{
  xprefix = "-";
  fsx = fsx[1..$];
}

if (fsx.length == 7)
  sx = xprefix ~ fsx[0] ~ "." ~ fsx[1..$];
else
  sx = xprefix ~ "0." ~ fsx;

sx is then the string you want to mixin.