It seems pure ain't so pure after all

[Don Clugston]

On 01/10/12 07:40, Tommi wrote:
> import std.stdio;
>
> int pow2(int val) pure
> {
>      if (__ctfe)
>          return 6;
>      else
>          return val * val;
> }
>
> void main()
> {
>             assert(pow2(3) == 9);
>      static assert(pow2(3) == 6);
>
>      writeln("9 = 6 ... I knew it! '6' was faking it all along");
>      readln();
> }

You don't need the if (__ctfe) to show this behaviour. Nor do you even 
need CTFE at all (though it would be a bit less obvious). You could 
demonstrate it in C++ too.

Any code that behaves differently when compiled with -O, will do this as 
well. Constant folding of floating point numbers does the same thing, if 
the numbers are represented in the compiler in a different precision to 
how the machine calculates them. I believe that GCC, for example, uses 
very much higher precision (hundreds of bits) at compile time.