I have a situation where I want to use a template if CTFE is possible and a function if it can only be processed at runtime. is there a version or static if which can check this?

"bmeck" <bmeck@stedwards.edu> wrote in message news:g956t5$18vg$1@digitalmars.com...
>I have a situation where I want to use a template if CTFE is possible and a function if it can only be processed at runtime. is there a version or static if which can check this?

I think you may be able to do this in D2 with __traits(compiles).  I can't test it, but you could try writing some code that would mixin a constant declaration with the expression you wish to evaluate.  I.e.

static if(__traits(compiles, mixin("enum x = yourExpression;")))
    // it can be evaluated at compile time
else
    // at runtime, or not at all if it's just an error

In D1 I don't think there's any way.

Well im using D1 so if anyone knows of a way that would be lovely, the basics are this:

template foo(char[] src)
{
    static if(src available at compile time for CTFE /*Ie is src const*/)
        blah;
    else
        blah;
}

Sorry for double post but this works if a lil hackey and a dislikable branch occurs at runtime slowing it a bit...

void main(char[][] args)
{
	Stdout(test("true")).newline;
	Stdout(test(args[0])).newline;

}

bool CTFE = true;
static this()
{
	CTFE = false;
}
char[] test(char[] src)
{
	if(CTFE)
	{
		return src;
	}
	else
	{
		return src;
	}
}

On Thu, 28 Aug 2008 01:46:06 -0400, bmeck <bmeck@stedwards.edu> wrote:

>Sorry for double post but this works if a lil hackey and a dislikable branch occurs at runtime slowing it a bit...
>
>void main(char[][] args)
>{
>	Stdout(test("true")).newline;
>	Stdout(test(args[0])).newline;
>
>}
>
>bool CTFE = true;
>static this()
>{
>	CTFE = false;
>}
>char[] test(char[] src)
>{
>	if(CTFE)
>	{
>		return src;
>	}
>	else
>	{
>		return src;
>	}
>}

No, both calls are executed at run time. Your test function does not qualify for CTFE because it references mutable global state. To ensure the function is executed at compile time put it in static context.

bool CTFE = true;
static this()
{
	CTFE = false;
}
char[] test(char[] src)
{
	if(CTFE)
	{
		return src;
	}
	else
	{
		return src;
	}
}

void main(char[][] args)
{
	pragma(msg, test("true"));
	writefln(test(args[0]));
}

test.d(13): Error: variable CTFE is used before initialization
test.d(25): Error: cannot evaluate test("true") at compile time
test.d(25): Error: string expected for message, not 'test("true")'