On Saturday, 3 August 2013 at 11:27:30 UTC, andrea9940 wrote:
Hello D world, I have just started using templates and mixins
to generate some boilerplate code and I have found a little
ambiguity:
This code compiles fine:
private template genTypes()
{
string eval() { ... }
const(char[]) genTypes = eval();
}
mixin(genTypes!()); // Working
But this does not, why ?
private template genTypes()
{
string eval() { ... }
string genTypes = eval();
}
mixin(genTypes!()); // Error: variable genTypes cannot be read
at compile time
I would expect both version to work.
( complete code at http://pastebin.com/FhmrgmZZ )
It is probably because you did not write what you meant.
First, take notice that:
const(char[]) //(1)
is different from
const(char)[] //(2)
1 is "full const", which means the compiler, since it is smart
enough, is able to say "hey, I can evaluate this variable at
compile time". In 2, however, while the "payload" of the array is
const, the array itself isn't. For example, you'll notice this is
perfectly legal:
private template genTypes()
{
string eval() { return "hello"; }
string genTypes = eval();
}
void main()
{
genTypes!() = "world";
}
As you can see, genTypes just resolves to a GLOBAL string, and
not to a manifest constant string. Because of this, mixin in
genTypes is not possible.
What you probably meant to write was:
private template genTypes()
{
string eval() { ... }
enum string genTypes = eval();
//or just
enum genTypes = eval();
}
This means genTypes will resolve statically to the string
returned by eval, and you can mix that in.
If you use "const" or "immutable" instead of "enum", then the
compiler will also generate a reference-able string for genTypes,
which you may or may not want. Hint, if in doubt, use enum,
that's what phobos does 100% of the time.
