Ok I think I am kind of getting this. The template name inside a template is
it's instantiation. I can do "CheckedInt.variable" and get back the value of
"variable" in the current instantiation.
The trouble is, when you do a call like CheckedInt() you will loose all other
data that you had before:
module binary_ops;
import std.stdio : writeln;
import std.traits;
import std.exception;
unittest
{
auto foo = CheckedInt!(int)(5);
auto bar = CheckedInt!(int)(5);
foo.x = 4;
bar.x = 5;
foo = foo + bar;
writeln(foo.x); // writes 0
writeln(bar.x); // writes 5
}
void main() { }
struct CheckedInt(N) if (isIntegral!N)
{
private N value;
int x;
this(N value)
{
this.value = value;
}
// addition
CheckedInt opBinary(string op)(CheckedInt rhs) if (op == "+")
{
auto result = value + rhs.value;
enforce(rhs.value >= 0 ? result >= value : result < value);
return CheckedInt(result);
}
}
Here I've lost the value of x. "return CheckedInt(result);" calls the
constructor of the already instantiated template, but because of the way D
works (afaik) it first has to deconstruct the object before constructing it
again. And that includes initializing all members to their .init value before
calling the constructor.
So, I don't like that return statement at all..
Steven Schveighoffer Wrote:
> On Tue, 24 Aug 2010 18:43:49 -0400, Andrej Mitrovic
> <andrej.mitrov...@whatever.com> wrote:
>
>
> > I wasn't refering to the mixin, but the call to CheckedInt(). mixin
> > compiles "value" ~ op ~ "rhs.value", which in this case evaluates to 5 +
> > 5 and the whole call becomes CheckedInt(10).
> >
> > What I don't understand is how you can construct a new CheckedInt struct
> > by calling it with CheckedInt(10), when I have to use a call like
> > CheckedInt!(int)(10) outside the struct (in main or in a unittest block).
>
> Inside a template instantiation, the template name without template
> parameters is equivalent to the current instantiation.
>
> It saves a lot of typing.
>
> -Steve
