On Wed, 25 Aug 2010 10:01:31 -0400, Andrej Mitrovic
<andrej.mitrov...@gmail.com> wrote:
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.
A struct is a value type, so you are making a copy regardless. Your
expectation that foo = ... does not wholly replace foo is incorrect.
You could do this:
this(N value, int x)
{
this.value = value;
this.x = x;
}
...
return CheckedInt(result, x);
...
and then you're x comes through.
Value types are always passed by value unless you use pointers or ref.
BTW, this has nothing to do with CheckedInt being a shortcut for
CheckedInt!N inside the instantiated template.
-Steve
