TDPL: Operator Overloading

[Yao G.]

On Tue, 24 Aug 2010 17:19:25 -0500, Andrej Mitrovic  
<andrej.mitrovich at whatever.com> wrote:

> [snip]
>
> struct CheckedInt(N) if (isIntegral!N)
> {
>     private N value;
>    this(N value)
>     {
>         this.value = value;
>     }
>    @property
>     auto Value()
>     {
>         return value;
>     }
>    // addition
>     CheckedInt opBinary(string op)(CheckedInt rhs) if (op == "+")
>     {
>         auto result = value + rhs.value;
>        enforce(rhs.value >= 0 ? result >= value : result < value);
>         return result;
>     }
> }
>
> I don't understand why he's trying to return result here (unless it was  
> a mistake). Result is going to have the type of "private N value",  
> whatever N may be, and this is conflicting with the return type which is  
> CheckedInt. So, this won't compile.

That's a bug. The return value should be CheckedInt(result);

>
> He has the same returns for subtraction and multiplication, but for  
> others like division, shift and bitwise overload he has this:
>
> // division and remainder
>     CheckedInt opBinary(string op)(CheckedInt rhs)
>     if (op == "/" || op == "%")
>     {
>         enforce(rhs.value != 0);
>         return CheckedInt(mixin("value" ~ op ~ "rhs.value"));
>     }
>
> This looks correct. If I change the add overload from the code to this:
>
> // addition
>     CheckedInt opBinary(string op)(CheckedInt rhs) if (op == "+")
>     {
>         auto result = value + rhs.value;
>        enforce(rhs.value >= 0 ? result >= value : result < value);
>         return CheckedInt(mixin("value" ~ op ~ "rhs.value"));
>     }
>
> Then the return statement calls a CheckedInt constructor and I get back  
> a CheckedInt struct with the right value in the private variable "value".
>
> What I don't understand is how the constructor can be called like that.  
> In my example the mixin would convert the code to:
>
> return CheckedInt(10);
>
> But if I ever tried a call like "CheckedInt(10)" in a unittest block, it  
> wouldn't work. So how does this magic work?
>

http://www.digitalmars.com/d/2.0/mixin.html

-- 
Yao G.