Operator overloading, structs

[Don]

Denis Koroskin wrote:
> On Thu, 04 Jun 2009 22:10:58 +0400, Andrei Alexandrescu 
> <SeeWebsiteForEmail at erdani.org> wrote:
> 
>> Max Samukha wrote:
>>> On Thu, 04 Jun 2009 10:41:31 -0500, Andrei Alexandrescu
>>> <SeeWebsiteForEmail at erdani.org> wrote:
>>>
>>>> You are mistakenly presupposing that if() takes a bool. In reality 
>>>> if() accepts a bool, an integral, a floating-point type, a pointer, 
>>>> an array, or a class reference.
>>>  or delegate
>>
>> I was sure I forgot something... and hash too. Anything that can be 
>> compared against 0 or null.
>>
>> Andrei
> 
> Is it considered a good practice?
> 
> Technically, the following construct is not exactly portable:
> 
> float f = ..;
> if (f) {
> }
> 
> because C (nor D) standard doesn't guaranty that float(0) will be 
> implemented as "all bits set to 0" on target platform (although it 
> currently holds true).

That _is_ guaranteed. Even on not-quite-conformant systems.
Actually, -0.0 is not implemented as "all-bits-zero", yet this works:
void main(){
    float f = -0.0;
    if (f) assert(0);
}
There's no portability problem here.

To fix the original issue, we'd just need to allow something like:

struct S
{
    bool opEquals(int x) { return (x!=0); }
}

void main()
{
    S x;
    if (x) assert(0);
}

That is, if x is a struct, convert "if(x)" into "if(x==0)"
and convert "if(!x)" into "if(x!=0)". Usual rules would then apply, so 
any opEquals with a parameter which could be implicitly cast from 0 
would work.

That wouldn't allow smart-pointer structs to implement if (x) without 
requiring them to allow comparisons with integers, so it might be 
necessary to add a second step: try "if(x==null)" instead.

Alternatively, introduce a "bool opNull()" member function for structs.
Both solutions are easy to implement.