The problem with opX_r (or more powerful template specialization)

[Sean Kelly]

Bill Baxter wrote:
> Sean Kelly wrote:
>> Bruno Medeiros wrote:
>>> Sean Kelly wrote:
>>>> Oskar Linde wrote:
>>>>>
>>>>> 1) Change the opX_r overloading rules, so that
>>>>>
>>>>> opX_r(T)(T x) {...}
>>>>> opX(T)(T x) {...}
>>>>>
>>>>> can coexist. opX_r is picked only of no opX overload is found.
>>>>
>>>> I thought operators worked this way:
>>>>
>>>>     x = y - z;
>>>>
>>>> 1. Call y.opSub(z) if it exists.
>>>> 2. If not, call z.opSub_r(y).
>>>>
>>>> ie. I had thought that the opX_r operations were for situations 
>>>> where the called object was on the right-hand side of the 
>>>> operation.  Assuming this were true, there should be no problem in 
>>>> having opX and opX_r coexist peacefully.
>>>>
>>>
>>> Nope. From the spec:
>>> "1. The expression is rewritten as both:
>>> a.opfunc(b)
>>> b.opfunc_r(a)
>>> If any a.opfunc or b.opfunc_r functions exist, then overloading is 
>>> applied across all of them and the best match is used."
>>>
>>> So those two forms (direct and reverse) compete on equal priority.
>>
>> So if a implemented an opSub(b) and b implemented an opSub_r(a) I'd 
>> get a compiler error about an ambiguous overload?  This seems somewhat 
>> sub-optimal.  Why was it implemented this way?
> 
> I think it makes sense.  Given the expression  "a + b", it's not clear 
> which add operation should be used.

Hrm, true enough.

> You can disambiguate by calling the method explicitly: a.opAdd(b).
> 
> Better would be if D had out-of-class operators.  Then you could just 
> have a free function - T opAdd(A a, B b) and you don't have to worry 
> about whether a or b owns this particular combination of addition.  I 
> believe the general rule of thumb in C++ is that operators that don't 
> modify their arguments (like +-/*) should be stand-alone functions.

That's the most flexible certainly, but it often means friend functions 
so data is accessible for the operation.  I think it's often easier to 
implement addition simply by creating a temporary and using AddAssign, 
expression templates aside.


Sean