Re: Problem with templates and operator overloading

Re: Problem with templates and operator overloading

[Oliver Kullmann]

> 
> I'm having trouble with the following piece of c++ code.  Is this a bug
> in gcc or am I doing something wrong?  The code compiles and runs
> without problems in icc.
> 
> GCC Version: 3.3.2
> 
> struct doubler {
>    double operator*(double val) {
>       return 2.0 * val;
>    }
> };
> 
> template<typename T> T operator*(double lhs , T rhs) {
>    return rhs * lhs;
> }
> 
> int main() {
>    doubler a;
>    a * 3.0;
>    return 0;
> }
> 
> Compiling this gives the following error message.
> 
> test3.cc: In function `T operator*(double, T)':
> test3.cc:7: error: `T operator*(double, T) [with T = double]' must have
> an
>    argument of class or enumerated type
> 
> Thanks.
>

I can't find no fault with this code: 

When overload resolution happens (section 13.3 of the Standard),
then according to clause 13.3.1.2 the set of member candidates
for operator * consists of doubler::operator *(double), while the
set of non-member candidates is empty. So overload resolution
should unambiguously choose the member operator of class doubler.

Instead, gcc seems to ignore the member operator, and considers as only
candidate the operator template, resulting in a failing overload
resolution.

To me g++ seems to be wrong.

Oliver

Re: Problem with templates and operator overloading

[Eljay Love-Jensen]

Hi Casey,

I recommend taking out your template operator entirely, and put in a specific:

inline double operator * (double lhs, doubler rhs)
{
  return rhs * lhs;
}

Or alternatively (and especially if your operator struct isn't mathematically oriented), avoid overloading operators entirely and use a method invocation instead.

struct doubler
{
  double calc(double val) { return 2.0 * val; }
};

Or a functor invocation.

struct doubler
{
  double operator () (double val) { return 2.0 * val; }
};

HTH,
--Eljay

Problem with templates and operator overloading

[Casey Goodlett]

I'm having trouble with the following piece of c++ code.  Is this a bug
in gcc or am I doing something wrong?  The code compiles and runs
without problems in icc.

GCC Version: 3.3.2

struct doubler {
   double operator*(double val) {
      return 2.0 * val;
   }
};

template<typename T> T operator*(double lhs , T rhs) {
   return rhs * lhs;
}

int main() {
   doubler a;
   a * 3.0;
   return 0;
}

Compiling this gives the following error message.

test3.cc: In function `T operator*(double, T)':
test3.cc:7: error: `T operator*(double, T) [with T = double]' must have
an
   argument of class or enumerated type

Thanks.

-- 
Casey Goodlett (cgoodle@clemson.edu)