Re: Help with g++ 6 abs() and templates

[Jonathan Wakely]

On 20/02/16 03:32 +0000, Jonathan Wakely wrote:
> On 19/02/16 20:18 -0700, Orion Poplawski wrote:
> >octave has template functions that call abs() on templated 
> >variables, something like:
> >
> >template <class T>
> >T myfunc(T arg) {
> >T x = abs(arg);
> >}
> >
> >
> >This leads to errors when instantiated for unsigned types because 
> >abs(unsigned type) is not defined as is doesn't make sense to do.
> >That's fine and all, but how then does one write generic template 
> >code as above that works for signed and unsigned types.
>
>
> template <class T>
>  typename std::enable_if<std::is_signed<T>::value, T>::type
>  generic_abs(T arg) {
>    return abs(arg);
>  }
>
> template <class T>
>  typename std::enable_if<!std::is_signed<T>::value, T>::type
>  generic_abs(T arg) {
>    return arg;
>  }
>
> template <class T>
>  T myfunc(T arg) {
>    T x = generic_abs(arg);
>  } 
Or maybe better, figure out which version of abs() it used to call
before the GCC 6 changes, which was probably abs(int), and just cast
to that.

If it worked before without an overloaded abs() then implicitly
converting to int must have been OK (or a lurking bug). So just:

template <class T>
T myfunc(T arg) {
T x = abs((int)arg);
}