Template operator() specialization for a special type?-CodePudding

I have a Vector class that is template based. I have a unit conversion template that is based on Scott Meyers Dimensional Analysis in C . I'm not going to list it as it is complex, unless the problem can't be solved without those details. I'm using the casting operator for when I need to pass the data to outside functions.

template<typename T>
struct XYZVector 
{
  operator Point3D() { return Point3D(x, y, z); }
}

If I use T as a double it works fine. But when I do this I need a specialization.

Point3D vecOut = XYZVector<Unit::usFoot>(x,y,z);

Will fail because when converting a Unit object you need to use an as() method.

operator Point3D() { return Point3D( x.as<T>(), y.as<T>(), z.as<T>() ); }

So I need someway to say if T is a Unit use this line, other wise use the next line.

template<typename T>
struct XYZVector
{
  //if typename T is a Unit type use this
  operator Point3D() { return Point3D( x.as<T>(), y.as<T>(), z.as<T>() ) }
  //otherwise use this
  operator Point3D() { return Point3D(x, y, z); }
}

Is this possible?

CodePudding user response：

if constexpr to the rescue.

operator Point3D() { 

     if constexpr (std::is_same_v<std::remove_cvref_t<T>,Unit>){
          return Point3D(x.as<T>(),y.as<T>(),z.as<T>());
     }else{
          return Point3D(x,y,z);
     }
}

CodePudding user response：

I would use overloads as customization point:

// overload for built-in type (no ADL)
double to_double(double d) { return d;}

template <typename T>
struct XYZVector
{
    // ...
    operator Point3D() const {
        return Point3D(to_double(x),
                       to_double(y),
                       to_double(z));
    }
};

// The ones which can be found by ADL can be placed after
namespace Unit {
    double to_double(usFoot u) { return u.as<double>(); } 
}