c++ cli - C++/CLI: how to overload an operator to accept reference types?

Question

I am trying to create a CLI value class c_Location with overloaded operators, but I think I have an issue with boxing. I have implemented the operator overloading as seen in many manuals, so I'm sure this must be right. This is my code:

value class c_Location
{
public:
  double x, y, z;
  c_Location (double i_x, double i_y, double i_z) : x(i_x), y(i_y), z(i_z) {}

  c_Location& operator+= (const c_Location& i_locValue)
  {
    x += i_locValue.x;
    y += i_locValue.y;
    z += i_locValue.z;
    return *this;
  }
  c_Location operator+ (const c_Location& i_locValue)
  {
    c_Location locValue(x, y, z);
    return locValue += i_locValue;
  }
};

int main()
{
  array<c_Location,1>^ alocData = gcnew array<c_Location,1>(2);
  c_Location locValue, locValue1, locValue2;
  locValue = locValue1 + locValue2;
  locValue = alocData[0] + alocData[1];  // Error C2679 Binary '+': no operator found which takes a right-hand operand of type 'c_Location'
}

After searching for a longer time, I found that the error comes from the operand being a reference type, as it is an array element of a value type, and the function accepting only value types as it takes an unmanaged reference. I now have 2 possibiblities:

1. adding a unboxing cast to c_Location and so changing the faulty line in main() to
   locValue = alocData[0] + (c_Location)alocData[1];
2. modifying the operator+ overloading so that it takes the parameter by value instead of by reference:
   c_Location operator+ (const c_Location i_locValue)

both options work, but as far as I can see, they both have disadvantages:
opt 1 means that I have to explicitly cast wherever needed.
opt 2 means that the function will create a copy of the parameter on its call and therefore waste performance (not much though).

My questions: Is my failure analysis correct at all or does the failure have another reason?
Is there a better third alternative?
If not: which option, 1 or 2, is the better one? I currently prefer #2.

Answer 1

The rules are rather different from native C++:

• the CLI demands that operator overloads are static members of the class
• you can use the const keyword in C++/CLI but you get no mileage from it, the CLI does not support enforcing const-ness and there are next to no other .NET languages that support it either.
• passing values of a value type ought to be done by value, that's the point of having value types in .NET in the first place. Using a & reference is very troublesome, that's a native pointer at runtime which the garbage collector cannot adjust. You'll get a compile error if you try to use your operator overload on a c_Location that's embedded in a managed class. If you want to avoid value copy semantics then you should declare a ref class instead. The hat^ in your code.
• any interop type you create in C++/CLI should be declared public so it is usable from other assemblies and .NET languages. It isn't entirely clear if that's your intention, it is normally the reason you write C++/CLI code.

You could make your value class look like this instead:

public value class c_Location
{
public:
  double x, y, z;
  c_Location (double i_x, double i_y, double i_z) : x(i_x), y(i_y), z(i_z) {}

  static c_Location operator+= (c_Location me, c_Location rhs)
  {
    me.x += rhs.x;
    me.y += rhs.y;
    me.z += rhs.z;
    return me;
  }
  static c_Location operator+ (c_Location me, c_Location rhs)
  {
    return c_Location(me.x + rhs.x, me.y + rhs.y, me.z + rhs.z);
  }
};

Untested, ought to be close. You'll now see that your code in main() compiles without trouble.