Since implement the assign operator for reference types eliminates the
ability to assign a reference object to a reference variable of the same
type or base class of that type, I assume that implementing the assign
operator ( = ), to assign the value of a type from one object to another
rather than the reference, should only be done for value types. Is there
any other reason for implementing the assign operator for a type ?

It can be useful for plumbing types. See msclr/auto_handle.h for an example.

Marcus

"Edward Diener" <eddielee_no_spam_here (AT) tropicsoft (DOT) com> wrote

Edward Diener wrote:
I don't think so. Assigning handles is like assigning pointers in
unmanaged code. operator= should be implemented to work on references.

MyRefClass^ object1;
MyRefClass^ object2;
object1 = object2; // assigns handles, does not copy the object
*object1 = *object2; // this calls operator=

With the stack syntax, you can assign objects in a more comfortable manner:

MyRefClass object1, object2;
object1 = object2; // this calls operator=

Either way, it only works in languages that support operator overloading
(such as C++/CLI).

No, the opposite. It can only be done to ref classes. You can't have an
assignment operator for a value type, just like you can't have a copy
constructor either. Value types in .NET are like PODs in C++. When you
assign a value type, the framework does a raw memory memcpy. You can't
have custom assignment behavior there.

Tom

Tamas Demjen wrote:
This is different from all other operators. Are you sure this is how the
assignment operator works ?

This makes sense.

OK, I assume then I will get a compiler error if I try to implement the
assignment error for a value type.

Edward Diener wrote:

Yes, if it's implemented correctly. In ISO C++ it should be
T& operator=(const T&);

In C++/CLI it should be
T% operator=(T%);
or
T% operator=(const T%);

const is optional here. Since you can't declare a const member function,
passing arguments by const makes little sense (unfortunately). You would
be forced to cast the const-ness out quite frequently.

But that's irrelevant. The input argument is a T%, not a T^. Just like
in native C++, you don't write operator=(const T*). You don't want to
override the "copy a pointer" behavior. A pointer (or handle) is always
a value type, and making a copy of that should not involve any side
effects or custom behavior. If you're about to define
T^ operator=(T^);
then your original statement is correct, that would be very confusing,
and should be avoided. You don't want to do that, you don't want to
assign a custom behavior to a simple "pass by pointer" syntax.

Yes.

Tom

Edward Diener wrote:

I don't see how it's different from ISO C++. I just tried this:

ref class MyRefClass
{
public:
MyRefClass% operator=(MyRefClass% source)
{
Console::WriteLine("MyRefClass:perator=");
return *this;
}
};

int main(array<System::String ^> ^args)
{
MyRefClass^ object1 = gcnew MyRefClass;
MyRefClass^ object2 = gcnew MyRefClass;
object1 = object2;
}

There's no operator= called. But if I change the last line to *object1 =
*object2, it gets called as expected. Just like in native C++, except
replace handles with pointers.

I also tried to shoot myself in the foot, but the compiler didn't let me:

ref class MyRefClass
{
public:
MyRefClass^ operator=(MyRefClass^ source)
// malformed operator=
{
Console::WriteLine("BAD MyRefClass:perator=");
return *this;
}
};

int main(array<System::String ^> ^args)
{
MyRefClass^ object1 = gcnew MyRefClass;
MyRefClass^ object2 = gcnew MyRefClass;
object1 = object2;
}

operator= is not called, the compiler doesn't let you overload the
handle assignment operator. A handle is a value type, and its assignment
can't be overloaded. In this case you don't get an error -- the
malformed operator= simply won't be called.

Tom

Tamas Demjen wrote:
OK, I understand this.

By analogy are you also saying that other operators should act the same
? So that for a + operator for a ref class I should have:

ref class X
{
public: static X% operator + ( X% first, X% second ) { // some code
returning a tracking reference to a new X }

// rather than

public: static X^ operator + ( X^ first, X^ second ) { // some code
returning a handle to a new X }

};

and the user should be doing:

X^ first;
X^ second;
X^ result;

*result = *first + *second;

rather than

result = first + second;

This is what I meant by the operator = being different from other
C++/CLI operators. In your assignment operator with a ref class you are
taking tracking references and returning a tracking reference, whereas
the second set of syntaxes above for my hypothetical operator + in a ref
class takes handles and returns a handle, while the first acts like your
assignment operator.

Edward Diener wrote:

You are right, in this case both operators are valid.

Yes, I agree.

Tom