C++/CLI comparing two clr poiinters for equality

  • Thread starter Thread starter Edward Diener
  • Start date Start date
E

Edward Diener

Now that operator overloading allows to ref classes to be compared for
equality using == syntax, how does one compare the actual ref pointers (
^ ) for equality instead ?

As an example:

SomeRefObject ^ obj1(..initialized somehow);
SomeRefObject ^ obj2(..initialized somehow);

if (obj1 == obj2) // This compares the objects themselves for equality
in C++/CLI I assume. How do I compare the ref pointers themselves ( ^ )
for equality ?
 
Edward Diener said:
Now that operator overloading allows to ref classes to be compared for
equality using == syntax, how does one compare the actual ref pointers (
^ ) for equality instead ?

As an example:

SomeRefObject ^ obj1(..initialized somehow);
SomeRefObject ^ obj2(..initialized somehow);

if (obj1 == obj2) // This compares the objects themselves for equality in
C++/CLI I assume.
Click to expand...

If SomeRefObject overwrites Equals, yes, otherwise no
How do I compare the ref pointers themselves ( ^ ) for equality ?
Click to expand...

Object::ReferenceEquals(obj1, obj2)
 
Marcus said:
If SomeRefObject overwrites Equals, yes, otherwise no
Click to expand...

So are you saying that operator overloading in C++/CLI for the ==
operator becomes equivalent to a ref class overloading the
Object::Equals method ?

Or is it that a ref class providing an overload for the Objecct::Equals
method now has that called when a C++/CLI programmer uses the == operator ?
Object::ReferenceEquals(obj1, obj2)
Click to expand...

Thanks ! That will always works.

However operator overloading in C++/CLI has me a bit confused. Does
C++/CLI operator overloading relate to any .NET framework virtual member
functions that can be called from other .NET languages ? if so which
operator oveloads and which member functions.

In general I have found the documentation for clr class operator
overloading in MSDN for C++/CLI to be sparse to the extreme and
explaining almost nothing. There is a single topic page which I can find
which has no explanation how C++/CLI relates to .net member functions.
Have I missed a more profuse explanation of this somewhere in the MSDN
documentation for C++/CLI ?
 
Sorry, I was quite tired when I wrote this. That's why I confused some
things here. (At least I was the only one to find it out :-).)

By default, operator == works on reference types like
Object::ReferenceEquals, however types can override operator ==.
In C++/CLI and C#, operator == NEVER maps to Object::Equals.

Here is an app that shows how it works:

// eqt.cpp
// compile with "CL /clr:safe eqt.cpp"

using namespace System;

ref class ClassWithEquals
{
int i;
public:
ClassWithEquals(int i) : i(i) {}

virtual bool Equals(Object^ o) override
{
if (o == nullptr)
return false;

ClassWithEquals^ that = dynamic_cast<ClassWithEquals^>(o);
if (that == nullptr)
return false;

return this->i = that->i;
}
};

ref class ClassWithEqualityOperator
{
int i;
public:
ClassWithEqualityOperator(int i) : i(i) {}

bool operator==(ClassWithEqualityOperator^ that)
{
if (that == (Object^)nullptr)
return false;

return this->i == that->i;
}
};

int main()
{
ClassWithEquals^ cwe1 = gcnew ClassWithEquals(1);
ClassWithEquals^ cwe2 = gcnew ClassWithEquals(1);

if (cwe1 == cwe2)
Console::WriteLine("cwe1 == cwe2");

ClassWithEqualityOperator^ cwoe1 = gcnew ClassWithEqualityOperator(1);
ClassWithEqualityOperator^ cwoe2 = gcnew ClassWithEqualityOperator(1);

if (cwoe1 == cwoe2)
Console::WriteLine("cwoe1 == cwoe2");
}



If o
 
Marcus said:
Sorry, I was quite tired when I wrote this. That's why I confused some
things here. (At least I was the only one to find it out :-).)

By default, operator == works on reference types like
Object::ReferenceEquals, however types can override operator ==.
In C++/CLI and C#, operator == NEVER maps to Object::Equals.
Click to expand...

Understood.

But I am gathering that if a class implements operator ==, then it
should also override Object::Equals to work in the same way in order to
be consistent.
Here is an app that shows how it works:

// eqt.cpp
// compile with "CL /clr:safe eqt.cpp"

using namespace System;

ref class ClassWithEquals
{
int i;
public:
ClassWithEquals(int i) : i(i) {}

virtual bool Equals(Object^ o) override
{
if (o == nullptr)
return false;

ClassWithEquals^ that = dynamic_cast<ClassWithEquals^>(o);
if (that == nullptr)
return false;

return this->i = that->i;
}
};

ref class ClassWithEqualityOperator
{
int i;
public:
ClassWithEqualityOperator(int i) : i(i) {}

bool operator==(ClassWithEqualityOperator^ that)
{
if (that == (Object^)nullptr)
return false;

return this->i == that->i;
}
};

int main()
{
ClassWithEquals^ cwe1 = gcnew ClassWithEquals(1);
ClassWithEquals^ cwe2 = gcnew ClassWithEquals(1);

if (cwe1 == cwe2)
Console::WriteLine("cwe1 == cwe2");

ClassWithEqualityOperator^ cwoe1 = gcnew ClassWithEqualityOperator(1);
ClassWithEqualityOperator^ cwoe2 = gcnew ClassWithEqualityOperator(1);

if (cwoe1 == cwoe2)
Console::WriteLine("cwoe1 == cwoe2");
}
Click to expand...

Thanks for the example.
 
Edward Diener said:
Understood.

But I am gathering that if a class implements operator ==, then it should
also override Object::Equals to work in the same way in order to be
consistent.
Click to expand...

This argument sounds plausible to me. If you follow this argument, then you
should also overide Object::GetHashCode, because some container classes in
the FCL expect the following behavior:

( obj1->Equals(obj2) ) => ( obj1->GetHashCode() ==
obj2->GetHashCode() )

Marcus
 
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