operator= with base type as lhs

[Talix]

Ok, this example is in Stroustrup C++ 3rd ed.:

complex operator+ (double a, complex b)
{
...
}

But when I have this code:

float& operator= (float& lhs, const EnumType& rhs)
{
lhs = rhs.getInt();
return lhs;
}

I get this error:
'operator =' must be a <Unknown> member

Any ideas why this is happening?

-Talix

P.S. For those of you looking at my other question, I assumed that using the base class EnumType was ok for this one, even though operator= functions aren't inherited, because this is not a member function, and so should work for all subclass types, right? :)

[PaulWendt]

I don't think that operator= has the flexibility to be a non member.
You'll have to do something like:


class A
{
// ...
A& operator=(const A& rhs);
// ...
};

class B : public A
{
// ...
B& operator=(const B& rhs);
B& operator=(const A& rhs);
// ...
};


This is

--Paul

[galathaea]

You can expose the operator= as a friend function in your class.

[Talix]

Originally posted by PaulWendt
I don't think that operator= has the flexibility to be a non member.


That would be frustrating, since I want my class to act like a string or an integer, and being assigned to those without the use of cumbersome member functions is a relatively important piece...

Originally posted by galathaea
You can expose the operator= as a friend function in your class.


I don't believe that would be useful, since I cannot declare the operator= function in my class since the left hand side needs to be a base type. I already have floats successfully being assigned to my class, now I want the reverse functionality.

Anyone else think there might be a way to do this? :confused:

-Talix

[PaulWendt]

operator= has to be a member of a class; it cannot be globally
overloaded the way you want. You CAN, however, implement
a cast function so that you can cast your user-defined datatype
to another type. This would make the conversion implicit so that
you could do the assignment you're looking for.

Instead of "explaining" it with techno-babble, here's an
example:


class A
{
// ...
operator float() const { return getInt(); };
// ...
};

A a;
a.setInt(3);
float f = a; // f will be 3.0


Be careful with this approach. You can sometimes find your class
behaving like a float when you don't WANT it to. From what you
describe, it seems like your class is going to be some variant-type
... or that it HAS to behave like other types. For this example,
you'll probably be safe ... but be careful :)

--Paul

[Talix]

PERFECT! :D

Thank you so much, that worked great. It was exactly what I was looking for.

This forum rules. :cool:

[Graham]

But beware the dangers of introducing user-defined conversions into your class. See More Effective C++ Item 5 - Be wary of user-defined conversion functions.

Personally, I'd go for a more explicit approach:

class A
{
public:
double as_double() {/* whatever */}
};

int main()
{
A a;

//......

double d = a.as_double();
}

More typing, fewer problems.

[PaulWendt]

Originally posted by Graham
But beware the dangers of introducing user-defined conversions into your class. See More Effective C++ Item 5 - Be wary of user-defined conversion functions.

Personally, I'd go for a more explicit approach:


I never thought I'd be saying this a couple of years ago, but I
wholeheartedly agree. For some reason, the original poster
didn't want any "cumbersome" function calls. Maybe he has weird
design requirements or something.... Either way, I'd agree with
Graham in that I think you should use the explicit conversion
functions if possible :)

--Paul