External cast operators

[Ulrich Heinen]

Let's say I have two classes A and B with similar goals but completely
different implementations . In my application code I want to use class A
exclusively, but instances of B should also be accepted. This can be
achieved when B provides an appropriate cast operator, e.g.

class A {
public:
/* ... */
protected:
/* ... */
};

class B {
public:
/* ... */
operator A() const;
protected:
/* ... */
};

void someMethod(cons t A & a) {
/* ... */
}

void someOtherMethod () {
A a;
B b;
someMethod(a);
someMethod(b); // invokes the cast operator to make A from B
}

You get the point. Now, when classes A and B are in two different libraries,
the introduction of the cast operator creates a dependency libB -> libA.
This could be avoided if the cast operator resides outside class B, i.e. in
the actual application. Is there a way to do this? I've tried

operator A(const B &) {
/* ... */
}

but this is apparently wrong. Any hints?

Thanks,

Ulrich Heinen

[Jeff Schwab]

Ulrich Heinen wrote:

Let's say I have two classes A and B with similar goals but completely
different implementations . In my application code I want to use class A
exclusively, but instances of B should also be accepted. This can be
achieved when B provides an appropriate cast operator, e.g.
<snip/>
You get the point. Now, when classes A and B are in two different libraries,
the introduction of the cast operator creates a dependency libB -> libA.
This could be avoided if the cast operator resides outside class B, i.e. in
the actual application. Is there a way to do this?

<snip/>

The usual approach is to derive A and B publicly from an abstract class
encapsulating their common interface.

struct A { /* ... */ };
struct A_impl_1: A { /* ... */ };
struct A_impl_2: B { /* ... */ };

void some_method( A const& a );

[Rolf Magnus]

Ulrich Heinen wrote:

Let's say I have two classes A and B with similar goals but completely
different implementations . In my application code I want to use class
A exclusively, but instances of B should also be accepted. This can be
achieved when B provides an appropriate cast operator, e.g.
You mean a conversion operator. Usually, one would define a conversion
constructor instead of a conversion operator though, unless you cannot
add a constructor to the target type (e.g. because it's part of a
library you use or because it's a built-in type).
class A {
public:
/* ... */
protected:
/* ... */
};

class B {
public:
/* ... */
operator A() const;
protected:
/* ... */
};

void someMethod(cons t A & a) {
/* ... */
}

void someOtherMethod () {
A a;
B b;
someMethod(a);
someMethod(b); // invokes the cast operator to make A from B
}

You get the point. Now, when classes A and B are in two different
libraries, the introduction of the cast operator creates a dependency
libB -> libA. This could be avoided if the cast operator resides
outside class B, i.e. in the actual application. Is there a way to do
this?
You can't make non-member conversion operators.
I've tried

operator A(const B &) {
/* ... */
}

but this is apparently wrong. Any hints?

Thanks,

Ulrich Heinen