Re: Compiler Generated Default Functions

"kanze" <>
27 Jul 2006 09:07:50 -0400
Allan W wrote:

kanze wrote:

The operator& is the hard one. It's defined for all
objects, of all types. The standard doesn't consider it an
implicitly defined operator; it says that the built in
operator is used if there is no user defined one. And in
fact, there is no way to define the operator to give the
semantics of the built-in operator.

How about...

    class Foo {
    // ...
        Foo* operator&() { return this; }
        const Foo* operator&() const { return this; }
    // ...

Technically, when the compiler implicitly generates a
function, it behaves like a user defined function; the
compiler generated operator=, for example, has an address,
participates in overload resolution, and introduces sequence
points. The global operator& which the compiler uses if
their is not a user defined one requires an lvalue (which a
user defined function cannot), doesn't have an address, and
doesn't introduce sequence points.

So a user-defined operator& can't be completely identical to
the built-in operator&. But "no way... to give the [same]
semantics" is still a bit overstated, isn't it?

I suppose it depends on what you understand by "semantics". To
tell the truth, I'm not sure whether this differs would fall
under that criteria or not. However:

    Foo f() ;

    Foo* p = &f() ;

The last line is illegal with the built-in &. I don't think you
can make it illegal with a user defined &.

I have found exactly one legal way to tell if operator& is
user-defined or not -- take the address.

    Foo* (Foo::*p)() = &(Foo::operator &);

This fails if operator& is not user-defined.

Is there any other difference, in legal code?

See my example above. If it fails to compile, operator& is not
user defined. If the goal is to test whether it is user
defined or not in meta-programming, your solution seems more
general and easier to exploit. If your goal is to provide a
user defined overload which is legal in the same cases the
built-in operator is legal, and only in those cases, however, I
don't think you can.

James Kanze GABI Software
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