C++ Logo


Advanced search

Re: [std-proposals] Force compilers to warn about double moves

From: Frederick Virchanza Gotham <cauldwell.thomas_at_[hidden]>
Date: Fri, 23 Jun 2023 08:34:09 +0100
On Thu, Jun 22, 2023 at 6:17 PM Hyman Rosen via Std-Proposals
<std-proposals_at_[hidden]> wrote:
> It may also be reasonable for someone to do
> void F(C &&o) {
> A_MightTake(move(o));
> B_MightTake(move(o));
> ...
> }
> to let a sequence of functions examine an object
> and maybe take it, maybe not. First one that
> wants it, gets it, the rest see a moved-from
> empty object.

Okay then how about my proposed compiler warning/error only kicks in
when then Rvalue reference is marked with postfix '&&' ?

So the following would fail to compile:

    void F(C &&o&&)
        A_MightTake( o );
        B_MightTake( o ); // possible double-move

And in order to alleviate this error, you would abandon the use of the
postfix '&&' operator, and you would simply mark the function
parameter as "&&o" instead of "&&o&&", and use 'std::move' as normal.

By the way, when it comes to marking a function parameter with both
prefix and postfix '&&', it would have no effect in a function
declaration, i.e. the following two function signatures are

    void F(C &&o);
    void F(C &&o&&);

There would be a difference however in the function definition as follows:

    void F(C &&o&&)
        // In here, 'o' is always an R-value (no need for 'move' or 'forward')

So if you decide to change a function parameter from "&&o" to "&&o&&",
then none of the header files need to change.

Received on 2023-06-23 07:34:22