[Bug c++/36744] function modifying argument received by-value affects caller's variable when passed as rvalue

["chris dot fairles at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #2 from chris dot fairles at gmail dot com  2008-07-06 19:33 -------
(In reply to comment #0)
> Consider:
> 
>   #include <iostream>
> 
>   struct S
>   {
>     S(): i(2) {}
//S(S const&) {}
S(S const&);
The cctor need only be defined as well to get the same behavior (gcc 4.4.0,
-std=c++0x, -fno-elide-constructors, -O0, -fno-inline).

>     int i;
>   };
> 
>   void f(S x) { x.i = 0; }

The argument type of f(S), so far as my very limited understanding of gimple
goes, is the class type S when the cctor is defined (also marks it for copy
construction). Without the def'n, it's transformed to a reference type (struct
S & restrict) which I assume is an rvalue ref and x just gets moved. Been
trying to locate the responsible code w/o luck.

Chris

> 
>   int main()
>   {
>     S y;
>     f(static_cast<S&&>(y));
>     std::cout << y.i << '\n';
>   }
> 
> Expected output: 2
> Actual output: 0
> 
> Thus, the assignment to the independent local variable x in f somehow modifies
> y. That can't be right, not even with that static_cast to S&&, can it?
> 


-- 

chris dot fairles at gmail dot com changed:

           What    |Removed                     |Added
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                 CC|                            |chris dot fairles at gmail
                   |                            |dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36744