* potential bug regarding rvalue refs?
@ 2014-01-29 18:58 Brian Budge
2014-01-29 19:40 ` Florian Weimer
2014-01-29 20:10 ` Jonathan Wakely
0 siblings, 2 replies; 4+ messages in thread
From: Brian Budge @ 2014-01-29 18:58 UTC (permalink / raw)
To: GCC-help
Hi all -
I have been playing around with universal references and wanted to see
what the difference is between something like
template <typename T>
void f(T t) {
f_impl(std::forward<foo>(t));
}
and
template <typename T>
void f(T &&t) {
f_impl(std::forward<foo>(t));
}
I wrote a quick program to start to try and tease out any differences,
but found some totally unintuitive behaviour that I think may be a
bug?
#include <iostream>
#if 0
struct foo {
int a;
foo(int b) : a(b) {}
};
#else
typedef int foo;
#endif
void f_impl(foo const &a) {
std::cerr << "f by const ref" << std::endl;
}
void f_impl(foo &&a) {
std::cerr << "f by rvalue ref" << std::endl;
}
template <typename T>
void f(T t) {
f_impl(std::forward<foo>(t));
}
void g_impl(foo const &a) {
std::cerr << "g by const ref" << std::endl;
}
void g_impl(foo &&a) {
std::cerr << "g by rvalue ref" << std::endl;
}
template <typename T>
void g(T &&t) {
g_impl(std::forward<foo>(t));
}
int main(int argc, char **args) {
foo lvalue = 5;
f(lvalue);
f(5);
g(lvalue);
g(5);
return 0;
}
When I compile this via g++ 4.8.2, I get the following output:
:./a.out
f by rvalue ref
f by rvalue ref
g by rvalue ref
g by rvalue ref
Although I'm far from an expert with rvalue refs and universal refs,
this seems like a bug to me. Thoughts?
Thanks,
Brian
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: potential bug regarding rvalue refs?
2014-01-29 18:58 potential bug regarding rvalue refs? Brian Budge
@ 2014-01-29 19:40 ` Florian Weimer
2014-01-29 20:10 ` Jonathan Wakely
1 sibling, 0 replies; 4+ messages in thread
From: Florian Weimer @ 2014-01-29 19:40 UTC (permalink / raw)
To: brian.budge; +Cc: gcc-help
On 01/29/2014 07:58 PM, Brian Budge wrote:
> f_impl(std::forward<foo>(t));
You need to write std::forward<T> instead of std::forward<foo> to get
the expected forwarding behavior.
> Although I'm far from an expert with rvalue refs and universal refs,
> this seems like a bug to me. Thoughts?
What output do you expected instead?
--
Florian Weimer / Red Hat Product Security Team
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: potential bug regarding rvalue refs?
2014-01-29 18:58 potential bug regarding rvalue refs? Brian Budge
2014-01-29 19:40 ` Florian Weimer
@ 2014-01-29 20:10 ` Jonathan Wakely
2014-01-29 20:56 ` Brian Budge
1 sibling, 1 reply; 4+ messages in thread
From: Jonathan Wakely @ 2014-01-29 20:10 UTC (permalink / raw)
To: Brian Budge; +Cc: GCC-help
This is a question about C++ and not specific to G++ so not really
on-topic on this list.
On 29 January 2014 18:58, Brian Budge wrote:
>
> Although I'm far from an expert with rvalue refs and universal refs,
> this seems like a bug to me. Thoughts?
G++ is correct, you're forwarding 't' as an rvalue, so overload
resolution chooses the function taking an rvalue reference.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: potential bug regarding rvalue refs?
2014-01-29 20:10 ` Jonathan Wakely
@ 2014-01-29 20:56 ` Brian Budge
0 siblings, 0 replies; 4+ messages in thread
From: Brian Budge @ 2014-01-29 20:56 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: GCC-help
On Wed, Jan 29, 2014 at 12:09 PM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> This is a question about C++ and not specific to G++ so not really
> on-topic on this list.
>
> On 29 January 2014 18:58, Brian Budge wrote:
>>
>> Although I'm far from an expert with rvalue refs and universal refs,
>> this seems like a bug to me. Thoughts?
>
> G++ is correct, you're forwarding 't' as an rvalue, so overload
> resolution chooses the function taking an rvalue reference.
Florian, Jonathan, thanks for the responses.
Brian
^ permalink raw reply [flat|nested] 4+ messages in thread
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