Re: Passing std::unique_ptr to std::thread's target

SG <>
Thu, 11 Aug 2011 03:03:35 CST
On 11 Aug., 03:04, Daniel Kr?gler <>

I have a guess, though: I assume that gcc does use std::bind internally
to implement the thread constructor. There exists currently a wording
problem described in LWG issue

for std::bind (and some other functions). The most recent proposed
resolution (not visible in the link above) suggests to change
[func.bind.bind] p3 as follows:

"Returns: A forwarding call wrapper g with a weak result type (20.8.2).
The effect of g(u1, u2, ..., uM) shall be INVOKE(fd,

<ins>std::forward<V2>(</ins>v2<ins>)</ins>, ...,
<ins>std::forward<VN>(</ins>vN<ins>)</ins>, result_of<FD cv <ins>&</ins>
(V1, V2, ..., VN)>::type), where cv represents the cv-qualifiers of g
and the values and types of the bound arguments v1, v2, ..., vN are
determined as specified below. [...]"

While LWG issue 2021 points to a defect in the wording of std::bind, the
intention clearly seems to be to provide lvalues of the "decayed"
functor and of the "decayed" functor arguments to the conceptual INVOKE

....and this is a good thing[tm].

so with or without the wording changes the problem will
probably remain for std::bind. This has the effect that if you replace
the line

std::thread thr(threadMain, std::move(outPtr));


std::bind(threadMain, std::move(outPtr))();

the code should be ill-formed. I find this decision a bit unfortunate
but I have been told that it must be necessary, because of
backward-compatibility reasons in regard to std::bind.

Not only that. If some bound arguments were to be forwarded as rvalues
you could only safely invoke the function _once_ because this might
invalidate the bound values. This is different from std::thread.
std::thread calls this function at most once. So, for std::thread it
is okay to finally invoke the function using rvalues as arguments.


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