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Re: Is removing necessity of std::move for local variables used for the last time a good proposal?

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[Dear moderator, please replace my previous message on this topic.]

SG <s.gesemann@googlemail.com> writes:

> So far, as you probably know, a compiler is not required nor allowed
> to do that. I think there is some value in further analyzing the
> impact of such a rule. We would not want this rule to break something.
> I've trouble coming up with examples that would break, but it does
> not mean that there are none. It would be nice to have a somewhat
> formal proof under reasonable assumptions that nothing could go wrong.

#include <memory>
#include <mutex>
std::mutex m;
int i;
void f1(std::shared_ptr<std::lock_guard<std::mutex> > p);
void f2()
{
    auto p = std::make_shared<std::lock_guard<std::mutex> >(m);
    ++i;
    f1(p);
    ++i;
}

If f1(p) automatically changed to f1(std::move(p)), then the
mutex would be unlocked already before the second ++i; statement.

The following example seems more realistic:

#include <cstdio>
#include <string>
void f1(std::string s) {}
int main()
{
    std::string s("hello");
    const char *p = s.c_str();
    f1(s);
    std::puts(p);
}

If f1(s) automatically changed to f1(std::move(s)), then the
pointer p would no longer be valid after f1 returns.


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Is removing necessity of std::move for local variables used for the  last time a good proposal? Piotr Nycz <piotrwn1@googlemail.com> - 2014-05-30 15:32 -0600
  Re: Is removing necessity of std::move for local variables used for  the  last time a good proposal? SG <s.gesemann@googlemail.com> - 2014-06-05 12:31 -0600
    Re: Is removing necessity of std::move for local variables used for  the  last time a good proposal? Kalle Olavi Niemitalo <kon@iki.fi> - 2014-06-07 01:25 -0600
    Re: Is removing necessity of std::move for local variables used for   the  last time a good proposal? Piotr Nycz <piotrwn1@googlemail.com> - 2014-06-07 02:33 -0600
      Re: Is removing necessity of std::move for local variables used for   the  last time a good proposal? Daniel Krügler <daniel.kruegler@googlemail.com> - 2014-06-08 01:32 -0600
        Re: Is removing necessity of std::move for local variables used for    the  last time a good proposal? Piotr Nycz <piotrwn1@googlemail.com> - 2014-06-17 07:25 -0600

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