[Bug c++/59070] New: Captured object is being moved from the lambda on returning it.

[Bug c++/59070] New: Captured object is being moved from the lambda on returning it.

[sir_nawaz959 at yahoo dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59070

            Bug ID: 59070
           Summary: Captured object is being moved from the lambda on
                    returning it.
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sir_nawaz959 at yahoo dot com

I'm using GCC 4.8.1

Here is the code which reproduces this bug:

   std::vector<std::string> items {"default"};

   auto add = [=](std::string item) mutable 
              { items.push_back(item); return items; };

   std::cout << add("one") << std::endl;
   std::cout << add("two") << std::endl;
   std::cout << add("three") << std::endl;

Imagine that operator<< is overloaded for std::vector which all items on ONE
line. The expected out is:

   default one 
   default one two 
   default one two three

But it actually outputs this:

   default one 
   two 
   three

So it seems on the first return, the captured vector is moved. 

However, if I define the lambda as:

   auto add = [=](std::string item) mutable 
              { items.push_back(item); auto & x = items; return x; } ;

OR as:

   auto add = [=](std::string item) mutable 
              { auto & x= items; x.push_back(item); return x; } ;


It prints the expected output.

[Bug c++/59070] Captured object is being moved from the lambda on returning it.

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59070

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2013-11-10
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Sarfaraz Nawaz from comment #0)
> Here is the code which reproduces this bug:

That code doesn't reproduce anything because it doesn't compile, please provide
*complete* testcases instead of expecting other people to fill in the gaps.

[Bug c++/59070] Captured object is being moved from the lambda on returning it.

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59070

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
         Resolution|---                         |WORKSFORME

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is a valid testcase:

#include <iostream>
#include <vector>
#include <string>

struct VecPrinter {
  const std::vector<std::string>& v;
};

std::ostream& operator<<(std::ostream& out, VecPrinter vp)
{
  for (auto& i : vp.v)
    out << i << ' ';
  return out;
}

int main()
{
   std::vector<std::string> items {"default"};

   auto add = [=](std::string item) mutable 
              { items.push_back(item); return items; };

   std::cout << VecPrinter{ add("one") } << std::endl;
   std::cout << VecPrinter{ add("two") } << std::endl;
   std::cout << VecPrinter{ add("three") } << std::endl;
}

And it works perfectly with 
gcc version 4.8.3 20131029 (prerelease) (GCC) 
gcc version 4.9.0 20131002 (experimental) (GCC)

[Bug c++/59070] Captured object is being moved from the lambda on returning it.

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59070

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #2)
> And it works perfectly with 
> gcc version 4.8.3 20131029 (prerelease) (GCC) 
> gcc version 4.9.0 20131002 (experimental) (GCC)

and
gcc version 4.7.4 20131030 (prerelease) (GCC)

[Bug c++/59070] Captured object is being moved from the lambda on returning it.

[sir_nawaz959 at yahoo dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59070

Sarfaraz Nawaz <sir_nawaz959 at yahoo dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |NEW
         Resolution|WORKSFORME                  |---

--- Comment #4 from Sarfaraz Nawaz <sir_nawaz959 at yahoo dot com> ---
Here is the complete testcase which compiles fine but gives incorrect output:




#include <iostream>
#include <vector>
#include <string>

std::ostream & operator<<(std::ostream & out, std::vector<std::string> const &
items)
{
    for(auto const & item : items ) 
        out << item << " ";
    return out;
}

int main() 
{
    std::cout << "\nGCC " << __VERSION__ << std::endl;

    std::vector<std::string> items {"default"};

    auto add = [=](std::string item) mutable { items.push_back(item); return
items; } ;

    std::cout << add("one") << std::endl;
    std::cout << add("two") << std::endl;
    std::cout << add("three") << std::endl;
}

$ g++-4.8 -std=c++11 -O2 -Wall -pedantic-errors main.cpp && ./a.out

GCC 4.8.1
default one 
two 
three

which is incorrect output

[Bug c++/59070] Captured object is being moved from the lambda on returning it.

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59070

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
      Known to work|                            |4.7.4, 4.8.3, 4.9.0
         Resolution|---                         |FIXED

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
But I already said it works with current releases, and obviously we don't have
a time machine to fix 4.8.1 now, so there's no point re-opening the bug.

GCC 4.8.2 20131017 (Red Hat 4.8.2-1)
default one 
default one two 
default one two three