boost::optional triggers -Wmaybe-uninitialized errors in g++

[Mathias Stearn <redbeard0531@…>]

Example taken from StackOverflow question 21755206*

#include <boost/optional.hpp>

::boost::optional<int> getitem();

int go(int nr)
{
  boost::optional<int> a = getitem();
  boost::optional<int> b;

  if (nr > 0)
    b = nr;

  if (a != b)
    return 1;

  return 0;
}

g++ -c -O2 -Wall /tmp/test.cpp
/tmp/test.cpp: In function ‘int go(int)’
/tmp/test.cpp:13:3: warning: ‘*((void*)& b +4)’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   if (a != b)
   ^

This has been reported as gcc bug 47679*.

Two workarounds that seem to work in my testing are value-initializing dummy_ in the aligned_storage default constructor or adding an empty asm statement that lists dummy_ as an output. Would it be possible to include a fix in mainline boost?

• Sorry, but trac flags this as spam if I include direct links.

[akrzemi1]

Could you give me an example of such an asm statement?

[anonymous]

Note that this GCC bug is fixed in GCC 5.1. Also as per th StackOverflow response, use a workaround by initializing an optional with a 'fabricated' empty value:

boost::optional<int> b = boost::make_opitonal(false, int());