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* [Bug c++/61856] New: Ternary operator in an NSDMI causes double evaluation
@ 2014-07-20 12:18 ville.voutilainen at gmail dot com
  2014-07-20 12:19 ` [Bug c++/61856] " redi at gcc dot gnu.org
                   ` (2 more replies)
  0 siblings, 3 replies; 4+ messages in thread
From: ville.voutilainen at gmail dot com @ 2014-07-20 12:18 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61856

            Bug ID: 61856
           Summary: Ternary operator in an NSDMI causes double evaluation
           Product: gcc
           Version: 4.10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ville.voutilainen at gmail dot com

First test:
#include <iostream>

#ifdef OK
struct ostream { } cout;

bool operator<<(ostream&, const char* s)
{ __builtin_printf("%s", s); return true; }
#else
using std::cout;
#endif

struct X { int a = (cout<<"hmm\n") ? 1 : 2;};

int main()
{
    X x;
}

With -DOK, this prints "hmm" once, otherwise it prints "hmm" twice. This
suggested
that perhaps the problem is specific to iostream, but wrapping the NSDMI into
a lambda fixes the double evaluation:

#include <iostream>

#ifdef OK
struct ostream { } cout;

bool operator<<(ostream&, const char* s)
{ __builtin_printf("%s", s); return true; }
#else
using std::cout;
#endif

struct X { int a = ([]{return bool(cout<<"hmm\n");}()) ? 1 : 2;};

int main()
{
    X x;
}

Simpler cases like just incrementing a global variable in the ternary
operator seem to work fine:

#include <iostream>

int global_x = 0;
struct X { int a = (global_x++) ? 1 : 2;};

int main()
{
    X x;
    std::cout << global_x << std::endl;
}

Perhaps this is some weird gimplification bug.


^ permalink raw reply	[flat|nested] 4+ messages in thread
end of thread, other threads:[~2015-06-21  9:06 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2014-07-20 12:18 [Bug c++/61856] New: Ternary operator in an NSDMI causes double evaluation ville.voutilainen at gmail dot com
2014-07-20 12:19 ` [Bug c++/61856] " redi at gcc dot gnu.org
2015-06-18 16:50 ` miyuki at gcc dot gnu.org
2015-06-21  9:06 ` ville.voutilainen at gmail dot com

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