https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79167
Bug ID: 79167
Summary: LHS of assignment operator side-effect sequenced
before RHS computation
Product: gcc
Version: 5.4.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ronen at barzel dot org
Target Milestone: ---
Created attachment 40556
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40556&action=edit
g++ --save-temps output
According to http://en.cppreference.com/w/cpp/language/eval_order,
20) In every simple assignment expression E1=E2 and every compound
assignment expression E1@=E2, every value computation and side-effect of E2 is
sequenced before every value computation and side effect of E1
But in this case, the size of the map gets incremented by the LHS before
m.size() gets evaluated on the RHS:
#include <map>
#include <string>
#include <cassert>
int main(int argc, char *argv[])
{
std::map<std::string, int> m;
m["bug"] = m.size();
assert(m["bug"] == 0);
return 0;
}
i.e. m["bug"] should be equal to 0 but it is getting set to 1.
version:
$ g++ --version
g++ (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$
To reproduce:
$ g++ bug.cpp
$ ./a.out
a.out: bug.cpp:9: int main(int, char**): Assertion `m["bug"] == 0' failed.
Aborted
$
