Operator precedence change - c++

Question

The following used to build OK on VS2008 but on VS2013, it moans. OK in g++.
#include <sstream>
int main()
{
int a = 2, b = 5;
std::ostringstream oss;
oss << a == 0 ? a : b;
}
The error message is
1>u:\precedence\precedence.cpp(7): error C2678: binary '==' : no operator found which takes a left-hand operand of type 'std::basic_ostream<char,std::char_traits<char>>' (or there is no acceptable conversion)
1> c:\program files\microsoft visual studio 12.0\vc\include\exception(497): could be 'bool std::operator ==(const std::exception_ptr &,const std::exception_ptr &)' [found using argument-dependent lookup]
1> c:\program files\microsoft visual studio 12.0\vc\include\exception(502): or 'bool std::operator ==(std::nullptr_t,const std::exception_ptr &)' [found using argument-dependent lookup]
1> c:\program files\microsoft visual studio 12.0\vc\include\exception(507): or 'bool std::operator ==(const std::exception_ptr &,std::nullptr_t)' [found using argument-dependent lookup]
1> c:\program files\microsoft visual studio 12.0\vc\include\system_error(402): or 'bool std::operator ==(const std::error_code &,const std::error_condition &) throw()' [found using argument-dependent lookup]
1> c:\program files\microsoft visual studio 12.0\vc\include\system_error(410): or 'bool std::operator ==(const std::error_condition &,const std::error_code &) throw()' [found using argument-dependent lookup]
1> while trying to match the argument list '(std::basic_ostream<char,std::char_traits<char>>, int)'
It works on both versions if the output statement is changed to
oss << (a == 0? a: b);
I can force a different error in VS2008 if a constant is not used: something like
int zero = 0;
oss << a == zero? a: b;
Just wondering why there is an error when a constant is not used.
Edit Managed to get an warning with g++ with -Wall.

Answer 1

The expression the compiler sees due to the precedence is, in all cases:
((oss << a) == 0) ? a : b;
Now oss << a returns oss. How can you compare oss with 0? It has an implicit conversion operator operator void *.
VS2013 presumably replaced that with explicit operator bool for C++11 (the whole point of which was so that these hard-to-find bugs don't occur). libstdc++ has not done this replacement to my knowledge. It must in order to conform with C++11.
Now why doesn't zero compile? zero is not a null pointer constant, but 0 is. Thus, the latter can be compared to the pointer returned by operator void *.
In any case, the simple way to fix this so it should work properly with any conforming implementation is to add parentheses:
oss << (a == 0 ? a : b);

Answer 2

It appears that the compiler is interpreting it different.
Based on the error, I would suspect that the compiler is interpreting the line of code like this
(oss << a) == 0 ? a : b;
So when you put in the parens there was no longer any way it could interpret it that way, and you got the expected results.
I suspect that this would also work.
oss << (a == 0) ? a : b;