Re: Oops! Comma operator is the hardest to understand in the C++ standard!
On Jun 22, 2:00 am, "Victor Bazarov" <v.Abaza...@comAcast.net> wrote:
Lighter wrote:
-----------------------
#include <iostream>
int main()
{
int *pi=0;
int x = (*pi, 10);
std::cout << x << std::endl;
return 0;
}
I tried with VS 2005. It didn't crash. That means the compiler doesn't
perform lvalue-to-rvalue conversion.
No, it doesn't mean squat. Undefined behaviour is just that, undefined.
If the compiler [by not following the Standard] actually performs the
conversion, there is no way to tell by observing the undefined behaviour
of the program above.
And traditionally, dereferencing a null pointer is undefined
behavior, regardless of whether there is an lvalue to rvalue
conversion afterwards, or not. Something like &*p is undefined
behavior in p is a null pointer, according to the C++ standard.
About the only sure way I can think of is something like:
int volatile* p = someMemoryMappedIOAddress ;
*p, 10 ;
If there is an lvalue to rvalue conversion, the compiler will
(should) generate code to access the memory mapped IO, which is
observable behavior (in theory, anyway). Without the lvalue to
rvalue conversion, no access.
--
James Kanze (GABI Software, from CAI) email:james.kanze@gmail.com
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