Re: Undefined behaviour with Non-static, non-polymorphic + null pointer?
Victor Bazarov wrote:
Alan Woodland wrote:
I'm fairly sure this is undefined behaviour, despite the fact that
it compiles and 'runs' (prints "this doesn't exist") on all my
platforms:
#include <iostream>
class foo {
public:
void bar() {
std::cout << "hello evil world!" << std::endl;
if (this) {
std::cout << "this exists" << std::endl;
}
else {
std::cout << "this doesn't exist!" << std::endl;
}
}
};
int main() {
foo *inst = 0;
inst->bar();
Here you're "using" the pointer that has an invalid value (does not
point to any object). That's undefined behavour. I could not quickly
locate the exact passage in the Standard that says that it is, but I
am sure you can find a mention of it in the archives, just search for
"dereference null pointer".
return 0;
}
Can someone please quote chapter and verse on this one and help me win
my current "this is a very bad idea" argument I'm having? I'd have
expected it to be forbidden under some general rule, and no exceptions
to have been made for it? Or is it actually really legal and defined
because nothing ever dereferences the this pointer?
The expression
inst->bar()
is in fact
(*inst).bar()
which already dereferences the null pointer 'inst'.
Thanks. It's funny, I'd never actually though about the implications of
that in this context before. Just found the following quote which ought
to convince certain people:
The Standard says that "p->" is
converted to "(*p)." (see section 5.2.5) and no matter how you slice it,
*p is a dereference. Dereferencing a null pointer results in undefined
behaviour.
Some compilers may ignore the conversion, but that's part of the
"undefined" part of the behaviour. You cannot rely on it happening on
all compilers - not even future releases of your current compiler.
Alan