Was reviewing some code in a project, and found a file where a developer had inserted the following in every single method:
class EventMapper {
public:
/*
* constructor
*/
EventMapper() {
if( !this ) return;
/* --SNIP--*/
}
/*
* destructor
*/
~EventMapper() {
if( !this ) return;
/* --SNIP-- */
}
/*
* initialises the event mapper
*/
void initialise() {
if( !this ) return;
/* --SNIP-- */
}This is an awesome way to mask a null-pointer dereference bug and make it even more of a pain to find!
I'm also wondering how often you will get a null this pointer in practice.
It seems quite likely that you would instead receive an arbitrary (but still incorrect) pointer value...
I'm also wondering how often you will get a null this pointer in practice.
It seems quite likely that you would instead receive an arbitrary (but still incorrect) pointer value...
if ((unsigned)(this) < 0x1000) return;I'm also wondering how often you will get a null this pointer in practice.
It seems quite likely that you would instead receive an arbitrary (but still incorrect) pointer value...
You should never get a null or incorrect "this" pointer. The fact that you have access to "this" already means the object exists in memory and is valid.
#include <iostream>
class Test {
public:
void printthis();
};
void Test::printthis() {
std::cout << this << std::endl;
}
void lalala(Test* p) {
p->printthis();
}
int main() {
lalala(0);
return 0;
}
If that guy also got such wrong code in his project... 
I think I wrote that exact same hack back in the 90's when I was first learning C++.
Somewhere I had a NULL pointer and was calling functions on it, which means you end up inside that function with a NULL this value.
I diagnosed this symptom (this is null), and "fixed" it by inserting the if statement from the OP... nevermind the actual cause of the bug... 
You should never get a null or incorrect "this" pointer. The fact that you have access to "this" already means the object exists in memory and is valid.
Unfortunately, not true. Try the following program:
#include <iostream>
struct X
{
void print() {
std::cout << "pointer: " << (void*)this << std::endl;
}
};
int main() {
X *x = NULL;
x->print();
X *y;
y->print();
}
On my machine, that produces the following:
$ clang++ null.c++
$ ./a.out
pointer: 0
pointer: 0x7fff5d4e5b68
Which goes back to my argument that the common case is not in fact NULL, but an arbitrary pointer value, because in my mind it is much more likely to accidentally call a function on an undefined pointer than a pointer which I have explicitly initialised to NULL (unless of course your dev platform always nulls pointers by default).
You should never get a null or incorrect "this" pointer. The fact that you have access to "this" already means the object exists in memory and is valid.
struct Foo
{
int Member;
void DoStuff()
{
std::cout << this << std::endl;
}
};
struct Bar
{
int padding;
int morepadding;
Foo data;
};
int main()
{
Bar* theobject = NULL;
theobject->data.DoStuff();
}
