Checking whether (this == nullptr) in a class method [duplicate]

Question

I would like to replace code like this:

if ((obj != nullptr) && obj->is_valid())

with

if (obj->is_valid())

where

class Obj { bool is_valid() { if (this == nullptr) return false; // some more logic ... } ... };

Obviously there are 2 conditions:

obj is always accessed via pointer
Obj::is_valid() is never virtual

This is based on the fact, that a non-virtual method accepts this as its 1-st argument, so that

obj->is_valid();

is rewritten as

Obj::is_valid(obj);

While this code does work as expected with gcc-5.4.0, my question is whether this is a legitimate C++ code, that will be interpreted / optimized correctly by other (older / newer) C++ compilers?

It's going to do what you expect it to do nearly everywhere (without any optimization at least), but there's no guarantee, as it is undefined behavior. — Blaze
– Blaze, Commented Oct 1, 2018 at 8:34
@Blaze It won't on newer versions of gcc with optimization enabled. That's why answers belong in the answer section, not in comments, so we can downvote misinformation like that. — Baum mit Augen
– Baum mit Augen ♦, Commented Oct 1, 2018 at 8:36
Thanks, I forgot about optimization. I'll add that to the comment. — Blaze
– Blaze, Commented Oct 1, 2018 at 8:39

YSC · Accepted Answer · 2018-10-01 13:51:58Z

7

Don't.

if (this == nullptr)

It would not provide any security as calling a member function (method) on a null pointer is undefined behavior in the first place. This is bringing a sensation of security without the security, which is worse than nothing.

edited Oct 1, 2018 at 13:51

answered Oct 1, 2018 at 8:32

YSC

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2 Comments

Scheff's Cat Over a year ago

I found this on cppreference: Null pointers: dereferencing a null pointer is undefined behavior.

Scheff's Cat Over a year ago

While reading (and upvoting) your answer, I was searching for something like an "authoritative" source... (and thought, it might be worth to provide the link to cppreference I found).

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