How to use NULL in a .cpp file?

Question

I'm studying the only instance now(in c++), and I tried the following code in my .cpp file.

#include "OnlyInstance.h"

OnlyInstance* OnlyInstance::instance = NULL;
..........

but the compiler told me "error C2065: 'NULL' : undeclared identifier". So is that means, I should

#include <stdio.h>

in front of it? But I also included stdio.h in the main file. So stdio.h will be included many times when compiling, right? How can I use NULL correctly?

Another question is that, I know I can use 0 instead of NULL, but which way is more recommended in c++?

As an aside, as you are initializing a static pointer with NULL, you could also simply omit the initializer. — Deduplicator
– Deduplicator, Commented Feb 10, 2015 at 21:45

Baum mit Augen · Accepted Answer · 2015-02-10 22:00:24Z

11

NULL is defined in various headers, for example in <cstddef>.

You should however prefer nullptr over NULL if your compiler supports this.

Including a header multiple times is no problem (if the header is not horribly broken, which the standard headers are not).

edited Feb 10, 2015 at 22:00

answered Feb 10, 2015 at 21:40

Baum mit Augen♦

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2 Comments

Deduplicator Over a year ago

which it should for C++11 and later.

Cong Wang Over a year ago

But if I do #include <stdio.h> in OnlyInstance.cpp and main.cpp at the same time, then #include 'OnlyInstance.h' in main.cpp. What will the compiler do? stdio.h will be included twice? Thank you!

Rosme · Accepted Answer · 2015-02-10 21:41:13Z

5

NULL is only a define with a value of 0. It is exactly the same.

But if you have access to C++11, I would recommend using nullptr.

answered Feb 10, 2015 at 21:41

Rosme

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4 Comments

Deduplicator Over a year ago

Actually, it can be something else. Which is one of the reasons to prefer nullptr.

Mats Petersson Over a year ago

Technically, it is a 0 of the same size as a pointer, so it may be 0 or 0L or 0LL depending on what integer type matches a pointer. Otherwise, things like printf("%p", NULL); will go wrong.

Deduplicator Over a year ago

@MatsPetersson: That would be nice, sometimes, for hiding errors. It isn't guaranteed though.

Mats Petersson Over a year ago

Ok, it may not apply to ALL compilers, but it certainly worked better in my experience than the printf("%p", 0); that some test-code I had to deal with when I implemented printf - it took a while to identify the problem, as it was hidden a little better than the code in this comment...

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