Can you use strcmp() to check for nullptr value?

Question

Is this piece of code any good to check if a pointer or variable hold a nullptr value?

if(strcmp(variable/pointer, nullptr) == 0){
    Do something cool
}

I'm trying to work on some code and I want to know the best way to check if there is a nullptr value present. Thanks.

@Mekanic I think [cppreference.com](http://en.cppreference.com/w/) is a good reference. For example, here's the entry for [`strcmp`](http://en.cppreference.com/w/cpp/string/byte/strcmp). Its two parameters must be pointers to null-terminated strings. — Blastfurnace, Jun 10 '17 at 05:49
So you considered it and didn't do it. Why not? And what does being a baby have to do with it? — user207421, Jun 10 '17 at 05:50
@Mekanic It's not personal. In general it's expected that people will check documentation and do some debugging before posting questions. You'll help yourself and ask better questions when you're really stuck. — Blastfurnace, Jun 10 '17 at 06:02
@Blastfurnace Ok well I respect that... Thanks for your help anyway. — Mekanic, Jun 10 '17 at 06:03

Curious · Answer 1 · 2017-06-10T05:44:13.653

1

I think what you want is

if (!pointer || !variable) { 
    // do something cool
}

!pointer is essentially the same thing as pointer == nullptr and is considered by some to be better style. See Can I use if (pointer) instead of if (pointer != NULL)?

strcmp is meant to be for C string comparisons. https://www.tutorialspoint.com/c_standard_library/c_function_strcmp.htm

edited Jun 10 '17 at 05:44

answered Jun 10 '17 at 05:39

Curious

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Does that check for the same thing as using strcmp()? – Mekanic Jun 10 '17 at 05:40
1

`strcmp` is meant for c-string comparisons, if you want to check if the pointers are pointing to null (or that the pointers are `nullptr`) then you want to do what I said – Curious Jun 10 '17 at 05:41
Ok, now does that deserve a downvote if your new to C++? – Mekanic Jun 10 '17 at 05:42
@Mekanic I did not downvote.. I don't downvote without giving sufficient explanation for the downvote – Curious Jun 10 '17 at 05:42

score 1 · Answer 2 · answered Jun 10 '17 at 05:53

You cannot use strcmp to check whether a variable is equal to nullptr. From strcmp documentation:

Parameters
lhs, rhs - pointers to the null-terminated byte strings to compare

Not only do the input arguments to strcmp need to point to byte strings, they must point to null-terminated byte strings.

Besides, you are making things may more complex than they need to be. You can just use:

if ( variable != nullptr )
{
    // Do something
}
else
{
    // Do something else
}

score 0 · Answer 3 · answered Jun 10 '17 at 07:05

strcmp() has undefined behaviour if either argument is a null pointer. Therefore it is necessary to check for null before calling strcmp().

For example, to only call strcmp() if both intended arguments are not null

 if (first && second && !strcmp(first, second))
 {
      // first and second are both non-null, and contain matching strings
 }

or (more verbosely, C++11 and later)

 if (first != nullptr && second != nullptr && strcmp(first, second) == 0)
 {
      // first and second are both non-null, and contain matching strings
 }

In C++, you'd be better off using the std::string type, rather than messing around with arrays of char, string termination, and functions like strcmp().

Can you use strcmp() to check for nullptr value?

3 Answers3