c++ - Can I use NULL as substitution for the value of 0?

Question

Am I allowed to use the NULL pointer as replacement for the value of 0?

Or is there anything wrong about that doing?

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Like, for example:

int i = NULL;

as replacement for:

int i = 0;

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As experiment I compiled the following code:

#include <stdio.h>

int main(void)
{
    int i = NULL;
    printf("%d",i);

    return 0;
}

Output:

0

Indeed it gives me this warning, which is completely correct on its own:

warning: initialization makes integer from pointer without a cast [-Wint-conversion] 

but the result is still equivalent.

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• Am I crossing into "Undefined Behavior" with this?
• Is it permissible to utilize NULL in this way?
• Is there anything wrong with using NULL as a numerical value in arithmetical expressions?
• And what is the result and behavior in C++ for this case?

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I have read the answers of What is the difference between NULL, '' and 0 about what the difference between NULL, and 0 is, but I did not get the concise information from there, if it is quite permissible and also right to use NULL as value to operate with in assignments and other arithmetical operations.

Answer 1

Am I allowed to use the NULL pointer as replacement for the value of 0?

No, it is not safe to do so. NULL is a null-pointer constant, which could have type int, but which more typically has type void * (in C), or otherwise is not directly assignable to an int (in C++ >= 11). Both languages allow pointers to be converted to integers, but they do not provide for such conversions to be performed implicitly (though some compilers provide that as an extension). Moreover, although it is common for converting a null pointer to an integer to yield the value 0, the standard does not guarantee that. If you want a constant with type int and value 0 then spell it 0.

• Am I might crossing into Undefined Behavior with this?

Yes, on any implementation where NULL expands to a value with type void * or any other not directly assignable to int. The standard does not define the behavior of your assignment on such an implementation, ergo its behavior is undefined.

• is it permissible to operate with the NULL in that way?

It is poor style, and it will break on some systems and under some circumstances. Inasmuch as you appear to be using GCC, it would break in your own example if you compiled with the -Werror option.

• Is there anything wrong about to use NULL as numerical value in arithmetical expressions?

Yes. It is not guaranteed to have a numerical value at all. If you mean 0 then write 0, which is not only well defined, but shorter and clearer.

• And how is the result in C++ to that case?

The C++ language is stricter about conversions than is C and has different rules for NULL, but there, too, implementations may provide extensions. Again, if you mean 0 then that's what you should write.