Let's say a is a pointer, and after allocating memory for it, I want to check if the memory was allocated successfully, I've seen two ways doing this :
if(a != NULL)
if(a)
What is the difference between the first and second statements ?
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4They'll end up as the same thing. – Steve Oct 19 '17 at 12:16
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NULL is a macro that is typically defined as 0, so first expression is same as (a != 0), second one also checks if a != 0, so its the same edit:typically – Paweł Dymowski Oct 19 '17 at 12:20
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Yes, if *`a`* is a pointer, it might be different if `a` is not a pointer depending on the definition of `NULL`. – Antti Haapala -- Слава Україні Oct 19 '17 at 12:21
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@PawełDymowski incorrect, NULL is a macro that is define to be the *implementation-defined null-pointer constant*. – Antti Haapala -- Слава Україні Oct 19 '17 at 12:21
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`if(a != NULL)` and `if(a)` is bool value. - `if(a != NULL)` in that case we directly sets condition to not null. - `if(a)` but in this case when a is not null if bool value will be true ans run the if statement and when a is null the bool value will be false and if will not run. SO both end up with same result. – Salman Oct 19 '17 at 12:25
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1They do the same thing. The only difference that I see is the readability. – Glenn Oct 19 '17 at 12:26
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How does this indicate memory allocation? – matt Oct 19 '17 at 12:30
4 Answers
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is the condition :
if(a != NULL)the same as the conditionif(a)?
They achieve the same purpose. The only real difference is in readability.
Their effect is the same, since they will result in the same thing.
NULL is a macro that is almost always 0, so:
if(a != NULL)
is equivalent to:
if(a != 0)
which is pretty similar to:
if(a)
since it will check if the expression a evaluates to true.
So, if a is a pointer, they will be the same. If it's not, then it will depend on how NULL is defined (which as I said is almost always 0).
gsamaras
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According to c faq :
if(p) is equivalent to if(p != 0)
and this is a comparison context, so the compiler can tell that the (implicit) 0 is actually a null pointer constant, and use the correct null pointer value. There is no trickery involved here; compilers do work this way, and generate identical code for both constructs. The internal representation of a null pointer does not matter.
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(a != NULL) return a boolean which is the same thing for (a). An if statement return false if the content equals 0 and true otherwise. NULL means 0 so if a == 0 the if return false. So it's exactly the same thing for the compiler.
Hope it helps you !
cydef
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There's no functional difference; they are equivalent and they both test for null pointer.
Any preference is purely subjective. Just use one you like or the what's followed by your fellow programmers in your project/organization.
P.P
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