Null aug

Add a comment. Active Oldest Votes. Null Pointers The integer constant literal 0 has different meanings depending upon the context in which it's used. As such, even on this funny architecture, the following ways are still valid ways to check for a null pointer: if! However, you may see something similar to this code: if!

References See Question 5. Thanks for pointing to the FAQ list. However, see also c-faq. No, you won't compare ptr to all-bits-zero. This is not a memcmp , but this is a comparison using a builtin operator. Aswell as with the other two versions with NULL and 0. Those three do the same things. You are taking the builtin comparison operator as a thing that would compare bit-strings. But that's not what it is. It compares two values, which are abstract concepts.

You make a good point about the comparison operator. I brushed up on C Anyone doing this deserves to be shot. Show 18 more comments. NULL is a macro, defined in as a null pointer constant. A null character is a byte which has all its bits set to 0. Chris Tang 7 7 silver badges 14 14 bronze badges. You missed 0. All three define the meaning of zero in different context. These three are always different when you look at the memory: NULL - 0x or 0x' 32 vs 64 bit NUL - 0x00 or 0x ascii vs 2byte unicode '0' - 0x20 I hope this clarifies it.

Nasko Nasko 1, 9 9 silver badges 7 7 bronze badges. I definitely could have made it explicit. Thanks — Nasko. Edit : Added more on the character literal. Eugene Yokota Eugene Yokota 92k 45 45 gold badges silver badges bronze badges. It's a character literal, which is an integer constant expression - so if it can be said to have a size then it's the the size of an int which has to be at least 2 bytes.

In C it is 4 on your computer just because of auto-cast which is int as default. Also size of 'a' is 4 out from a varable. Apologies to Ogden Nash, but the three "l" nulll means check your spelling. The special pointer value that means the pointer points nowhere is "NULL". The two terms are not interchangeable in meaning.

Intrastellar Explorer 1, 3 3 gold badges 20 20 silver badges 61 61 bronze badges. A one-L NUL, it ends a string. Kids Definition of null. Legal Definition of null. Get Word of the Day daily email! Test Your Vocabulary.

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Merriam-Webster's Words of the Week - Jan. Ask the Editors 'Everyday' vs. What Is 'Semantic Bleaching'? How 'literally' can mean "figuratively". Literally How to use a word that literally drives some pe Is Singular 'They' a Better Choice? The awkward case of 'his or her'.

New Year, Recondite Vocabulary Take the quiz. These are two semantically very different cases that must be handled differently.

Or else the outcome might be life-threatening. Just suppose that the patient has allergies, but an allergy test has not yet been done and the software tells the doctor that 'there are no allergies'. Hence we need additional information. We need to know why the function returns null. It would be tempting to say: Well, to differentiate, we return null if an allergy test has not yet been performed, and we return an empty list if there are no allergies. The different semantics for returning null versus returning an empty list would need to be well documented.

And as we all know, comments can be wrong i. There is no protection for misuses in client code that calls the function. For example, the following code is wrong, but it compiles without errors. Moreover, the error is difficult to spot for a human reader. And there is no protection against forgetting any one of these changes.

If, later on, there is another case to be distinguished e. There are different ways to do this, depending on the programming language we use. In order to differentiate the cases, we define a parent type AllergyTestResult , as well as three sub-types that represent the three cases NotDone , Pending , and Done :. Note: If you think that the above code is quite verbose and a bit hard to write, then you are not alone.

Some modern languages allow us to write conceptually similar code much more succinctly. And null-safe languages distinguish between nullable and non-nullable values in a reliable way at compile-time — there is no need to comment the nullability of a reference or to check whether a reference declared to be non-null has accidentally been set to null. If we need to know why there is no value associated with a reference, then additional data must be provided to differentiate the possible cases.

The first instruction declares a String variable s1 and assigns it the value "foo". The more interesting instruction is the last one. No value is explicitly assigned to s3. Hence, it is reasonable to ask: What is the state of s3 after its declaration? What will happen if we write s3 to the OS output device? It turns out that the state of a variable or class field declared without assigning a value depends on the programming language.

Moreover, each programming language might have specific rules for different cases. For example, different rules apply for reference types and value types, static and non-static members of a class, global and local variables, and so on.

The best option is the last one. As an example, Java applies the last option for local variables. Hence, the following code results in a compile-time error at the second line:. The basic rule is simple: null should only be allowed when it makes sense for an object reference to have 'no value associated with it'.

For example, suppose type person with fields name and dateOfFirstMarriage :. Every person has a name. Field name is non-nullable. It is illegal to assign null to it. On the other hand, field dateOfFirstMarriage doesn't represent a required value. Not everyone is married. Hence it makes sense for dateOfFirstMarriage to have 'no value associated with it'. Therefore dateOfFirstMarriage is a nullable field.

If a person's dateOfFirstMarriage field points to null then it simply means that this person has never been married. There is no way to reliably state that null can never be assigned to a given object reference. In some languages it is possible to use annotations, such as the non-standard annotations Nullable and NonNullable in Java.

Here is an example:. However, such annotations are not used by the compiler to ensure null-safety. Still, they are useful for the human reader, and they can be used by IDEs and tools such as static code analyzers.

Consider a function that reads configuration data from a file. Many languages C , Java, etc. Instead, the function's signature should be changed in order to make it clear that the function might fail:. Allow null only if it makes sense for an object reference to have 'no value associated with it'.

At run-time, the above code results in the infamous null pointer error , because we try to execute a method of a reference that points to null. It is the most frequent bug in many software applications, and has been the cause for countless troubles in the history of software development.

Tony Hoare, the inventor of null , calls it the 'billion-dollar mistake'. But Tony Hoare Turing Award winner in and inventor of the Quicksort algorithm , also gives a hint to a solution in his speech :.

Contrary to some common belief, the culprit is not null per se. The problem is the lack of support for null handling in many programming languages. For example, at the time of writing May , none of the top ten languages in the Tiobe index natively differentiates between nullable and non-nullable types. Therefore, some new languages provide compile-time null-safety and specific syntax for conveniently handling null in source code.

In these languages, the above code would result in a compile-time error. Software quality and reliability increases considerably, because the null pointer error delightfully disappears. Hence, null pointer errors cannot occur.