The difference between null and 0 in C language
In C language, the difference between NULL and 0 is as follows: NULL is a null pointer constant, and 0 is an integer constant; the data type of NULL is void *, and the data type of 0 depends on the context; NULL and 0 cannot be directly compared for equality, and the type of 0 needs to be explicitly converted; NULL is used to represent a null pointer, and 0 is used to represent various values, such as zero value or null pointer; in old code, 0 may be used as a null pointer, but for For safety, it is recommended to use NULL to initialize pointer variables. 0 can only be used for integer variables.
The difference between NULL and 0 in C language
In C language,NULL Although 0 are constants, they represent different concepts:
1. Value and type
-
NULLis a special pointer constant, which represents a null pointer pointing to a non-existent memory address. -
0is an integer constant whose value is equal to zero.
2. The data type of data type
-
NULLisvoid *, which is Pointer to typevoid. The data type of -
0depends on its context, it can be an integer or float such asint,long,floatPoint type.
3. Compare
-
NULLand0cannot be directly compared for equality, because their Different types. - Requires explicit conversion of
0tovoid *type for comparison:NULL == (void *)0
4. Purpose
-
#NULLis usually used to initialize pointers, indicating that they do not point to any valid memory address. -
0is used to represent various values, such as:- Integer constant: represents a zero value.
- Pointer constant: represents a null pointer (not recommended).
- Boolean values:
0means false, non-zero means true (although there is no explicit boolean type in C).
5. Notes
- Before C99,
NULLwas defined as0. Therefore, in old code,0may be used as a null pointer constant. - For pointer variables, it is safer to initialize with
NULLbecause it clearly indicates that the pointer points to an invalid address, whereas0may be mistaken for a valid address. - For integer variables,
0is a valid zero value, whileNULLcan only be used for pointers.
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