Why can't structs be compared for NULLness in C?

user_9914653 0
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I have the following struct:

typedef struct School
{
    int numOfStudents;
} School;

For example if I have a struct:

School s;

I want to check whether the struct is null like this:

if(s) {
    printf("null");
}

This will not compile and error message is as follows:

error: used struct type value where scalar is required

Why can't I check structs for NULLness inside if statement in C?

struct null c
7 Answers

In the case of:

School s;

s is definitely not NULL, because it is not a pointer. It is however, an uninitialized instance of a struct, and the value of s.numOfStudents is undefined.

user_1044580

NULL applies to pointer types, and s isn't a pointer, it's an instance of the School type.

If you declared s as a pointer to School, such as

School *s;

then the null check would make sense:

if ( s )
{
  // s points to a valid instance of School
}
user_1345540

Because null applies to pointers and s is not a pointer.

BTW, s is an automatic object that lives on the stack and cannot be null.

user_14170630

NULL is a special pointer value that is defined to not point anywhere. You can't check if s is NULL because s is not a pointer.

If your intent is to see if s has been initialized, you can't do that either because there's no special value that tells you that a variable has been initialized. For example a uninitialized int could contain 0, 1, 34634, -265356657, or any other value.

Your best bet is to initialize the variable with a known value and check for that.

School s = { -1 };

if (s.numOfStudents == -1) {
    printf("not used yet\n");
}
user_16871190

A struct consists of data elements. Each element can be null (if appropriate for the type of element), but the struct itself cannot be [compared to] null, even if it has only one data element.

user_31003120

NULL is used only by pointers. For example something like school *s == NULL would be valid since s is a pointer to type school. However in your case s is not a pointer therefore it cannot be compared to NULL.

Admin1

People perform NULL checks on pointers to determine whether or not the pointer is valid. We would need to call a function to determine whether or not a struct was valid, and the answer would depend on knowledge of the exact contents and their meaning. For example...

typedef struct
{
    int numOfStudents;
} School;

int null_check(School* item)
{
    /* This test assumes that a School is invalid if it has no students.*/
    return (!item || !item->numOfStudents);
}

/*...*/
School s;
/*...*/
if(null_check(&s)) {
    printf("null");
}

Note that while a pointer is either pointing at something or not, the question of whether a data structure is valid is potentially much more complex, and should not be confused with the question of whether the structure has been initialized or has every data member set to zero.

user_98294030
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