如何检查初始化的结构是否为空？

Question

So I want to check if my struct is empty or not. I declare my struct as a variable but DO not allocate memory.

text* t;

Later on I want to check with an if statement whether my struct is empty or not. if (t!=NULL) does not seem to work as t has an address. Also doing something like if (t->head != NULL) gives me a segfault as I haven't allocated any memory for the struct.

I don't want to malloc as soon as I declare t . Is there a way to check if my struct is empty?

Thanks in advance!

Answer 1

Just make sure you initialise the pointer to NULL :

text* t = NULL;

then later you can malloc on demand, eg:

if (t == NULL)
{
    t = malloc(sizeof *t);
    // NB: check for error here - malloc can fail!
}
t->head = foo;

Answer 2

Simply initialize the pointer whan it is defined

text* t = NULL;

In this case you can check whether an object that should be pointed to by the pointer was allocated

if ( t != NULL ) { /* some action */ }

or you can write

if ( t && t->head ) { /* some action */ }

Take into account that if a pointer is defined outside any function that is if it has the static storage duration then it initialized implicitly by the compiler to a null pointer constant.

Answer 3

You do not declare the struct, but a pointer to the struct. You even define the pointer (bring it into life and reserve memory space for it).

To declare (and define) a variable of that struct (presumed it has been declared somewhere before with typedef):

text my_struct_var;

This will reserve the space at compile time. The struct is initialized with 0 by the run-time system before main() is called (this may also be done by the OS functions which load the program). This means all fields are set to 0.

If you stick to to pointer-approach, you first have to malloc() ("memory allocate") the required space for such an object (or an array of such objects) and assign it to the pointer. This allocation is called "dynamic", as the meory is allocated at run-time under program control. Similar, it can be free() d by the program code after which it must never be accessed again.

The allocated memory is not initialized, thus it will have random data. Therefore, you cannot test for anything inside.

A priciple problem with dynamic memory allocation is that it might fail (no momory available). Therefore, you always have to check the result of malloc() if a valid pointer has been returned:

text *my_text_struct_ptr = malloc(sizeof(text));
if ( my_text_struct_ptr == NULL ) {
    // no memory allocated treat correspondingly
    // most basic reaction: exit(1) the program immediately
}

That might be what you call "the struct is empty". However, it is the pointer to the struct which is invalid ("points to nowhere"). For some more advanced data structures such a null pointer sometimes is a synptom ofthe structure bein empty (eg a list).

Further readings: about null pointers, declaration vs. definition, pointers and structs.