为结构指针分配内存

Question

Why is it that when we declare a structure pointer we need to allocate memory

struct A{
///
};
int main(void)
{
    struct A *var=(struct A*)malloc(sizeof(struct A));
//
//
}

but when we declare a structure variable we do not need to allocate any memory?

struct A var;

Answer 1

This is true for any pointers, not just pointers to structures. The reason being, when you declare a variable (of type int, char or the type of some struct A), you tell the compiler to create a new variable/instance. So, the compiler automatically allocates memory for that variable. But when you are declaring a pointer to some int or some struct A, you are essentially telling the compiler that you need a reference to some variable, not an a new variable of that type entirely. To illustrate this:

struct A{};
int a,b; // New variable a and b
struct A c,d; // New variables c,d of type struct A

// Now take a look at this:

int *px;
px = &a; // px referencing to a, no new int variable created;
px = &b; // px referencing to b, no new int variable created;

struct A* py;
py = &c; // py referencing to c, no new struct A variable created;
py = &d; // py referencing to d, no new struct A variable created;

Now, if you just declare a pointer A* p, here p is not referencing to anything. So, if you want p to refer to a new instance of struct A, you have to write explicitly:

c
p = (struct A*)malloc(sizeof(struct A));

Answer 2

• struct A var declares a variable var on stack area of main memory with internal structure as declared in the struct A .

• struct A * var also declares a variable var on stack area of main memory but the var is a pointer now, and as you may know, a pointer in C is used to store address of a variable, so the var need to know what is the address, the statment (struct A*)malloc(sizeof(struct A)); gives you the address.

Answer 3

You don't necessarily have to allocate memory when declaring a pointer (it can point to any existing object of the same type), but in order to use a pointer without causing undefined behavior, it has to point to an object of that type. Since pointers are used extensively in data structures, it is more often than not that allocated objects are the ones most commonly pointed to, which is why it seems like you have to allocate memory.