Can a structure be assigned to another using assignment operator?

Answer 1

Yes, provided both structures are of the same type:

struct S {/*...*/};

S a, b;

// ...

a = b;

The built-in structure assignment operator is typically implemented as a memory copy (memcpy) operation as this is the most efficient means of performing member-wise copy, as opposed to performing individual assignment operations upon each member.

Be aware that pointer members are shallow-copied not deep-copied:

struct S {

void* ptr;

size_t size;

};

S a;

a.size = 100 * sizeof(int); // allow for storage of 100 integers

a.ptr = malloc (a.size); // allocate memory

At this point, object 'a' owns the memory allocated to a.ptr.

S b;

b = a;

At this point, ownership of the memory is ambiguous because both a.ptr and b.ptr are referring to the same memory address. Shared memory is best avoided because releasing that memory will invalidate all objects that share the same memory:

free (a.ptr);

free (b.ptr); // Undefined behaviour: releasing the same memory twice!

To avoid this, we must manually perform a deep-copy:

S b;

b.size = a.size; // copy the size member

b.ptr = malloc (b.size); // allocate new memory

memcpy (b.ptr, a.ptr, a.size); // deep-copy

Note that the onus is entirely upon the programmer to ensure resource ownership is unambiguous. The compiler cannot help you.

In object oriented languages like C++, we use "smart" pointers (resource handles) rather than "naked" pointers. In this way, the onus of resource ownership shifts to the language itself.

Answer 2

Yes, but you need a copy constructor or an assignment operator method. And you can only do this in C++, not in C.