Why is it allowed to overwrite a const variable using a pointer to it using memcpy?

Question

Why is it allowed to change a const variable using a pointer to it with memcpy?

This code:

const int i=5;
int j = 0;
memcpy(&j, &i, sizeof(int));         


        

Answer 1

The reason why is because the C language allows any pointer type to be implicitly casted to/from the type void*. It is designed that way because void pointers are used for generic programming.

So a C compiler is not allowed to stop your code from compiling, even though the program invokes undefined behavior in this case. A good compiler will however give a warning as soon as you implicitly try to cast away a const qualifier.

C++ has "stronger typing" than C, meaning that it would require an explicit cast of the pointer type for this code to compile. This is one flaw of the C language that C++ actually fixed.