Lets say that you have a struct of

struct intvalue {
    int a;
    int b;
};

Just to say that since I am using int(eger) values so that is what I am incrementing by in the pointer arithmetic which is why I am casting the pointer to a integer value.

So lets say that we create a variable of intvalue and setup the values as below

intvalue testvalue;
testvalue.a = 4;
testvalue.b = 5;

We can then pull out the value of a or b, but using memcpy and just outputting to a int(eger) variable as below, the key is the ((int*)&testvalue)+1, this will first convert the testvalue variable to a pointer to memory location and then (int*) casts that pointer to a int pointer, because internally that is what it is, and then just add 1 to it, which points to the second value ( in this case the value of b which is 5)

    int avalue;
    // convert to a int pointer type and then add one to it (to the next array element as such).
    memcpy(&avalue, ((int*)&testvalue)+1,sizeof(int));
    cout << "a value (or is it the b value :) ) " << avalue << endl;

The output would be

a value (or is it the b value :) ) 5

because I am pointing to the second value which is b and thus 5 .

Of course if you just wanted the value of first int (the value of a in this case) you do not add the 1 to the memory location, for example

    memcpy(&avalue, ((int*)&testvalue),sizeof(int));

this time I am just converting the testvalue (casting) to a int pointer and thus pointing to the start of the struct and that is where the int a variable is living .

So the memcpy is just a binary data copy and nothing else, there is no type checking.

Here is a example in c++ code that will copy a newly created struct(ure) from one place to another.

void* memcpy (void *copied_to, const void *copier, size_t num);

where the return is also the copied_to value, since we are not altering the copier then a const(ant) can be applied and the last parameter is num which is the size of the data you want to copy.

#include <stdio.h>
#include <string.h>
 
// diferent type of object, e.g. not a int,char
typedef struct 
{
    int bob;
} insertType;
 
int main()
{
  // create two variables of insertType with default values for bob of 10 and 20
  insertType t1 = { 10 };
  insertType t2 = { 20 };
 
  // output the declared values
  printf("t1 = %d\nt2 = %d\n", t1.bob, t2.bob);
 
  // do a memory copy of t2 into t1 only copy as big as the variable size.
  memcpy(&t1, &t2, sizeof(insertType));
 
  // output the new values
  printf("t1 = %d\nt2 = %d\n", t1.bob, t2.bob);
  return 0;
}

here is the output

t1 = 10
t2 = 20
t1 = 20
t2 = 20

also note that the size_t is a unsigned integral type (normal of int, of the base operating system)