C/C++ Pointer to a POD struct also points to the 1st struct member

Question

Can I assume that a C/C++ struct pointer will always point to the first member?
Example 1:

typedef struct {
 unsigned char  array_a[2];
 unsigned char  array_b[5];
}test;
//..
test var;
//..

In the above example will &var always point to array_a? Also in the above example is it possible to cast the pointer to an unsigned char pointer and access each byte separately?
Example 2:

function((unsigned char *)&var,sizeof(test));
//...
//...
void function(unsigned char *array, int len){
 int i;
 for( i=0; i<len; i++){
    array[i]++;
 }
}

Will that work correctly?

Note: I know that chars are byte aligned in a struct therefore I assume the size of the above struct is 7 bytes.

Answer 1

For C structs, yes, you can rely on it. This is how almost all "object orientated"-style APIs work in C (such as GObject and GTK).

For C++, you can rely on it only for "plain old data" (POD) types, which are guaranteed to be laid out in memory the same way as C structs. Exactly what constitutes a POD type is a little complicated and has changed between C++03 and C++11, but the crux of it is that if your type has any virtual functions then it's not a POD.

(In C++11 you can use std::is_pod to test at compile-time whether a struct is a POD type.)

EDIT: This tells you what constitutes a POD type in C++: http://en.cppreference.com/w/cpp/concept/PODType

EDIT2: Actually, in C++11, it doesn't need to be a POD, just "standard layout", which is a lightly weaker condition. Quoth section 9.2 [class.mem] paragraph 20 of the standard:

A pointer to a standard-layout struct object, suitably converted using a reinterpret_cast, points to its initial member (or if that member is a bit-field, then to the unit in which it resides) and vice versa. [ Note: There might therefore be unnamed padding within a standard-layout struct object, but not at its beginning, as necessary to achieve appropriate alignment. — end note ]