struct_layout_question.cpp

#include <iostream>
#include <cstdlib>
/**
 * low level programming question, exp. C/C++ programmer
 * should know: 
 * How are struct and ptr related? How is this
 * different if all the named variables in the struct
 * are homogeneous in type? How about when the struct
 * has many types?
 */

/**
 * Selected ref. material:
 * 
 * From C99 §6.7.2.1:
 * 12. Each non-bit-field member of a structure or union 
 * object is aligned in an implementation- defined manner 
 * appropriate to its type.
 * 
 * 13. Within a structure object, the non-bit-field 
 * members and the units in which bit-fields reside 
 * have addresses that increase in the order in which 
 * they are declared. A pointer to a structure object, 
 * suitably converted, points to its initial member 
 * (or if that member is a bit-field, then to the unit 
 * in which it resides), and vice versa. There may be 
 * unnamed padding within a structure object, but not 
 * at its beginning.
 * 
 * C++ Specifics 
 * ref: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf
 * 
 * One critical way structs deviate from classes in layout:
 * 9.2.15. The order of allocation of non-static data members with 
 * different access control is unspecified (11).
 * 
 * Other similarities with C structs:
 * 9.2.18. Two standard-layout struct (Clause 9) types are 
 * layout-compatible if they have the same number of non-static 
 * data members and corresponding non-static data members (in 
 * declaration order) have layout-compatible types (3.9).
 * 
 * 9.2.21. 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. 
 */

/** make example struct **/
struct foo
{
    int a;
    int b;
}F = { 2, 3 };

int
main( int argc, char **argv )
{
    /**
     * NOTE: you probably never want to do this, but
     * if you're doing low level work, it can be useful
     * at times.
     */
    const int * const ptr( reinterpret_cast< int* >( &F ) );
    std::cout << ptr[ 0 ] << " - " << ptr[ 1 ] << "\n";
    return( EXIT_SUCCESS );
}