Why depends the result on order of declaration of class, or struct members?

Question

struct test
{
char a; int c; char b;
};
struct test1
{
char a;char b; int c;
};
class test2
{
char a; int c; char b;
};

class test3
{
char a; char b; int c;
};

int main()
{
std::cout << sizeof(test) << std::endl;
std::cout << sizeof(test1) << std::endl;
std::cout << sizeof(test2) << std::endl;
std::cout << sizeof(test3) << std::endl;
std::cin.get();
return 0;
}

Answer 1

Thanks @C++ Soldier (Babak) .

Answer 2

This SO Wiki addressed the question

"Why does the sizeof operator return a size larger for a structure than the total sizes of the structure's members?"

Like so

"This is because of padding added to satisfy alignment constraints. [...] ¹"

Note: See the example there.

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¹ https://stackoverflow.com/questions/119123/why-isnt-sizeof-for-a-struct-equal-to-the-sum-of-sizeof-of-each-member