Cross-platform discussion of union and struct type sizes

This question of the textbook online a lot, I will not say more here to give a: Click to open the link

This question is the most likely to cause a dispute, why? With the platform or the compiler has a relationship, first look at the following code:

#include <iostream>using namespace std;union u1{    char a[9];    int b;}; Union u2{    U1 A;    Double b;}; Union u3{    Char a[10];    Double b;}; struct s1{    char a[13];    Double C;}; struct s2{    S1 A;    U1 b;}; struct s3{    char b;    int C;    Char d[13];}; int main () {    cout << "U1:" << sizeof (U1) << Endl;    cout << "U2:" << sizeof (U2) << Endl;    cout << "U3:" << sizeof (U3) << Endl;    cout << "S1:" << sizeof (S1) << Endl;    cout << "S2:" << sizeof (S2) << Endl;    cout << "S3:" << sizeof (S3) << Endl;    return 0;}

I tested it in two ways, with the following results:

(1) The results of the g++ compilation under Ubuntu system are as follows:

(2) Under Windows System, the results of CMake compilation are as follows:

Specifically not very clear, may be Ubuntu did some optimization, welcome to discuss

Cross-platform discussion of union and struct type sizes