C ++结构中的奇怪行为

Question

I know that Union members share memory space, so I expect following code to output 9 & 9. However, I get 12 & 9. Why?

union Sample_union {
    int x;
    char array [9];
};
int main(){
    Sample_union sample;
    cout<<sizeof(sample)<<endl;
    char test [9];
    cout<<sizeof(test)<<endl;
}

I test following code in different compilers, as well.

Answer 1

This happens because of padding.

If the union contains an int the compiler in this case wants to be sure that in an array of unions all the int s will be aligned to a 4-byte boundary.

char s on the other hand don't have alignment requirements, so there's no point in padding an array of 9 chars.

For example with g++ if you add a double member the size of the union becomes 16 (because double alignment is 8 bytes).