C++/C 内存大小

#include <stdio.h>

struct test1{
char a1;
int a2;
double a3;
};

struct test2{
char a1;
struct test1 t;
};

int main(void)
{
printf("%d\n",sizeof(struct test1));
printf("%d\n",sizeof(struct test2));

return 0;
}

用gcc编译 为16 20

用VC编译 为16 24