C++中struct 和 class的sizeof大小

//============================================================================
// Name        : Sizeof.cpp
// Author      : gwwu
// Version     :
// Copyright   : Your copyright notice
// Description : Hello World in C++, Ansi-style
//============================================================================

#include <iostream>
using namespace std;

/* 一个struct的大小是它的所有成员大小的和，
 * 一般会为了提高执行效率，增加额外的字节对齐*/
struct A {
    int i[100];
};

/* 对象的基本规则之一是每个对象必须有一个唯一的地址，
 * 因此，无数据成员的结构总应当有最小的非零长度*/
struct B {
    void f();
};

void B::f() {}

struct C {
    //int a;
    short b;
    void c();
};
void C::c()
{
    cout << "I am c in C" << endl;
}

class D {
    void d();
};

class E {
    int e;
    void efunc();
};

int main() {
    cout << "sizeof struct A = " << sizeof(A) << " bytes" << endl; // prints !!!Hello World!!!
    cout << "sizeof struct B = " << sizeof(B) << " bytes" << endl;
    cout << "sizeof struct C = " << sizeof(struct C) << " bytes" << endl;
    cout << "sizeof class D = " << sizeof(D) << " bytes" << endl;
    cout << "sizeof class E = " << sizeof(E) << " bytes" << endl;
    return 0;
}

编译运行：

sizeof struct A = 400 bytes
sizeof struct B = 1 bytes
sizeof struct C = 2 bytes
sizeof class D = 1 bytes
sizeof class E = 4 bytes