C++ | class size

 1 #include <iostream>
 2 
 3 using namespace std;
 4 
 5 class A {};
 6 
 7 class B {};
 8 
 9 class C : public A {
10     virtual void func() = 0;
11 };
12 
13 class D : public B, public C {};
14 
15 class E {
16 public:
17     virtual void e() {};    
18 private:
19     char abc[3];
20 };
21 
22 class F : public E {
23 public:
24     virtual void f() {};
25 };
26 
27 class G : virtual public E {};
28 
29 class H {
30 public:
31     virtual void h() {};
32 private:
33     char abc[3];
34 };
35 
36 class I : public E, public H {};
37 
38 class J : virtual public E {};
39 
40 class K : public G, public J {};
41 
42 class L {
43     static int l;
44     static void la() { static int m; }
45 };
46 
47 int main() {
48     cout << "sizeof(A) = " << sizeof(A) << endl; // 空类，大小为1
49     cout << "sizeof(B) = " << sizeof(B) << endl; // 空类，大小为1
50     cout << "sizeof(C) = " << sizeof(C) << endl; // 虚函数表指针 （64位机器指针为8个字节）
51     cout << "sizeof(D) = " << sizeof(D) << endl; // 虚函数表指针
52     cout << "sizeof(E) = " << sizeof(E) << endl; // 字节对齐+虚函数表指针(8 +　8)
53     cout << "sizeof(F) = " << sizeof(F) << endl; // 同E
54     cout << "sizeof(G) = " << sizeof(G) << endl; // 虚继承，还包含一个父类的指针，所以是父类指针 + 虚函数表指针 + 字节对齐 = 24
55     cout << "sizeof(H) = " << sizeof(H) << endl; // 同E
56     cout << "sizeof(I) = " << sizeof(I) << endl; // 两个虚函数指针分别指向E和H + 两个数组字节对齐 = 32
57     cout << "sizeof(J) = " << sizeof(J) << endl; // 同G
58     cout << "sizeof(K) = " << sizeof(K) << endl; // 一个指向父类(E)的指针 + 两个虚函数指针分别指向G和J + 一个字节对齐 = 32
59     cout << "sizeof(L) = " << sizeof(L) << endl; // 静态变量不占空间
60     return 0;
61 }

 1 sizeof(A) = 1
 2 sizeof(B) = 1
 3 sizeof(C) = 8
 4 sizeof(D) = 8
 5 sizeof(E) = 16
 6 sizeof(F) = 16
 7 sizeof(G) = 24
 8 sizeof(H) = 16
 9 sizeof(I) = 32
10 sizeof(J) = 24
11 sizeof(K) = 32
12 sizeof(L) = 1