Predict the output of the following program?
1 #include<iostream>
2 using namespace std;
3
4 class Empty
5 {
6 };
7
8 int main()
9 {
10 cout << sizeof(Empty) << endl;
11 return 0;
12 }
Output： 1
Size of an empty class is not zero. It is 1 byte generally. It is non-zero to ensure that the two different objects will have different addresses.
See the following example.
1 #include<iostream>
2 using namespace std;
3
4 class Empty
5 {
6 };
7
8 int main()
9 {
10 Empty a, b;
11
12 if (&a == &b)
13 {
14 cout << "impossible " << endl;
15 }
16 else
17 {
18 cout << "Fine " << endl;
19 }
20
21 return 0;
22 }
Output: Fine
For the same reason (different objects should have different addresses), “new” always returns pointers to distinct objects.
See the following example.
1 #include<iostream>
2 using namespace std;
3
4 class Empty
5 {
6 };
7
8 int main()
9 {
10 Empty* p1 = new Empty;
11 Empty* p2 = new Empty;
12
13 if (p1 == p2)
14 {
15 cout << "impossible " << endl;
16 }
17 else
18 {
19 cout << "Fine " << endl;
20 }
21 return 0;
22 }
Output： Fine
Now guess the output of following program (This is tricky).
1 #include<iostream>
2 using namespace std;
3
4 class Empty
5 {
6 };
7
8 class Derived: Empty
9 {
10 int a;
11 };
12
13 int main()
14 {
15 cout << sizeof(Derived);
16 return 0;
17 }
Output (with GCC compiler): 4
Note that the output is not greater than 4. There is an interesting rule that says that an empty base class need not be represented by a separate byte. So compilers are free to make optimization in case of empty base classes.
As an excercise, try the following program on your compiler.
1 #include <iostream>
2 using namespace std;
3
4 class Empty
5 {
6 };
7
8 class Derived1 : public Empty
9 {
10 };
11
12 class Derived2 : virtual public Empty
13 {
14 };
15
16 class Derived3 : public Empty
17 {
18 char c;
19 };
20
21 class Derived4 : virtual public Empty
22 {
23 char c;
24 };
25
26 class Dummy
27 {
28 char c;
29 };
30
31 int main()
32 {
33 cout << "sizeof(Empty) " << sizeof(Empty) << endl;
34 cout << "sizeof(Derived1) " << sizeof(Derived1) << endl;
35 cout << "sizeof(Derived2) " << sizeof(Derived2) << endl;
36 cout << "sizeof(Derived3) " << sizeof(Derived3) << endl;
37 cout << "sizeof(Derived4) " << sizeof(Derived4) << endl;
38 cout << "sizeof(Dummy) " << sizeof(Dummy) << endl;
39
40 return 0;
41 }
运行结果如下所示。
补充：
运行如下代码，运行结果为： 1
1 #include <cstdio>
2 #include <iostream>
3 using namespace std;
4 class Test
5 {
6 int arr[0];
7 };
8
9 int main()
10 {
11 Test t;
12 cout<<sizeof(t);
13 return 0;
14 }
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2013-11-25 20:39:10