【1】默认构造函数
关于默认构造函数，请参见随笔《类中函数》
请看测试代码：
1 #include <iostream>
2 using namespace std;
3
4 // 空类
5 class empty
6 {
7 };
8
9 // 一个默认构造函数，一个自定义构造函数
10 class Base
11 {
12 public:
13 Base()
14 {
15 cout << " default Base construct " << endl;
16 m_nValue = 100;
17 };
18 Base(int nValue)
19 {
20 cout << " custom Base construct " << endl;
21 m_nValue = nValue;
22 };
23
24 private:
25 int m_nValue;
26 };
27
28 // 一个默认复合构造函数
29 class custom
30 {
31 public:
32 custom(int value = 100)
33 {
34 cout << " default && custom construct " << endl;
35 m_nValue = value;
36 }
37
38 private:
39 int m_nValue;
40 };
41
42 void main()
43 {
44 empty* pEmpty1 = new empty;
45 empty* pEmpty2 = new empty();
46
47 Base* pBase1 = new Base;
48 Base* pBase2 = new Base();
49 Base* pBase3 = new Base(200);
50
51 custom* pCustom1 = new custom;
52 custom* pCustom2 = new custom();
53
54 delete pEmpty1;
55 delete pEmpty2;
56
57 delete pBase1;
58 delete pBase2;
59 delete pBase3;
60
61 delete pCustom1;
62 delete pCustom2;
63 }
64 // Result:
65 /*
66 default Base construct
67 default Base construct
68 custom Base construct
69 default && custom construct
70 default && custom construct
71 */
至此足以。
【2】加括号与不加的区别
（1）加括号
1. 若括号为空，即无实参项，那么理解为调用默认构造函数；
2. 若括号非空，即有实参项，可以理解为调用重载构造函数，或默认复合构造函数。
（2）不加括号
调用默认构造函数，或默认复合构造函数。