new对象时,类名后加括号与不加括号的区别

【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:

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 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 */
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  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。

原文地址:https://www.cnblogs.com/fclbky/p/5441289.html