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什么时候需要在类的构造函数中使用初始化列表

2024-07-09 12:38:48   222人阅读

1，如果基类没有default构造函数，则意味着其不能自己初始化。如果其被派生，派生类的构造函数要负责调用基类的构造函数，并传递给它需要的参数。下例中Base

2，如果类成员没有默认构造函数。下例中Elem4

2，如果类的成员变量中含有const成员变量，如果不使用列表，在构造函数中是不能对其赋值的，会导致编译失败。下例中b

3，如果类的成员变量中含有引用，引用必须被初始化。下例中c

4，需要提高效率的时候，如果不使用初始化列表，而放在构造函数体内赋值的方法，则变量先被默认构造函数初始化，然后又调用copy构造函数。

 1 /////////////////////////////////////////////////////////////////////////////// 2 // 3 //  FileName    :   constructor.h 4 //  Version     :   0.10    created    2013/11/09 00:00:00         5 //  Author      :   Jimmy Han 6 //  Comment     :   7 // 8 /////////////////////////////////////////////////////////////////////////////// 9 #include <iostream>10 using namespace std;11 12 class Elem1{13     public:14     Elem1(int x){15         cout << "Elem1() was called." << endl;16     }17     ~Elem1(){18         cout << "~Elem1() was called." << endl;19     }20 };21 22 class Elem2{23     public:24     Elem2(int x){25         cout << "Elem2() was called." << endl;26     }27     ~Elem2(){28         cout << "~Elem2() was called." << endl;29     }30 };31 32 class Elem3{33     public:34     Elem3(int x){35         cout << "Elem3() was called." << endl;36     }37     ~Elem3(){38         cout << "~Elem3() was called." << endl;39     }40 };41 42 class Elem4{43     public:44     Elem4(int x){45         cout << "Elem4() was called." << endl;46     }47     ~Elem4(){48         cout << "~Elem4() was called." << endl;49     }50 };51 52 53 class Base{54 public:55     Base(int):_elem2(Elem2(2)), _elem1(Elem1(1)), _elem3(Elem3(3)){56         cout << "Base() was called." << endl;57     }58     ~Base(){59         cout << "~Base() was called." << endl;60     }61 private:62     Elem1 _elem1;63     Elem2 _elem2;64     Elem3 _elem3;65 };66 67 class Derive : public Base{68 public:69     //if there is no default constructor for base class, it has to be called explicit in derive class70     //four scenarios to use initialization list must:71     //1. Base. base class don‘t have Base();72     //2. const int b. class member is const 73     //3. int& c. class member is reference74     //4. _elem4. class member dont‘ have default constructor.75     Derive():Base(1), _elem4(Elem4(4)), b(3), c(b){76         cout << "Derive() was called." << endl;77     }78 private:79     Elem4 _elem4;80     const int b;81     const int& c;82 83 };
 1 /////////////////////////////////////////////////////////////////////////////// 2 // 3 //  FileName    :   constructor_client.cc 4 //  Version     :   0.10 5 //  Author      :   Ryan Han 6 //  Date        :   2013/11/19 7 //  Comment     :   8 // Elem1() was called. 9 // Elem2() was called.10 // Elem3() was called.11 // Base() was called.12 // Elem4() was called.13 // Derive() was called.14 // ~Elem4() was called.15 // ~Base() was called.16 // ~Elem3() was called.17 // ~Elem2() was called.18 // ~Elem1() was called.19 //20 ///////////////////////////////////////////////////////////////////////////////21 #include <iostream>22 #include "constructor.h"23 using namespace std;24 25 int main(){26     Derive d1;27 28     return 0;29 }

 


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