C++-什么时候需要在类的构造函数中使用初始化列表

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。

 ///////////////////////////////////////////////////////////////////////////////
 //
 //  FileName    :   constructor.h
 //  Version     :   0.10    created    2013/11/09 00:00:00
 //  Author      :   Jimmy Han
 //  Comment     :
 //
 ///////////////////////////////////////////////////////////////////////////////
 #include <iostream>
 using namespace std;

 class Elem1{
     public:
     Elem1(int x){
         cout << "Elem1() was called." << endl;
     }
     ~Elem1(){
         cout << "~Elem1() was called." << endl;
     }
 };

 class Elem2{
     public:
     Elem2(int x){
         cout << "Elem2() was called." << endl;
     }
     ~Elem2(){
         cout << "~Elem2() was called." << endl;
     }
 };

 class Elem3{
     public:
     Elem3(int x){
         cout << "Elem3() was called." << endl;
     }
     ~Elem3(){
         cout << "~Elem3() was called." << endl;
     }
 };

 class Elem4{
     public:
     Elem4(int x){
         cout << "Elem4() was called." << endl;
     }
     ~Elem4(){
         cout << "~Elem4() was called." << endl;
     }
 };

 class Base{
 public:
     Base(int):_elem2(Elem2()), _elem1(Elem1()), _elem3(Elem3()){
         cout << "Base() was called." << endl;
     }
     ~Base(){
         cout << "~Base() was called." << endl;
     }
 private:
     Elem1 _elem1;
     Elem2 _elem2;
     Elem3 _elem3;
 };

 class Derive : public Base{
 public:
     //if there is no default constructor for base class, it has to be called explicit in derive class
     //four scenarios to use initialization list must:
     //1. Base. base class don't have Base();
     //2. const int b. class member is const
     //3. int& c. class member is reference
     //4. _elem4. class member dont' have default constructor.
     Derive():Base(), _elem4(Elem4()), b(), c(b){
         cout << "Derive() was called." << endl;
     }
 private:
     Elem4 _elem4;
     const int b;
     const int& c;

 };

 ///////////////////////////////////////////////////////////////////////////////
 //
 //  FileName    :   constructor_client.cc
 //  Version     :   0.10
 //  Author      :   Ryan Han
 //  Date        :   2013/11/19
 //  Comment     :
 // Elem1() was called.
 // Elem2() was called.
 // Elem3() was called.
 // Base() was called.
 // Elem4() was called.
 // Derive() was called.
 // ~Elem4() was called.
 // ~Base() was called.
 // ~Elem3() was called.
 // ~Elem2() was called.
 // ~Elem1() was called.
 //
 ///////////////////////////////////////////////////////////////////////////////
 #include <iostream>
 #include "constructor.h"
 using namespace std;

 int main(){
     Derive d1;

     return ;
 }