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(2)), _elem1(Elem1(1)), _elem3(Elem3(3)){
        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(1), _elem4(Elem4(4)), b(3), 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 0;
}