成员初始化表的作用实例解析

来源：互联网发布：mac make alias 编辑：程序博客网时间：2024/06/03 19:31

在类的初始化过程中，成员初始化表是一种更高效的方式，从下面例子可以看出以下两种初始化方法的效率区别

第一种：

BigClass(const MemClass& a):_a(a){}

第二种：

BigClass(const MemClass& a)
  {
   _a = a;
  }

对于第一种成员类不调用默认构造函数，直接调用复制构造函数

对于第二种成员类首先调用默认构造函数，然后调用=操作符重载来完成复制

其区别类似于

MemClass a（MemClass（b））；

和

MemClass a；

MemClass b；

a = b；

以下是完整代码

#include <iostream>

using namespace std;

class MemClass
{
private:
  int num;
public:
  MemClass()
  {
   cout<<"default Constructer of MemClass is called,Addr:"<<this<<"   size:"<<sizeof(*this)<<endl;
  }
  MemClass(int n)
  {
   cout<<"Constructer of MemClass is called ADDR:"<<this<<" size:"<<sizeof(*this)<<endl;
   num = n;
  }
  MemClass(const MemClass &a)
  {
   cout<<"copy construct of MemClass is called Addr:"<<this<<"   size:"<<sizeof(*this)<<endl;
   this->num = a.num;
  }
  MemClass& operator= (const MemClass& roe)
  {
   cout<<"operator= of MemClass is called Addr:"<<this<<"   size:"<<sizeof(*this)<<endl;
   if (this == &roe) return *this;
   this->num = roe.num;
   return *this;
  }
  ~MemClass()
  {
   cout<<"MemClass is destructed Addr:"<<this<<"   size:"<<sizeof(*this)<<endl;
  }
};

class BigClass
{
private:
  MemClass _a;
public:
  BigClass()
  {
   cout<<"default Constructor of BigClass Addr:"<<this<<" size:"<<sizeof(*this)<<endl;
  }
#ifdef CASE1
  BigClass(const MemClass& a):_a(a)
  {
   cout<<"Constructor of BigClass Addr:"<<this<<" size:"<<sizeof(*this)<<endl;
  }
#else
  BigClass(const MemClass& a)
  {

    cout<<"Constructor of BigClass Addr:"<<this<<" size:"<<sizeof(*this)<<endl;
   _a = a;
  }
#endif
  ~BigClass(){cout<< "BigClass is destructed Addr:"<<this<<"   size:"<<sizeof(*this)<<endl;}
};

int main()
{
BigClass big2(MemClass(1));
return 0;
}

以下是程序结果