实例演示C++对象构造

 前言：
        在c++类中，两个已存在的对象使用“=”赋值，这属于类的赋值，我们可以不用声明opreator =()，编译器会自动给类实现；
         使用一个对象来新建一个新的对象的构造函数，称为 复制构造函数/拷贝构造函数。

#include <iostream>
#include <string>
#include <vector>
using namespace std;

struct X
{   //simple test class
 int val;
 void out( const string& s,int nv )
     {
         cerr<<this<<"->"<<s<<":"<<val<<"( "<<nv<<")\n";   //注意，下面的构造函数都是先输出再给val赋值的,所以val是不确定的值

     }
 X(  )           //default constructed function
 {
     out( "X(  )" ,0);
     val=0;

 }
 X( int v )
 {
     out( "X( int )" ,v);
     val=v;

 }
 X( const X& x )       //a copy constructed funtion
 {
     out( "X( X& )",x.val );
     val=x.val;

 }
 X& operator =( const X& a )     //a evaluated  function
 {
     out( "X::operator=(  )",a.val);
     val=a.val;
     return *this;

 }
 ~X(  )                                 //deconstructed function
 {
     out( "~X(  )",0 );

 }

};

X glob( 2 );           //a global variable

X copy( X a )
{return a;}

X copy2( X a )
{X aa=a;return aa;}

X& ref_to( X& a )
{return a;}

X* make( int i )
{X a( i );return new X( a );}

struct XX
{
 X a;
 X b;
};

int main(  )
 {
     X loc( 4 );
     X loc2=loc;
     loc=X( 5 );
     loc2=copy( loc );
     loc2=copy2( loc );
     X loc3( 6 );
     X& r=ref_to( loc );
     delete make( 7 );
     delete make( 8 );
     vector<X> v( 4 );
     XX loc4;
     X *p=new X( 9 );
     delete p;
     X* pp=new X[ 5 ];
     delete [  ] pp;

     return 0;

 }