默认复制构造函数的浅探

• 头文件stringbad1.h
  

  #include <iostream>
   #ifndef STRINGBAD_H_
   #define STRINGBAD_H_
   
   class StringBad{
   private:
       char * str;
       int len;
       static int num_strings;
   public:
       StringBad();
       StringBad(const char * s); 
       ~StringBad();
       //friend function
       friend std::ostream & operator<<(std::ostream & os, 
               const StringBad & s); 
   };
   
   #endif

• 类定义stringbad1.cc
  

   #include "stringbad1.h"
   #include <cstring> // for strlen and strcpy
   using std::cout;
   
   int StringBad::num_strings = 0;
   
   StringBad::StringBad()
   {
       len = 4;
       str = new char[len];
       std::strcpy(str, "C++");
       num_strings++;
       //for your information
       cout << num_strings << ": "" << str <<  
           """ << "default object created.
  ";
   }
   
   //note:this is an bad idea to change "s"
   StringBad::StringBad(const char * s)
   {
       len = std::strlen(s);
       str = new char[len + 1]; 
       std::strcpy(str, s); 
       num_strings++;
       //FYI
       cout << num_strings << ": "" << str <<
            """ << "object created.
  ";
   }
   
   StringBad::~StringBad()
   {
       cout << """ << str << """ << "deleted."
      cout << """ << str << """ << "deleted."
            << --num_strings << " left.
  ";
       delete [] str;
   }
   
   std::ostream & operator<<(std::ostream & os,
           const StringBad & st)
   {
       os << st.str;
       return os;
   }

• 函数usestring1.cc
  

   #include <iostream>
   #include "stringbad1.h"
   using std::cout;
   
   int main(void)
   {
       using std::endl;
       {   
           cout << "Starting an inner block.
  ";
           StringBad headline("The first string!");
           StringBad sports = "basketball";
           cout << "Sport: "    << sports   << endl;
           cout << "headline: " << headline << endl;
   
           cout << "Initialize one object to anther:
  ";
           StringBad sailor = sports;
           cout << "Sailor: "    << sailor    << endl;
           cout << "Sport2: "    << sports   << endl;
   
           cout << "Assign one object to anther: 
  ";
           StringBad knot;
           knot = headline;
           cout << "knot: "      << knot << endl;
           cout << "headline2: " << headline << endl;
   
           cout << "Exing the block.
  ";
       }   
       cout << "end of main().
  ";
   
       return 0;
   }
  /***************************************************
    * Starting an inner block.
    * 1: "The first string!"object created.
    * 2: "basketball"object created.
    * Sport: basketball
    * headline: The first string!
    * Initialize one object to anther:
    * Sailor: basketball
    * Sport2: basketball
    * Assign one object to anther: 
    * 3: "C++"default object created.
    * knot: The first string!
    * headline2: The first string!
    * Exing the block.
    * "The first string!"deleted.2 left.
    * "basketball"deleted.1 left.
    * " �8"deleted.0 left. 
    * * ***********************************************/

   通过程序输出。我们看到3: "C++"default object created. ，这一句其实时由于 StringBad knot; 而创建的。由此我们知道在声明对象时：调用默认构造函数。 这不是什么惊天动地的结论，但我们仔细想想，为什么会出现这句：" �8"deleted.0 left. 。这句是由于内存上的错误造成的，为什么会这样？看这句：knot = headline; 。这句将headline的值赋给knot，但knot本身的值是C++，也就是说knot是标识C++的唯一标记，但将headline赋值给knot时，knot也就必须要放弃对C++的标识而接受headline的内容。所以本来“C++”所占用的内存就无法被正确用析构函数释放。