默认复制构造函数的浅探

头文件stringbad1.h

 1 #include <iostream>
 2  #ifndef STRINGBAD_H_
 3  #define STRINGBAD_H_
 4  
 5  class StringBad{
 6  private:
 7      char * str;
 8      int len;
 9      static int num_strings;
10  public:
11      StringBad();
12      StringBad(const char * s); 
13      ~StringBad();
14      //friend function
15      friend std::ostream & operator<<(std::ostream & os, 
16              const StringBad & s); 
17  };
18  
19  #endif

类定义stringbad1.cc

 1  #include "stringbad1.h"
 2  #include <cstring> // for strlen and strcpy
 3  using std::cout;
 4  
 5  int StringBad::num_strings = 0;
 6  
 7  StringBad::StringBad()
 8  {
 9      len = 4;
10      str = new char[len];
11      std::strcpy(str, "C++");
12      num_strings++;
13      //for your information
14      cout << num_strings << ": "" << str <<  
15          """ << "default object created.
";
16  }
17  
18  //note:this is an bad idea to change "s"
19  StringBad::StringBad(const char * s)
20  {
21      len = std::strlen(s);
22      str = new char[len + 1]; 
23      std::strcpy(str, s); 
24      num_strings++;
25      //FYI
26      cout << num_strings << ": "" << str <<
27           """ << "object created.
";
28  }
29  
30  StringBad::~StringBad()
31  {
32      cout << """ << str << """ << "deleted."
33     cout << """ << str << """ << "deleted."
34           << --num_strings << " left.
";
35      delete [] str;
36  }
37  
38  std::ostream & operator<<(std::ostream & os,
39          const StringBad & st)
40  {
41      os << st.str;
42      return os;
43  }

函数usestring1.cc

 1  #include <iostream>
 2  #include "stringbad1.h"
 3  using std::cout;
 4  
 5  int main(void)
 6  {
 7      using std::endl;
 8      {   
 9          cout << "Starting an inner block.
";
10          StringBad headline("The first string!");
11          StringBad sports = "basketball";
12          cout << "Sport: "    << sports   << endl;
13          cout << "headline: " << headline << endl;
14  
15          cout << "Initialize one object to anther:
";
16          StringBad sailor = sports;
17          cout << "Sailor: "    << sailor    << endl;
18          cout << "Sport2: "    << sports   << endl;
19  
20          cout << "Assign one object to anther: 
";
21          StringBad knot;
22          knot = headline;
23          cout << "knot: "      << knot << endl;
24          cout << "headline2: " << headline << endl;
25  
26          cout << "Exing the block.
";
27      }   
28      cout << "end of main().
";
29  
30      return 0;
31  }
32 /***************************************************
33   * Starting an inner block.
34   * 1: "The first string!"object created.
35   * 2: "basketball"object created.
36   * Sport: basketball
37   * headline: The first string!
38   * Initialize one object to anther:
39   * Sailor: basketball
40   * Sport2: basketball
41   * Assign one object to anther: 
42   * 3: "C++"default object created.
43   * knot: The first string!
44   * headline2: The first string!
45   * Exing the block.
46   * "The first string!"deleted.2 left.
47   * "basketball"deleted.1 left.
48   * " �8"deleted.0 left. 
49   * * ***********************************************/

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。