C++ write String constructors copy constructors destructors and assignment functions

C++ write String constructors, copy constructors, destructors, and assignment functions

Write the constructor, destructor and assignment functions of class String. The prototype of class String is known as:

class String
{
public:
String(const char *str = NULL); //  Ordinary constructor 
String(const String &other); //  Copy constructor 
~ String(void); //  The destructor 
String & operate =(const String &other); //  The mutator 
private:
char *m_data; //  To hold strings 
}; 

#include <iostream> 
class String 
{ 
public: 
  String(const char *str=NULL);// Ordinary constructor  
  String(const String &str);// Copy constructor  
  String & operator =(const String &str);// The mutator  
  ~String();// The destructor  
protected: 
private: 
  char* m_data;// To hold strings  
}; 
 
// Ordinary constructor  
String::String(const char *str)
{ 
  if (str==NULL)
  { 
    m_data=new char[1]; // Automatically request end marks for empty strings '\0' The space of  
    if (m_data==NULL)
    {// Whether the memory application is successful  
     std::cout<<" Application for memory failed! "<<std::endl; 
     exit(1); 
    } 
    m_data[0]='\0'; 
  } 
  else
  { 
    int length=strlen(str); 
    m_data=new char[length+1]; 
    if (m_data==NULL)
    {// Whether the memory application is successful  
      std::cout<<" Application for memory failed! "<<std::endl; 
      exit(1); 
    } 
    strcpy(m_data,str); 
  } 
} 

// Copy constructor  
String::String(const String &other)
{ // The input parameter is const type  
  int length=strlen(other.m_data); 
  m_data=new char[length+1]; 
  if (m_data==NULL)
  {// Whether the memory application is successful  
    std::cout<<" Application for memory failed! "<<std::endl; 
    exit(1); 
  } 
  strcpy(m_data,other.m_data); 
} 

// The mutator  
String& String::operator =(const String &other)
{// The input parameter is const type  
  if (this == &other) // Check self assignment  
  { return *this; }

  delete [] m_data;// Free up the original memory resources  

  int length=strlen(other.m_data);   
  m_data= new char[length+1]; 
  if (m_data==NULL)
  {// Whether the memory application is successful  
    std::cout<<" Application for memory failed! "<<std::endl; 
    exit(1); 
  } 
  strcpy(m_data,other.m_data); 

  return *this;// Returns a reference to this object  
} 

// The destructor  
String::~String()
{ 
  delete [] m_data; 
} 
 
void main()
{ 
  String a; 
  String b("abc"); 
  system("pause"); 
} 

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