c++ deep copy constructor [duplicate] - c++

Question

This question already has answers here:
Rule-of-Three becomes Rule-of-Five with C++11? [closed]
(9 answers)
Closed 5 years ago.
this is my code:
#include<iostream>
#include<cstring>
using namespace std;
class String
{
private:
char *s;
int size;
public:
String(const char *str = NULL); // constructor
String& operator=(String &c){
size = strlen(c.s);
s = new char[size+1];
strcpy(s, c.s);
}
~String() { delete [] s; }// destructor
void print() { cout << s << endl; }
void change(const char *); // Function to change
};
String::String(const char *str)
{
size = strlen(str);
s = new char[size+1];
strcpy(s, str);
}
void String::change(const char *str)
{
delete [] s;
size = strlen(str);
s = new char[size+1];
strcpy(s, str);
}
int main()
{
String str1("learnc++");
String str2 = str1;
str1.print(); // what is printed ?
str2.print();
str2.change("learnjava");
str1.print(); // what is printed now ?
str2.print();
return 0;
}
it can be compiled, and the result is:
learnc++
learnc++
learnjava
learnjava
in addition to that,there is:
*** Error in `./code': double free or corruption (fasttop): 0x0000000000f7f010 ***
BTY, if I delete "delete [] s;" in String::change,the result just becomes:
learnc++
learnc++
learnc++
learnjava
and no error appers, and why ?
the code is from geek foe feeks, I have changes some strings, and the code can be run in its IDE, but in my ubuntu 14.04, it cannot.

Answer 1

Your class is not following the Rule of Three because it is missing a proper copy constructor.
String str2 = str1; is just syntax sugar for String str2(str1);, so it uses the copy constructor, not your operator= (which has a memory leak, BTW).
Since you did not provide a copy constructor, the compiler provided one for you, but it does not make a deep copy of the char* data. It just copies the pointer itself, which causes the behaviors you are seeing.
A proper implementation would look more like this:
#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
class String
{
private:
char *s;
int size;
public:
String(const char *str = NULL);
String(const String &src);
~String();
String& operator=(const String &rhs);
void print() const;
};
String::String(const char *str)
{
size = strlen(str);
s = new char[size+1];
strcpy(s, str);
}
String::String(const String &src)
{
size = src.size;
s = new char[size+1];
strcpy(s, src.s);
}
String::~String()
{
delete [] s;
}
void String::print() const
{
cout << s << endl;
}
String& String::operator=(const String &rhs)
{
if (&rhs != this)
{
String tmp(rhs);
swap(s, tmp.s);
swap(size, tmp.size);
}
return *this;
}
int main()
{
String str1("learnc++");
String str2 = str1;
str1.print();
str2.print();
//str2.change("learnjava");
str2 = "learnjava";
str1.print();
str2.print();
return 0;
}
If you are using C++11 or later, you can use this implementation instead, which follows the Rule of Five by adding move semantics:
#include <iostream>
#include <cstring>
#include <utility>
using namespace std;
class String
{
private:
char *s;
int size;
public:
String(const char *str = nullptr);
String(const String &src);
String(String &&src);
~String();
String& operator=(String rhs);
void print() const;
};
String::String(const char *str)
{
size = strlen(str);
s = new char[size+1];
strcpy(s, str);
}
String::String(const String &src)
{
size = src.size;
s = new char[size+1];
strcpy(s, src.s);
}
String::String(String &&src)
{
size = src.size;
s = src.s;
src.s = nullptr;
src.size = 0;
}
String::~String()
{
delete [] s;
}
void String::print() const
{
cout << s << endl;
}
String& String::operator=(String rhs)
{
swap(s, rhs.s);
swap(size, rhs.size);
return *this;
}

Answer 2

Add a copy constructor.
String(const String& c){
size = strlen(c.s);
s = new char[size+1];
strcpy(s, c.s);
}