1. 14.4
Copy Constructors

2. Copy Constructors
Special constructor used when a newly created object is initialized to the data of another object of same class
Default copy constructor copies field-to-field
Default copy constructor works fine in many cases

3. Copy Constructors Problem: what if object contains a pointer?
class SomeClass
{ public:
SomeClass(int val = 0)
{value=new int; *value = val;}
int getVal();
void setVal(int);
private:
int *value; }

4. Copy Constructors
What we get using memberwise copy with objects containing dynamic memory:
SomeClass object1(5);
SomeClass object2 = object1;
object2.setVal(13);
cout << object1.getVal(); // also 13 13
object1
object2
value
value

5. Programmer-Defined Copy Constructor
Allows us to solve problem with objects containing pointers:
SomeClass::SomeClass(const SomeClass &obj) {
value = new int;
*value = obj.value; }
Copy constructor takes a reference parameter to an object of the class

6. Programmer-Defined Copy Constructor
Each object now points to separate dynamic memory:
SomeClass object1(5);
SomeClass object2 = object1;
object2.setVal(13);
cout << object1.getVal(); // still 5 5 13
object1
object2
value
value

7. Programmer-Defined Copy Constructor
Since copy constructor has a reference to the object it is copying from,
SomeClass::SomeClass(SomeClass &obj)
it can modify that object.
To prevent this from happening, make the object parameter const:
SomeClass::SomeClass
(const SomeClass &obj)

9. MyString Design MyString class: Represent a sequence of characters
the length of the string is important
data member:
s : pointer
len
methods:
constructor: int n or char *
length,
print,
destructor

10. Declare the Class my_string
class MyString{ public: MyString(int n); MyString(const char * str); ~ MyString(); int length() const ; //const member function void print() const; private: char * s; int len; };

11. Constructors
MyString :: MyString(int n) { s = new char[n+1]; s[0] = ‘\0'; len =n; } MyString :: MyString(const char * str) len = strlen(str); s = new char[len+1]; strcpy(s,str);

12. Overload Operator + (MyString) member function
Prototype:
MyString operator +(const MyString &b) const;
Definition:
MyString MyString ::operator+ (const MyString &b) const
{ cout << "member +" << endl;
char * temp = new char[this->len + b.len +1];
strcpy(temp, this->s);
strcat(temp, b.s);
MyString str(temp);
return str; }

13. Returning Local Object from a function
Returning an object invokes the copy constructor while returning a reference doesn't.
If a method or function returns a local object, it should return an object, not a reference.
It is an error to return a pointer to a local object. Once the function completes, the local object are freed. The pointer would be a dangling pointer that refers to a nonexistent object.