Notes

Outline

Copy Constructors


	Shallow Copy:
		The data members of one object are copied into the data members of another object without taking any dynamic memory pointed to by those data members into consideration. (“memberwise copy”)
	Deep Copy:
		Any dynamic memory pointed to by the data members is duplicated and the contents of that memory is copied (via copy constructors and assignment operators -- when overloaded)

Copy Constructors


	In every class, the compiler automatically supplies both a copy constructor and an assignment operator if we don't explicitly provide them.
	Both of these member functions perform copy operations by performing a memberwise copy from one object to another.
	In situations where pointers are not members of a class, memberwise copy is an adequate operation for copying objects.
	However, it is not adequate when data members point to memory dynamically allocated within the class.

Copy Constructors


	Problems occur with shallow copying when we:
		initialize an object with the value of another object: name s1; name s2(s1);
		pass an object by value to a function or when we return by value:
		name function_proto (name)
		assign one object to another:
		s1 = s2;

Copy Constructors


	If name had a dynamically allocated array of characters (i.e., one of the data members is a pointer to a char),
		the following shallow copy is disastrous!

Copy Constructors


	To resolve the pass by value and the initialization issues, we must write a copy constructor whenever dynamic member is allocated on an object-by-object basis.
	They have the form:

	class_name(const class_name &class_object);

	Notice the name of the “function” is the same name as the class, and has no return type
	The argument’s data type is that of the class, passed as a constant reference (think about what would happen if this was passed by value?!)

Copy Constructors


	//name.h interface
	class name {
	public:
	name(char* = ""); //default constructor
	name(const name &); //copy constructor
	~name(); //destructor
	name &operator=(name &); //assignment op
	private:
	char* ptr; //pointer to name
	int length; //length of name including nul char
	};

	#include "name.h" //name.c implementation
	name::name(char* name_ptr) { //constructor
	length = strlen(name_ptr); //get name length
	ptr = new char[length+1]; //dynamically allocate
	strcpy(ptr, name_ptr); //copy name into new space
	}
	name::name(const name &obj) { //copy constructor
	length = obj.length; //get length
	ptr = new char[length+1]; //dynamically allocate
	strcpy(ptr, obj.ptr); //copy name into new space
	}

Copy Constructors


	Now, when we use the following constructors for initialization, the two objects no longer share memory but have their own allocated

Copy Constructors


	Copy constructors are also used whenever passing an object of a class by value: (get_name returns a ptr to a char for the current object)

	int main() {
	name smith("Sue Smith"); //constructor with arg used

	//call function by value & display from object returned
	cout <<function(smith).get_name() <<endl;
	return (0);
	}

	name function(name obj) {
	cout <<obj.get_name() <<endl;
	return (obj);
	}

Copy Constructors


	Using a copy constructor avoids objects “sharing” memory -- but causes this behavior

	This should convince us to avoid pass by value whenever possible -- when passing or returning objects of a class!

Copy Constructors


	Using the reference operator instead, we change the function to be: (the function call remains the same)

	name &function(name &obj) {
	cout <<obj.get_name() <<endl;
	return (obj);
	}