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How to implement a copy constructors in C++?
Here we will see how the copy constructors are implemented in C++. Before discussing that we should know what is the copy constructor.
The copy constructor is a constructor which creates an object by initializing it with an object of the same class, which has been created previously. The copy constructor is used to −
- Initialize one object from another of the same type.
- Copy an object to pass it as an argument to a function.
- Copy an object to return it from a function.
If a copy constructor is not defined in a class, the compiler itself defines one. If the class has pointer variables and has some dynamic memory allocations, then it is a must to have a copy constructor. The most common form of copy constructor is shown here −
classname (const classname &obj) { // body of constructor }
Here, obj is a reference to an object that is being used to initialize another object.
Example
#include <iostream> using namespace std; class Line { public: int getLength( void ); Line( int len ); // simple constructor Line( const Line &obj); // copy constructor ~Line(); // destructor private: int *ptr; }; // Member functions definitions including constructor Line::Line(int len) { cout << "Normal constructor allocating ptr" << endl; // allocate memory for the pointer; ptr = new int; *ptr = len; } Line::Line(const Line &obj) { cout << "Copy constructor allocating ptr." << endl; ptr = new int; *ptr = *obj.ptr; // copy the value } Line::~Line(void) { cout << "Freeing memory!" << endl; delete ptr; } int Line::getLength( void ) { return *ptr; } void display(Line obj) { cout << "Length of line : " << obj.getLength() <<endl; } // Main function for the program int main() { Line line(10); display(line); return 0; }
Output
Normal constructor allocating ptr Copy constructor allocating ptr. Length of line : 10 Freeing memory! Freeing memory!
Let us see the same example but with a small change to create another object using existing object of the same type −
Example
#include <iostream> using namespace std; class Line { public: int getLength( void ); Line( int len ); // simple constructor Line( const Line &obj); // copy constructor ~Line(); // destructor private: int *ptr; }; // Member functions definitions including constructor Line::Line(int len) { cout << "Normal constructor allocating ptr" << endl; // allocate memory for the pointer; ptr = new int; *ptr = len; } Line::Line(const Line &obj) { cout << "Copy constructor allocating ptr." << endl; ptr = new int; *ptr = *obj.ptr; // copy the value } Line::~Line(void) { cout << "Freeing memory!" << endl; delete ptr; } int Line::getLength( void ) { return *ptr; } void display(Line obj) { cout << "Length of line : " << obj.getLength() <<endl; } // Main function for the program int main() { Line line1(10); Line line2 = line1; // This also calls copy constructor display(line1); display(line2); return 0; }
Output
Normal constructor allocating ptr Copy constructor allocating ptr. Copy constructor allocating ptr. Length of line : 10 Freeing memory! Copy constructor allocating ptr. Length of line : 10 Freeing memory! Freeing memory! Freeing memory!