I am very new to C++ and I don't understand why the copy constructor was called in the following code:
#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;
}
And when it runs:
Normal constructor allocating ptr
Copy constructor allocating ptr.
Length of line : 10
Freeing memory!
Freeing memory!
Why does it construct an other Line object after the first one with the simple constructor?
Thanks!
because you are passing Line as value
void display(Line obj)
{
cout << "Length of line : " << obj.getLength() <<endl;
}
You should pass it as constant reference, which is faster (and display shoud be a constant method so constant objects can call it).
void display(const Line &obj) const
{
cout << "Length of line : " << obj.getLength() <<endl;
}
(provided that you change to int getLength( void ) const;)
And in your case, think of redefining assignment operator
Line &operator=(const Line &other);
or affecting a line in another will copy pointer data and will crash on your second object deletion since memory will be freed twice.
If you want to make sure that default copy constructor / assignment operator cannot be called, just declare them in your private part:
private:
Line &operator=(const Line &other);
Line(const Line &other);
code trying to use them won't compile.
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