Doubt regarding constructors

Code:

/*
Program to study constructors using inheritance...
*/
#include<iostream.h>
#include<conio.h>
class base1
{
	int x;
	public:
		base1(){}
		base1(int a)
		{
			x=a;
			cout<<"Constructing base1..."<<x<<endl; getch();
		}
		~base1(){cout<<"Destroying base1..."<<endl; getch();}
};
class derived1
{
	int y; 
	public:
		derived1(){}
		derived1(int b)
		{
			y=b;
			cout<<"Constructing derived1..."<<y<<endl; getch();
		}
		~derived1(){cout<<"Destroying derived1..."<<endl; getch();}
};
class derived2:public base1,public derived1
{
	protected:
	int z;
	public:
		derived2(){}
		derived2(int a,int b,int c):
		base1(a),
		derived1(b)
		{
			z=c;
			cout<<"Constructing derived2..."<<z<<endl; getch();
		}
	~derived2(){cout<<"Destroying derived2..."<<endl; getch();}
};
int main()
{
	clrscr();
	derived2 d1;
	d1=derived2(1,2,3);
	cout<<"Executing getch()..."<<endl;
	getch(); return(0);
}

Hello everyone. I had a small doubt regarding the above program. The destructors of all three classes are being executed just before cout<<"Executing getch()..."<<endl; as well as after the getch() statement, in the main function. I was wondering why this is happening, as I was expecting the destructors to be executed only once, i.e. after the getch() statement in main(). Please help me out.

Compiler : Borland C++ 5.5

I executed your program, this is what I got.

Code:

Constructing base1...1
Constructing derived1...2
Constructing derived2...3
Destroying derived2...
Destroying derived1...
Destroying base1...
Executing getch()...
Destroying derived2...
Destroying derived1...
Destroying base1...
Press any key to continue

here's what happens:

Code:

derived2 d1;

the first constructor for derived2 gets called. you didn't chain the constructors, so no base class constructor gets called.
(add cout << "derived2 default" in the derived2() constructor) =>nothing gets printed on the output.

now, you ASSIGN d1 to a new derived2 object.

Code:

d1=derived2(1,2,3);

When you do this, few things happen:

* constructor call of the right handside member (this outputs the first 3 lines)

* assignement call of the left handside member => none has been defined, using default assignement operator by destroying d1 (this outputs the destructor calls of d1: next three lines )

* perform member by member copy between d1 and the new object

Now, the main() function returns => destruction of d1, SECOND call to its destructor, and hence the last three lines of output

Everything seems just fine to me...... but maybe what you thought you did was :

Code:

derived2 d1;    // declare a d1 object
d1 = derived2(1,2,3)   // construct the object

this is true in java:

Code:

derived2 d1;
d1 = new derived2(1,2,3); // only one constructor call

but not in c++. In C++ you have to do this instead:

Code:

derived2 d1(1,2,3);  // only one constructor call

in C++, you have to understand the basic of constructor / destructor calls (this was a good exercise) or you're in big trouble

little add-on: using getch() messes everything up.

this is how I've modified the end of your code:

Code:

class derived2: public base1,public derived1
{
	protected:
	int z;
	public:
		derived2(){cout <<"default derive2 constructor\n";}
		derived2(int a,int b,int c):
		base1(a),
		derived1(b)
		{
			z=c;
			cout<<"Constructing derived2..."<<z<<endl;
		}
	~derived2(){cout<<"Destroying derived2..."<<endl;}
};
int main()
{
	cout << "code: derived2 d1;\n";
	derived2 d1;
	cout << "code: d1=derived2(1,2,3);\n";
	d1=derived2(1,2,3);
	cout<<"Executing getch()..."<<endl;
	getch();
	return(0);
}

this is the output:

Code:

code: derived2 d1;
default derive2 constructor
code: d1=derived2(1,2,3);
Constructing base1...1
Constructing derived1...2
Constructing derived2...3
Destroying derived2...
Destroying derived1...
Destroying base1...
Executing getch()...
Destroying derived2...
Destroying derived1...
Destroying base1...
Press any key to continue

Thanks max_unreg. That explanation helped a lot.

Originally posted by max_unreg

you didn't chain the constructors, so no base class constructor gets called.

That's not true. The default constructors of the base class get called without explicit chaining, but they don't output anything either.