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);
}

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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:

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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

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here's what happens:

Code:

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derived2 d1;

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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:

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d1=derived2(1,2,3);

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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:

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

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this is true in java:

Code:

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

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

Code:

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

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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);
}

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this is the output:

Code:

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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

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Thanks max_unreg. That explanation helped a lot.

Quote:

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Originally posted by max_unreg

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

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