Constructor calling order in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call the base class constructor from the derived class.

C++

class Base {
public:
    Base() { std::cout << "Base constructor\n"; }
};

class Derived : public Base {
public:
    Derived() : [1]() { std::cout << "Derived constructor\n"; }
};

int main() {
    Derived d;
    return 0;
}

Drag options to blanks, or click blank then click option'

ADerived

BBase

Cthis

Dsuper

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Derived()' instead of 'Base()' in the initializer list.

Trying to call 'super()' which is not valid in C++.

The derived class constructor calls the base class constructor by naming the base class Base() in the initializer list.

2fill in blank

medium

Complete the code to call the parameterized base class constructor from the derived class.

C++

class Base {
public:
    Base(int x) { std::cout << "Base constructor with " << x << "\n"; }
};

class Derived : public Base {
public:
    Derived(int y) : [1](y) { std::cout << "Derived constructor with " << y << "\n"; }
};

int main() {
    Derived d(5);
    return 0;
}

Drag options to blanks, or click blank then click option'

ABase

Bthis

CDerived

Dsuper

Attempts:

3 left

💡 Hint

Common Mistakes

Calling 'Derived(y)' instead of 'Base(y)' in the initializer list.

Trying to call 'super(y)' which is invalid in C++.

The derived class constructor calls the base class constructor with an argument by naming the base class Base(y) in the initializer list.

3fill in blank

hard

Fix the error in the constructor calling order by completing the code.

C++

class A {
public:
    A() { std::cout << "A constructor\n"; }
};

class B : public A {
public:
    B() { std::cout << "B constructor\n"; }
};

class C : public B {
public:
    C() : [1]() { std::cout << "C constructor\n"; }
};

int main() {
    C c;
    return 0;
}

Drag options to blanks, or click blank then click option'

AB

BA

CC

Dsuper

Attempts:

3 left

💡 Hint

Common Mistakes

Calling A() directly from C's constructor, which is not the immediate base class.

Trying to call 'super()' which is not valid in C++.

Class C inherits from B, so its constructor should call B's constructor explicitly if needed. The base class A's constructor is called automatically by B's constructor.

4fill in blank

hard

Fill both blanks to create a constructor in Derived that calls the Base constructor with an argument and initializes member x.

C++

class Base {
public:
    Base(int a) { std::cout << "Base constructor with " << a << "\n"; }
};

class Derived : public Base {
    int x;
public:
    Derived(int a, int b) : [1](a), [2](b) { std::cout << "Derived constructor with " << b << "\n"; }
};

int main() {
    Derived d(10, 20);
    return 0;
}

Drag options to blanks, or click blank then click option'

ABase

Bx

Cy

DDerived

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Derived' instead of 'Base' to call the base constructor.

Initializing a non-existent member 'y' instead of 'x'.

The constructor initializer list first calls the base class constructor Base(a), then initializes the member variable x(b).

5fill in blank

hard

Fill the blanks to complete the constructor calling order in multiple inheritance.

C++

class A {
public:
    A() { std::cout << "A constructor\n"; }
};

class B {
public:
    B() { std::cout << "B constructor\n"; }
};

class C : public A, public B {
public:
    C() : [1](), [2]() { std::cout << "C constructor\n"; }
};

int main() {
    C c;
    return 0;
}

Drag options to blanks, or click blank then click option'

AA

BB

CA::A

DB::B

Attempts:

3 left

💡 Hint

Common Mistakes

Using scoped names like A::A() instead of just A().

Reversing the order of base constructors.

The constructor of C calls the constructors of both base classes A and B, in the order the bases appear in the inheritance list (A before B).

Practice

1. In C++, when creating an object of a derived class, which constructor is called first?

easy

A. Member objects' constructors

B. Derived class constructor

C. Base class constructor

D. Destructor of the base class

Solution

1. Step 1: Understand constructor order in inheritance

   When an object of a derived class is created, the base class constructor runs first to initialize the base part.

2. Step 2: Identify the first constructor called

   After the base class constructor, member objects and then the derived class constructor run.

3. Final Answer:

   Base class constructor -> Option C

4. Quick Check:

   Base constructor runs first [OK]

Hint: Base constructor always runs before derived [OK]

Common Mistakes:

• Thinking derived constructor runs first
• Confusing member object construction order
• Assuming destructors run during construction

2. Which of the following is the correct syntax to call a base class constructor from a derived class constructor in C++?

easy

A. Derived() { Base(); }

B. Derived() : Base {}

C. Derived() -> Base() {}

D. Derived() : Base() {}

Solution

1. Step 1: Recall base constructor call syntax

   In C++, the base class constructor is called in the initializer list of the derived constructor using a colon and parentheses.

2. Step 2: Match correct syntax

   Derived() : Base() {} uses the correct syntax: Derived() : Base() {}. Others are invalid or incorrect.

3. Final Answer:

   Derived() : Base() {} -> Option D

4. Quick Check:

   Use colon and parentheses for base constructor call [OK]

Hint: Use colon and parentheses to call base constructor [OK]

Common Mistakes:

• Calling base constructor inside derived constructor body
• Using arrow (->) instead of colon
• Omitting parentheses after base class name

3. Consider the following code:

class A {
public:
  A() { std::cout << "A "; }
};
class B {
public:
  B() { std::cout << "B "; }
};
class C : public A {
  B b;
public:
  C() { std::cout << "C "; }
};

int main() {
  C obj;
  return 0;
}

What is the output when this program runs?

medium

A. A B C

B. B A C

C. C A B

D. A C B

Solution

1. Step 1: Determine constructor call order

   First, base class A constructor runs printing "A ". Then member object b of class B is constructed printing "B ". Finally, derived class C constructor runs printing "C ".

2. Step 2: Combine outputs in order

   The output is "A B C " exactly in that sequence.

3. Final Answer:

   A B C -> Option A

4. Quick Check:

   Base, members, derived = A B C [OK]

Hint: Base then members then derived constructor order [OK]

Common Mistakes:

• Assuming members construct before base
• Mixing order of output strings
• Ignoring member object construction

4. Given this code snippet:

class Base {
public:
  Base() { std::cout << "Base "; }
};
class Derived : public Base {
public:
  Derived() { std::cout << "Derived "; }
};

int main() {
  Derived d();
  return 0;
}

What is the output when this program runs?

medium

A. No output

B. Derived Base

C. Base Derived

D. Compilation error

Solution

1. Step 1: Analyze object declaration

   The line 'Derived d();' is a function declaration, not an object instantiation, so no constructor runs.

2. Step 2: Understand program output

   Since no object is created, no constructors run and no output is printed.

3. Final Answer:

   No output -> Option A

4. Quick Check:

   Function declaration, no object created [OK]

Hint: Watch out for most vexing parse: 'Derived d();' is a function [OK]

Common Mistakes:

• Thinking 'Derived d();' creates an object
• Expecting constructor output
• Ignoring C++ parsing rules

5. You have a class hierarchy:

class X {
public:
  X() { std::cout << "X "; }
};
class Y {
public:
  Y() { std::cout << "Y "; }
};
class Z : public X {
  Y y1, y2;
public:
  Z() { std::cout << "Z "; }
};

int main() {
  Z obj;
  return 0;
}

What is the exact output when this program runs?

hard

A. Y Y X Z

B. X Y Y Z

C. X Z Y Y

D. Y X Y Z

Solution

1. Step 1: Identify constructor call order

   First, base class X constructor runs printing "X ". Then member objects y1 and y2 of class Y are constructed in declaration order, printing "Y " twice.

2. Step 2: Derived class constructor runs last

   Finally, derived class Z constructor runs printing "Z ".

3. Final Answer:

   X Y Y Z -> Option B

4. Quick Check:

   Base, members in order, derived = X Y Y Z [OK]

Hint: Members construct in declaration order after base [OK]

Common Mistakes:

• Mixing member construction order
• Assuming derived runs before members
• Ignoring multiple member objects