What is order of calling constructor and destructor in C++?

Order of execution of constructors and destructors in C++ is frequently asked interview question. For this question, we have to answer c++ constructor call order and destructor call order  in inheritance relationship with a program example.

[NOTE: During answering c++ inheritance destructor order question, it is better to include answer for multiple inheritance constructor call order also that I have explained as a notes in bottom of this post and here the Constructor Destructor call order for multiple composed objects in C++ class ]

Answer: C++ constructor call order will be from top to down that is from base class to derived class and c++ destructor call order will be in reverse order.

Below is the example of constructor destructor call order for multi-level inheritance, in which Device class is the base class and Mobile class is derived from Device base class and then Android class is derived from Mobile class as a base class.

When we create the object of Android class, order of invocation of constructor and destructor will be as below
Constructor: Device
Constructor: Mobile
Constructor: Android
Destructor : Android
Destructor : Mobile
Destructor : Device

Example of order of constructor and destructor in c++ in multilevel inheritance.

#include
using namespace std;

//base class
class Device{

public:
	Device(){cout<<"Constructor: Device\n";}
	~Device(){cout<<"Destructor : Device\n";}
};

//derived class
class Mobile:public Device{

public:

	Mobile(){cout<<"Constructor: Mobile\n";}
	~Mobile(){cout<<"Destructor : Mobile\n";}
};

//derived class
class Android:public Mobile{

public:

	Android(){cout<<"Constructor: Android\n";}
	~Android(){cout<<"Destructor : Android\n";}
};

//--------------------TEST--------------------------------------

int main()
{
	Android _android; // create the object that will call required constructors	

	return 0;
}

NOTES:

In multiple inheritance, for exampleclass Gadgets:public Mobile,public Tablet”, order of constructor call will be of Mobile, Tablet and then derived class i.e. Gadgets.

Example: C++ multiple inheritance constructor call order

#include
using namespace std;


class Mobile{

public:

	Mobile(){cout<<"Constructor: Mobile\n";}
	~Mobile(){cout<<"Destructor : Mobile\n";}
};

class Tablet{

public:

	Tablet(){cout<<"Constructor: Tablet\n";}
	~Tablet(){cout<<"Destructor : Tablet\n";}
};

//Gadgets is the derived class using Multiple inheritance features with //Mobile and Tablet class
class Gadgets:public Mobile,public Tablet{

public:
	Gadgets(){cout<<"Constructor: These are my Gadgets\n";}
	~Gadgets(){cout<<"Destructor : Destroying Gadgets\n";}
};

//--------------------TEST--------------------------------------

int main()
{
	Gadgets myGadgets; // create the object that will call required constructors	

	return 0;
}

OUTPUT:
Constructor: Mobile
Constructor: Tablet
Constructor: These are my Gadgets
Destructor : Destroying Gadgets
Destructor : Tablet
Destructor : Mobile

Recommend to read another important interview question: Why to use virtual destructor in a class in C++?

In short, virtual desturctor is used to maintain the destructors call order in a polymorphic class in an inheritance hierarchy.

What is order of calling constructor and destructor in C++?

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