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[ Option A ]

A constructor is a special function inside a class that runs automatically when an object of that class is created. Its main purpose is to initialize the object data members.
A class is just a blueprint or logical design, and it does not do anything on its own. Only when an object is created from that class does memory get allocated, and at that moment, the constructor is automatically called to set initial values.

[ Option C ]

A copy constructor is a special constructor that is used to create a new object using an existing object. It takes a reference to an object of the same class as its parameter.
Syntax:
ClassName(ClassName &obj)
{
    // copy data members
}

[ Option B ]

In C++, when an object is created and initialized using another object of the same class, the copy constructor is called.
In statement myclass m3=m1;

• The m3 is being created and initialized using the existing object m1.
• This is not assignment. It is initialization at the time of object creation. Therefore, the copy constructor of myclass is executed.

Since no user-defined copy constructor is provided, the compiler generates a default copy constructor, which performs a member-wise copy of a and b from m1 to m3.

[ Option C ]

• If you do not write a destructor, the compiler generates a default destructor, which is public and empty.
• A destructor is a special member function of a class that is automatically called when an object goes out of scope or is deleted using delete. The purpose of destructor is to clean up resources (like memory, file handles, network connections, etc.) that the object was using.
• Generally, a custom destructor is required when a class allocates memory using the new operator, opens files or databases, or acquires other system resources. In such cases, it is necessary to release these resources. If they are not released properly, it can lead to memory leaks or resource leaks, which may cause the program to slow down or crash.

[ Option B ]

In C++, a constructor is a special member function of a class that is automatically and guaranteed to execute whenever an object of that class is created.

• The constructor has the same name as the class.
• It is used to initialize data members of the object.
• It does not have a return type, not even void.

[ Option C ]

The line Sample (string s) : name (s), is a parameterized constructor of the class Sample, and : name(s) is an example of a member initializer list in C++. It means that the data member name will be initialized with the value of s before the constructor body runs.

[ Option D ]

Here a custom copy constructor is defined, but it copies only one data member.
In this object m1 is created using the default constructor, which does not initialize the data members a and b. When m1.setData(10) is called, both a and b of m1 are assigned the value 10. 
Next, the statement myclass m2=(m1); creates a new object m2 using the copy constructor. However, the copy constructor copies only the member b from m1 to m2 and does not copy a. As a result, in object m2, the value of b becomes 10, while a remains uninitialized and contains a garbage value.
When m2.showData() is executed, it prints the value of a followed by b, resulting in Garbage 10.

[ Option B ]

In C++, if a class does not explicitly define a constructor, the compiler automatically provides a default constructor. A default constructor is a constructor that takes no arguments. It does not initialize members to specific values unless explicitly coded.

[ Option B ]

In Object-Oriented Programming System (OOPS), a constructor is a special member function that automatically gets called when an object of a class is created. Its main purpose is to initialize newly created objects. There are several types of constructors, each serving a distinct purpose in object initialization.

Default Constructor: This is a constructor that takes no arguments and initializes objects with default values.

class Demo {
    int x;
public:
    Demo() { x = 0; } // Default Constructor.
};

Parameterized Constructor: It takes one or more parameters, allowing the object’s data members to be initialized with user-defined values at the time of object creation.

class Demo {
    int x;
public:
    Demo(int a) { x = a; } // Parameterized Constructor.
};

Copy Constructor: The constructor which takes object as an argument of its own types, called copy constructor. It creates a new object as a copy of an existing object.

class Demo {
    int x;
public:
    Demo(int a) { x = a; } // Parameterized Constructor.
    Demo(Demo &obj) { x = obj.x; } // Copy Constructor.
};

There is no such thing as a 'friend constructor' in mainstream object-oriented programming languages like C++, Java. We can have friend functions or friend classes in C++, but constructors cannot be declared as friend.