Question

Consider the following class declaration: class Thing \\{ private: \(\operatorname{int} x\) int \(\mathrm{y}\) static int \(z\) public: Thing() \\[ \\{x=y=z ; \quad\\} \\] static void putThing(int a) \\[ \\{z=a ;\\} \\] Assume a program containing the class declaration defines three Thing objects with the following statement: Thing one, two, three; How many separate instances of the x member exist? How many separate instances of the y member exist? How many separate instances of the z member exist? What value will be stored in the \(x\) and \(y\) members of each object? Write a statement that will call the PutThing member function before the objects above are defined.

Short answer

Answer: There are 3 unique instances of x and y members for each object, and a single instance of the z member shared across all objects. The x and y members have a value of 0 for each object, and the z member also has a value of 0.

Step-by-step solution

Analyze the class and its members

In the given class 'Thing', we have three data members: x, y, and z. The scope of x and y is "private", which means they can only be accessed within the class. In contrast, the scope of z is "public" and "static", which implies that there will be only a single instance of z shared across every object of this class. The constructor of the class initializes all three members with the same value.

Determine the instances of x, y, and z members

We have three objects of the class Thing: one, two, and three. Since x and y are private non-static class members, each object will have its separate instance of x and y resulting in a total of 3 instances for x and 3 instances for y. In contrast, z is a public and static class member, which means that all the objects of class Thing will share the same instance of z. As a result, there will only be a single instance of z.

Determine the values of x and y members

The constructor of class Thing initializes x, y, and z to the same value. Therefore, each object's x and y members will have the same value as z. Since z is not initialized explicitly in the class declaration, the value of z will be 0 (as it is a global variable). Thus, the value of the x and y members for each object will be 0.

Write a statement to call PutThing member function

The PutThing member function is a 'public static' method, which means it can be accessed and called without needing an object of the class. Thus, to call the PutThing member function before creating the instances one, two, and three, we can use the following statement: Thing::putThing(5);

Key concepts

static member variables

In C++, a static member variable is shared across all instances of a class, meaning it has only one copy no matter how many objects of that class are created. Static variables are extremely useful when you need consistent data across all objects. For example, consider a class called Counter with a static variable that counts how many objects are created.

The unique characteristic of a static member is that it is initialized once and maintains its value throughout the life of the program. In the 'Thing' class, the static variable z exists as a single instance. Thus, regardless of how many Thing objects are instantiated, there will only be one z, and changes made to it through any object or class method would affect all objects. This is why when we call Thing::putThing(5), we set z to 5 for the entire class, and every object sees this updated value.

private and public access specifiers

Access specifiers in C++ define how the members of a class can be accessed. There are three types: public, protected, and private.

In the 'Thing' class example, x and y are specified as private, which restricts their access to the Thing class's own methods. You cannot access them directly from outside the class, such as from the main function or through objects. On the contrary, z being public, is accessible from outside the class. However, since it is also static, it should be accessed via the class name rather than through an object instance (Thing::z).

class constructors

A class constructor is a special method in C++ that is automatically called when an object of a class is created. Its main purpose is to initialize the class's member variables. Constructors can be overloaded with different sets of parameters to allow for various initialization complexities.

In our textbook exercise with the 'Thing' class, the default constructor has no parameters and initializes x, y, and the static variable z to zero, although this is implicit for static variables as they are zero-initialized by default. It's essential to understand that the constructor is unique in that it does not return a value — not even void — and it has the same name as the class.

objects instantiation

Object instantiation is the process where an actual object is created from a class blueprint in C++. When you instantiate an object, memory is allocated for all non-static members, and the constructor is called to initialize these members.

In the provided exercise, instantiation happens when we declare Thing one, two, three;. At this point, three separate objects are created, and each has its own copy of x and y, as these are non-static members. As described earlier, they would all have the same value, which is zero in this case because that's what the constructor sets them to. Understanding how instantiation works is critical for managing resources and ensuring that your program behaves as expected.