Question

What is the output of the following C++ code? int x; int y; int *p = &x int *q = &y x = 35; y = 46; p = q; *p = 27; cout << x << " " << y << endl; cout << *p << " " << *q << endl;

Short answer

The output is "35 27\n27 27".

Step-by-step solution

Understanding the Variables and Pointers

We start by identifying the variables and pointers used in the code. We have two integer variables `x` and `y`, and two pointers `p` and `q` that initially point to these variables, respectively, with `p` pointing to `x` and `q` pointing to `y`.

Assigning Initial Values

Next, we assign initial values to the variables. The code sets `x = 35` and `y = 46`. At this point, `x` holds 35 and `y` holds 46.

Changing Pointer References

The line `p = q;` assigns the address stored in `q` to `p`. This means that now `p` also points to `y`. Therefore, both `p` and `q` point to `y`.

Modifying the Value Using a Pointer

Then, the line `*p = 27;` assigns the value 27 to the variable that `p` is pointing to, which is `y`. As a result, `y` becomes 27, while `x` remains unchanged as 35.

Printing Variable Values

The first `cout` statement outputs the values of `x` and `y`, which are `x = 35` and `y = 27`, due to the previous changes.

Printing Dereferenced Pointers

The second `cout` statement outputs the values pointed to by `p` and `q`. Both pointers point to `y`, which is 27, so the output is `27 27`.

Key concepts

Pointer Reference

In C++, a pointer is a special kind of variable that stores the memory address of another variable. Initially, in our example, we have two pointers, `p` and `q`.
They are declared to hold addresses of integer variables, specifically `int *p = &x;` and `int *q = &y;`.

• This declares `p` as a pointer which stores the address of `x` and `q` as a pointer to hold the address of `y`.
• The `&` operator is used to get the memory address of a variable. Therefore, `p` stores the address of `x`, and `q` stores the address of `y`.

The initial connection between pointers and variables is often referred to as setting a reference for the pointer. In this context, a correct reference means a pointer is correctly associated with the intended variable.

Pointer Dereferencing

Dereferencing a pointer means accessing or modifying the value of the variable to which the pointer refers. This is done using the `*` operator.
For example, when `*p = 27;` is executed in our code, the compiler accesses the value stored at the memory location held in pointer `p` and changes it.

• Here `p` is pointing to `y`, so `*p = 27;` changes the value of `y` to 27.
• Dereferencing allows us to read or modify the value stored in the addressed location.

It's vital to ensure that the pointer is correctly referenced before dereferencing it; otherwise, it might lead to undefined behavior or errors.

Variable Assignment

Variable assignment is a straightforward but essential concept in programming where we allocate a specific value to a variable. In the given code, `x = 35;` and `y = 46;` are examples of direct variable assignment.

• Assignments like these initialize the variables `x` and `y` with integers 35 and 46, respectively.
• It's crucial to perform such direct assignments before manipulating the variables using pointers, ensuring they have defined values.

Direct variable assignments become fundamental steps before performing more complex pointer manipulations, making sure the pointed variable has meaningful data.

Pointer Manipulation

Manipulating pointers involves changing where the pointers point to, or changing the values at those locations. In the line `p = q;` of the code, we perform pointer manipulation by changing what `p` points to.

• This makes `p` point to the same memory location as `q`, so both pointers now refer to `y`.
• This manipulation allows for more dynamic and powerful data handling.

Pointer manipulation is crucial for tasks requiring dynamic memory operations or complex data structures. However, careful handling is necessary to avoid errors, such as memory leaks or corruptions, due to incorrect or unintended pointer operations.