Question

What is the output of the following C++ code? int x; int y; int *p = &x int *q = &y *p = 35; *q = 98; *p = *q; cout << x << " " << y << endl; cout << *p << " " << *q << endl;

Short answer

The output is: 98 98 98 98

Step-by-step solution

Understand Pointer Initialization

The code begins by declaring two integers, `x` and `y`, and two pointers, `p` and `q`. The pointer `p` is initialized to point to the memory address of `x` and `q` to the address of `y`. This setup allows us to modify the values of `x` and `y` using the pointers.

Assign Values Using Pointers

The line `*p = 35;` assigns the value 35 to the location in memory pointed to by `p`, which is `x`. Therefore, `x` becomes 35. The line `*q = 98;` assigns the value 98 to the location pointed to by `q`, which is `y`. Now, `y` is 98.

Copy Value from One Memory Location to Another

The operation `*p = *q;` assigns the value at the location pointed to by `q` (which is `y`, holding the value 98) to the location pointed to by `p` (which is `x`). Thus, `x` now becomes 98 instead of 35.

Output the Values of `x` and `y`

The first `cout` statement, `cout << x << " " << y << endl;`, displays the current values of `x` and `y`. Since both `x` and `y` are 98, it prints `98 98`.

Output the Values Pointed by `p` and `q`

The second `cout`, `cout << *p << " " << *q << endl;`, outputs the values pointed to by `p` and `q`. Since both pointers point to `98`, it outputs `98 98`.

Key concepts

Pointer Initialization

In C++, pointers are powerful tools that allow for direct memory access and manipulation. Initialization is the first step when working with pointers. When you declare a pointer, you reserve space in memory to store the memory address it points to. Consider the example from the problem statement:
```cpp int x; int *p = &x; ```

• Here, `x` is an integer variable, which holds its own value.
• The pointer `p` is declared and initialized to store the memory address of `x` using the `&` operator.

This operation means that `p` doesn't hold a traditional value; instead, it holds a reference to where `x` is stored in memory. Proper pointer initialization is crucial, as using an uninitialized pointer can lead to undefined behavior, often resulting in segmentation faults or crashes. Initializing a pointer correctly ensures that it points to a legitimate, intended address, setting the stage for further operations like memory manipulation.

Memory Address Manipulation

Memory address manipulation involves changing the values stored at specified memory locations. With pointers, this becomes straightforward because pointers can "point" to specific addresses. In our example, once the pointers are initialized:
```cpp *p = 35; *q = 98; ```

• The statement `*p = 35;` tells the program to store the value 35 at the memory address pointed to by `p`, i.e., the address of `x`. Consequently, `x` is set to 35.
• Similarly, `*q = 98;` directs the storage of 98 at the address pointed by `q`, which is `y`, so `y` becomes 98.

Effectively, using the dereference operator `*`, pointers allow us to manipulate data directly at specific locations. This ability to modify memory directly makes pointers extremely useful but potentially dangerous if misused, as accidental modifications of unintended addresses can occur.

Value Assignment using Pointers

Using pointers to assign values involves utilizing the memory address they point to. This can include copying values between different memory locations. Continuing with the given example, examine the operation:
```cpp *p = *q; ```

• This line uses pointer `p` to copy the value located at the address pointed to by `q` into the space occupied by `x`.
• Since `q` refers to `y`, which holds the value 98, this operation results in `x` also being set to 98.

Thus, by using pointers, you can efficiently copy and assign values between different positions in memory. Understanding how to properly use pointers for assignment operations is vital, as it offers a clear and concise way to manipulate multiple data points without utilizing direct variable references. However, careful consideration should always be given to ensure data integrity and correct memory handling when performing these operations.