Question

What is the output of the following C++ code? int num1; int num2; int *p = &num1 p = &num2 *p = 25; num1 = num2 + 6; p = &num1 num2 = 73; *p = 47; cout << *p << " " << num1 << " " << num2 << endl;

Short answer

The output is: 47 47 73

Step-by-step solution

Declare Variables and Pointer

The code begins by declaring two integer variables, `num1` and `num2`, and an integer pointer `p` that initially points to the address of `num1`.

Change Pointer Reference

The pointer `p` is then assigned to the address of `num2`. At this point, `p` points to `num2`, but `num2` has not been initialized with a value.

Assign Value to Pointer

The statement `*p = 25` assigns the value 25 to the location where `p` is pointing, i.e., `num2`. Thus, `num2` now holds the value 25.

Calculate `num1`

The statement `num1 = num2 + 6` assigns to `num1` the value of `num2 + 6`. Given `num2` is currently 25, `num1` becomes 25 + 6 = 31.

Point to `num1` Again

The pointer `p` is re-assigned to point to `num1` with `p = &num1`. Thus, `p` now points to `num1`.

Change `num2`

The statement `num2 = 73` changes the value of `num2` to 73.

Change `num1` via Pointer

The statement `*p = 47` assigns the value 47 to `*p`. Since `p` is pointing to `num1`, `num1` is now 47.

Output the Values

The final statement `cout << *p << " " << num1 << " " << num2 << endl;` outputs the current pointed value by `p`, `num1`, and `num2`. `*p` and `num1` both output 47, while `num2` outputs 73.

Key concepts

Variable Declaration

Variable declaration is one of the foundational concepts in C++ programming. It involves specifying a name and data type for a variable, which provides a way to store values in memory during the execution of a program. In the given C++ code, we see the declarations: `int num1;` and `int num2;`. These declarations inform the compiler that `num1` and `num2` are variables that will hold integer values. Initially, these variables are not assigned any specific values, just allocated space in memory.

• Data Type: Specifies what kind of value a variable can hold. Here, `int` indicates integers.
• Variable Name: Acts as an identifier for a location in memory to store data.

By declaring variables, you enable your program to make use of these memory spaces to perform operations and store results.

Pointer Assignment

In C++, pointers are variables that store memory addresses of other variables, allowing indirect manipulation of the variable to which they point. The pointer `p`, as seen in the code: `int *p = &num1;`, is declared to hold the address of an integer type. The `&` operator fetches the address of `num1` and assigns it to `p`.
Later in the code, `p = &num2;` changes the reference of the pointer to point to `num2` instead. This flexibility allows pointers to point to different variables at different times.

• Pointer Declaration: Indicates a pointer that can point to a specific data type.
• Assignment: Uses `&` to assign an address to the pointer, which then links it to a specific variable.

Pointer assignment is powerful as it directly enables memory manipulation by referring to addresses rather than names.

Memory Address

A memory address is a specific location in memory where data is stored. Every variable has a memory address associated with it, and pointers use these addresses to access and manipulate variable values.
When you declare a variable like `int num1;`, `num1` is allocated a specific spot in memory. Using an operator like `&num1`, you can retrieve the memory address of this variable. This address allows you to use pointers to modify the variable indirectly.

• Address-of Operator (&): Retrieves the memory address of a variable.
• Dereferencing Operator (*): Accesses the value at a pointer's memory address.

Understanding memory addresses and how pointers use them is crucial in effective memory management and manipulation in C++.

Output Stream

The output stream in C++ is a sequence of characters shared with an output device, like a console or window, using the `cout` object from the iostream library. This allows for printing information to the console for end-user interaction.
In the code, `cout << *p << " " << num1 << " " << num2 << endl;` is used for outputting values to the console. The `" "` within the `cout` statement signifies spaces between the numbers for clear readability.

• cout: The standard output stream object used to display text.
• Insertion Operator (<<): Directs data to the output stream.
• endl: A manipulator that ends the current line and flushes the buffer.

Output streams are essential in debugging, testing, and providing user feedback within a C++ application.