Question

What is the output of the following C++ code? (Assume that decimal numbers are output with two decimal places.) double *test1 = new double; double *test2 = new double; double *average; average = test1; *test1 = 45.00; *test2 = 90.00; test1 = test2; test2 = new double; *test2 = 86.00; *average = ((*test1) + (*test2)) / 2; cout << *test1 << " " << *test2 << " " << *average << endl;

Short answer

The output is: 90.00 86.00 88.00

Step-by-step solution

Variable Initialization

First, memory is dynamically allocated for `test1` and `test2` using `new double`, pointing them to different areas in memory. A pointer `average` is declared but not yet initialized. Then, `average` is initialized to point to the same memory location as `test1`, so they share the same memory reference.

Assign Values

The value `45.00` is assigned to the memory location pointed by `test1`, as `*test1 = 45.00;`. Similarly, the memory location pointed by `test2` receives the value `90.00`.

Pointer Reassignment

The `test1` pointer is updated to point to `test2`'s location. From this point on, dereferencing `test1` will access the value held at `test2`'s memory location. As a result, the value `45.00` is not accessible from `test1` anymore.

Allocate New Memory for Test2

A new memory block is allocated for `test2` using `new double`. After this line, any value previously stored and referenced by `test2` is left inaccessible (potential memory leak). The new memory is assigned the value `86.00`.

Calculate Average

The value of `*average` is recalculated as `((*test1) + (*test2)) / 2`. At this point, `*test1`, which is now `90.00` due to pointer reassignment in Step 3, and `*test2`, which is `86.00`, are averaged. Thus, `*average` becomes `88.00`.

Output Values

The program outputs the values stored at the locations pointed to by `test1`, `test2`, and `average`. Hence, the values `90.00`, `86.00`, and `88.00` are printed, each formatted to two decimal places due to the problem's requirement.

Key concepts

Dynamic memory allocation

In C++, allocating memory dynamically allows you to request a specific amount of memory during program execution. This is done using the `new` keyword. For example, `double *test1 = new double;` dynamically allocates memory for a double variable which `test1` can point to.

Dynamic memory is crucial when the size of the data is not known at compile time. It helps in creating flexible data structures like arrays, linked lists, etc.

• Use `new` to allocate memory.
• Be sure to delete the allocated memory when it's no longer needed using `delete` to prevent memory leaks.

Understanding dynamic memory allocation is fundamental in C++ as it enables efficient memory management and resource usage.

Pointer reassignment

Pointers in C++ not only store the address of a memory location but can also be reassigned to point to different addresses during the program execution. Initially, `test1` and `test2` point to different memory spaces allocated dynamically.

Pointer reassignment can lead to changes in which variable is indirectly modified when using a pointer. For example:

• `test1 = test2;` makes `test1` point to the memory location initially meant for `test2`.
• This change means any operation on `*test1` modifies the value at `test2`'s original location.

Reassigning a pointer without handling the current memory it points to can cause memory leaks. Thus, always ensure that any memory a pointer leaves should be properly deallocated if necessary.

Dereferencing pointers

Dereferencing is the process of accessing or modifying the data at a particular memory location that a pointer points to using the `*` operator. For instance, after `*test1 = 45.00;`, the value stored in the memory pointed by `test1` becomes 45.00.

When you dereference a pointer:

• You indirectly work with the value stored in the location the pointer indicates.
• This is particularly useful for modifying values or accessing dynamically allocated data.

In the exercise, dereferencing is used for both retrieving values and performing calculations. For example, the average calculation `*average = ((*test1) + (*test2)) / 2;` uses dereferenced values of `test1` and `test2`.

Memory management in C++

Effective memory management in C++ is vital to writing efficient and error-free programs. Since C++ doesn't use automatic garbage collection like some other languages, managing memory is typically a developer's responsibility.

This involves:

• Allocating memory when needed using `new`.
• Releasing memory using `delete` to avoid memory leaks.
• Ensuring that pointers are pointing to valid memory before dereferencing.

In the given exercise, memory is allocated for `test1` and `test2` using `new`. However, reassignment causes the first block of memory allocated to `test2` to become inaccessible, demonstrating a **memory leak**. Although the exercise does not explicitly show cleanup, you should always free any dynamically allocated memory when it is no longer in use by calling `delete`, like `delete test1;`, to maintain excellent memory management practices.