Question

What is the output of the following C++ code? int *p; int *q = new int; p = q; *q = 75; delete p; p = new int; *p = 62; q = new int; q = p; *q = 26; cout << *p << " " << *q << endl;

Short answer

The output is "26 26".

Step-by-step solution

Initialize Pointers

The code starts by declaring a pointer `p` and allocates memory dynamically to a pointer `q`. So `p` is uninitialized and `q` points to the address of a newly allocated integer.

Assign q to p

The pointer `p` is assigned to point where `q` is pointing, making both `p` and `q` point to the same memory location.

Assign Value 75 to *q

The statement `*q = 75;` assigns the value 75 to the memory location pointed by `q`. Since `p` also points to the same memory location, `*p` is also 75.

Delete Memory Block

The `delete p;` statement releases the memory that `p` (and `q`) was pointing to. Neither `p` nor `q` should be used to access this deleted memory anymore.

Reassign p to a New Memory Block

Allocate new memory for `p` using `p = new int;`, giving `p` a valid memory address to point to a new integer.

Assign Value 62 to *p

The assignment `*p = 62;` sets the memory referenced by `p` to store the value 62.

Allocating and Redirecting q

Initially, `q = new int;` allocates a new integer and assigns `q` to its address. Then `q = p;` discards this new allocated memory (causing a memory leak) and makes `q` point to the same memory location as `p`.

Assign Value 26 to *q

Finally, `*q = 26;` sets the value at the memory location pointed by both `p` and `q` to 26. Consequently, `*p` and `*q` are both 26.

Output Values

The `cout << *p << " " << *q << endl;` command prints the values stored at the locations pointed by `p` and `q`, outputting "26 26".

Key concepts

Dynamic Memory Allocation

In C++, dynamic memory allocation allows you to allocate memory during program execution rather than at compile time. This means you can request memory from the heap, and it isn't restricted by function scope. The line `int *q = new int;` highlights this concept by creating a new integer space in the heap memory. Using `new int`, you are telling the program to allocate memory for an integer and return a pointer to it.

Dynamic memory allocation gives you the flexibility to decide the size needed for storage at runtime. But, it also requires careful handling because it can lead to issues like memory leaks if not managed correctly.

Remember:

• Always free dynamically allocated memory using `delete` or `delete[]` to prevent memory leaks.
• Be cautious, as improper handling can lead to undefined behavior or crashes.

Pointer Assignment

Pointer assignment in C++ can seem straightforward, yet it plays a crucial role in manipulating memory addresses. In the given code, when `p = q;` is executed, the pointer `p` now points to the same memory location that `q` was pointing to. This means both `p` and `q` can access the same piece of data. Through pointer assignment, you enable multiple pointers to reference the same underlying data.

This can lead to efficient memory usage, but also requires care:

• If one pointer alters the data, all pointers pointing to that same memory space will reflect that change.
• Ensure pointers are assigned correctly to avoid unintended data modification or memory access violations.

Understanding pointer assignment is crucial for effective memory manipulation and efficient program design.

Memory Management

Memory management in C++ is critical in ensuring your application runs efficiently without exhausting available resources. In the provided exercise, proper memory management is demonstrated through the use of `new` and `delete`.

Initially, `q` is dynamically allocated an integer space, which is later deleted using `delete p;`. This prevents the program from holding onto memory that is no longer needed, which is known as a memory leak.

However, there's a subtle error in the code: a memory leak occurs when `q = new int;` and is not deleted before being reassigned to `p`. This unreleased memory is wasted and can degrade system performance.

Key points:

• Always pair `new` with `delete` to ensure memory blocks are freed when no longer required.
• Be vigilant about assigning pointers without deleting previously allocated memory, as this can lead to memory leaks.

Dereferencing Pointers

Dereferencing a pointer in C++ involves accessing the value stored at the memory address the pointer references. You use the `*` operator to dereference a pointer.

In the code snippet, `*q = 75` and `*p = 26` are examples of dereferencing. These statements set the values at the allocated memory location via `q` and `p` respectively.

Through dereferencing, you change or access the data stored in the memory location the pointer is pointing to. However, dereferencing requires caution:

• Ensure that the pointer is pointing to a valid memory location before dereferencing to prevent undefined behavior.
• Avoid dereferencing deleted or unassigned pointers, which can lead to runtime errors or crashes.

Mastering dereferencing is crucial for utilization of pointers in accessing and modifying data efficiently.