Question

What is the output of the following C++ code? int *p = new int; int *q = new int; *p = 26; *q = 10; cout << 2 * (*p) << " " << (*q + 3) << endl; p = q; *p = 42; cout << *p << " " << *q << endl; q = new int; *p = 25; *q = 18; cout << *p << " " << *q << endl;

Short answer

The output sequence is: "52 13", "42 42", "25 18".

Step-by-step solution

Analyze Initial Values

The code starts by initializing two integer pointers. `int *p = new int;` reserves memory for an integer and assigns the address to `p`. Similarly, `int *q = new int;` reserves memory for another integer and assigns the address to `q`. The values pointed to by `*p` and `*q` are later set to 26 and 10 respectively.

First Output Calculation

The code calculates and outputs `2 * (*p)` and `(*q + 3)`. Given `*p = 26`, `2 * (*p)` evaluates to 52. For `*q = 10`, `(*q + 3)` evaluates to 13. Therefore, the output is `52 13`.

Update Pointers

The pointer `p` is updated to point to the same address as `q` with the assignment `p = q;`. As a result, both `*p` and `*q` now refer to the value that `q` was originally pointing to, which was 10.

Modify the Shared Pointer Value

The statement `*p = 42;` modifies the value at the location both `p` and `q` are pointing to. Now, `*p` and `*q` both equal 42. The next line outputs `*p` and `*q`, which are `42 42`.

Allocate New Memory and Assign

A new memory location is allocated for `q` with `q = new int;`. Now, `*p` stays 42 since `p` still points to the old shared address, but `*q` is not yet assigned a meaningful value.

Assign New Values After Allocation

The code sets the values with `*p = 25;` and `*q = 18;`. Since `p` was still pointing to its old address, `*p` becomes 25. `*q = 18` sets the newly pointed location of `q` to 18. The final output is `25 18`.

Key concepts

Dynamic Memory Allocation in C++

Dynamic memory allocation in C++ is an essential concept, allowing programmers to request memory at runtime. This is a crucial feature because sometimes, we don’t know how many resources our program will need until it’s actually running.
Using pointers, we can dynamically allocate memory using the `new` keyword. For example, `int *p = new int;` reserves enough memory to store an integer and stores the address of this memory in the pointer `p`.
Dynamic memory helps in:

• Efficient memory usage because memory is allocated only when necessary.
• Flexibility, as it allows for dynamic data structures like linked lists and trees.
• Avoiding memory wastage since you can deallocate memory that is no longer needed using `delete`.

This powerful tool comes with the responsibility of managing the memory properly to avoid leaks or corruptions.

Understanding Pointer Arithmetic

Pointer arithmetic enables us to perform operations on pointer values, such as moving to next elements in an array. However, it’s also used in basic number manipulations.
In pointer arithmetic, when you add an integer to a pointer, it's multiplied by the size of the data type before the addition is performed, allowing you to traverse through consecutive memory cells.
An example in the original exercise hints at indirect usage of pointer arithmetic where operations are performed on values stored via pointers, such as `2 * (*p)` which doubles the value that `*p` points to.
Remember: pointer arithmetic is valid within the same array only; moving a pointer outside allocated memory leads to undefined behavior.
Using pointers correctly leads to powerful and efficient code, but requires considering memory layout and bounds.

Variable Assignment and Pointers

Variable assignment using pointers requires careful attention to the address and the value being handled.
When we write `int *p = new int;`, `p` not only holds a memory address but also allows access to the integer stored there. The expression `*p = 26;` assigns the value 26 to this location.
Assignments like `p = q;` redirect the pointer `p` to the same memory location as `q`, meaning that both `p` and `q` now point to the same data.
Key points to remember in pointer variable assignment:

• Pointer-to-pointer assignment changes which address a pointer variable holds.
• The use of `*` to manipulate the value at the pointer’s address.
• The potential for unintentional overwrites when pointers share addresses.

Proper cautions and checks should be in place during assignments to avoid altering unintended parts of the memory.

Efficient Memory Management

Managing memory efficiently ensures your application runs smoothly without wasting resources or causing memory leaks. In C++, memory that is allocated dynamically using `new` should also be deallocated using `delete`.
Memory management becomes crucial when your pointers start to point to new memory locations, as in `q = new int;`. Forgetting to free the previous memory that `q` was pointing to might lead to memory leaks.
Considerations in memory management include:

• Always deallocate memory which is no longer used by using `delete`.
• Reset pointers to `nullptr` after `delete` to avoid dangling pointers, which point to memory that has been freed.
• Implement checks before reassigning or deleting pointers to ensure you do not free unallocated memory.

With meticulous management, memory-related errors and resource wastage are minimized, enabling robust applications.