Question

What is the output of the following C++ code? int *length; int *width; length = new int; *length = 5; width = length; length = new int; *length = 2 * (*width); cout << *length << " " << *width << " " << (*length) * (*width) << endl;

Short answer

10 5 50

Step-by-step solution

Variable Initialization and Memory Allocation

In the given C++ code, two integer pointers, `length` and `width`, are declared. First, `length` is allocated memory using the `new` keyword, where it's assigned a memory space for an integer.

Assigning Values and Updating Pointers

The memory location pointed to by `length` is assigned the value `5`. Then, `width` is set to point to the same memory location as `length`. At this point, both `length` and `width` point to the same memory location containing the integer value `5`.

Memory Reallocation and New Value Assignment

The pointer `length` is assigned a new memory location by using `new int` again. This changes only the pointer, while `width` still points to the old memory location. The new memory location pointed to by `length` is assigned the value `2 * (*width)`, which calculates to `2 * 5 = 10`.

Output Evaluation

Lastly, the program outputs the value pointed to by `length`, the value pointed to by `width`, and their product. `*length` is `10`, `*width` is `5`, and their product is `10 * 5 = 50`. Hence, the output line in the program produces the string: `10 5 50`.

Key concepts

Memory Allocation

In C++, memory allocation is a pivotal concept when working with pointers. It involves designating specific areas of memory to store data that a program will use during its execution. When using dynamic memory allocation with pointers in C++, the `new` keyword plays an essential role. For instance, in the code `int *length;`, we start by declaring a pointer that can hold the address of an integer.

Then, by writing `length = new int;`, we instruct the program to allocate memory for an integer value dynamically. This allocated memory is on the heap, which is a particular memory area meant for storing objects dynamically. Allocating memory on the heap allows values to persist until they are explicitly deallocated, unlike stack memory which is routinely managed. This provides more control over the memory lifespan and usage but requires extra care to manage properly.

Pointer Arithmetic

Pointer arithmetic is an exciting facet of working with pointers in C++. It allows manipulation of the memory addresses stored by pointers. Essentially, you can perform arithmetic operations on pointers to traverse memory locations. However, one must handle this with caution, as incorrect operations can lead to undefined behavior.

In the example code, `width = length;` illustrates a form of pointer arithmetic, though it's more accurately pointer assignment. Here, `width` points to the same memory address as `length`, meaning they refer to the same variable in memory. If `length` changes what it points to, `width` still points to the original memory address. This is crucial when understanding how operations on one pointer may affect another if they are not independently managed.

Dynamic Memory Management

Dynamic memory management in C++ revolves around controlling the allocation and deallocation of memory at runtime. It's all about how we use the `new` and `delete` operators effectively together. Whenever we allocate memory using `new`, we must ensure it is eventually deallocated using `delete` to prevent memory leaks.

In the original solution, after the statement `length = new int;` is executed for the second time, the initial memory allocated is not linked to `length` anymore, yet `width` still holds that location. Without the use of `delete` to release the first allocation before redirecting `length` to a new memory location, this can lead to memory leaks. Properly managing this process ensures the program runs efficiently without exhausting system memory and causing slower execution or crashes.