Question

Given the declaration: int num = 6; int *p = # which of the following statements increment(s) the value of num? a. p++; b. (*p)++; c. num++; d. (*num)++;

Short answer

Options b and c increment the value of num.

Step-by-step solution

Understanding Declarations

First, we analyze the variable declarations. We have \[ \text{int num = 6;} \] which means that a variable 'num' is of type integer and is initialized with the value 6. Next, the declaration \[ \text{int *p = \#} \] creates a pointer 'p' of type integer pointer and assigns it the address of 'num'. This means 'p' now points to the variable 'num'.

Evaluating Option a: p++

The statement \[ \text{p++;} \] attempts to increment the pointer 'p' itself. This means 'p' will point to the next integer in memory, but it does not change the value stored in 'num'. Thus, the value of 'num' remains unchanged.

Evaluating Option b: (*p)++

The statement \[ \text{(*p)++;} \] dereferences the pointer 'p', accessing the value of 'num', and then increments that value by 1. Therefore, this operation will increment the value of 'num' from 6 to 7.

Evaluating Option c: num++

The statement \[ \text{num++;} \] directly increments the value stored in 'num'. Thus, 'num' is increased from 6 to 7. This option successfully increments 'num'.

Evaluating Option d: (*num)++

The statement \[ \text{(*num)++;} \] is invalid because 'num' is not a pointer. Attempting to dereference 'num' like a pointer will result in a compilation error. Hence, this does not change 'num'.

Key concepts

Pointer Increment

In C++, pointers are a powerful feature. They allow you to directly manipulate memory addresses. A pointer is a variable that stores the memory address of another variable. When you increment a pointer, such as by using

• p++;

You are updating the pointer to point to the next memory location for that data type. If p is an integer pointer, incrementing it makes it point to the next integer in memory. However, the data stored in the location p was previously pointing to remains unchanged. It merely makes p move to another address where a new integer is located. This means the actual value, like num in the example, remains the same after executing this increment.

Dereferencing Pointers

Dereferencing a pointer in C++ involves accessing the value stored at the memory address to which the pointer refers. This is done using the asterisk (*) operator. For example:

• (*p)++;

This statement increases the value stored in the address p is pointing to by 1. Here, p points to num, so (*p)++; increases the original num value. It's effectively the same as writing num++;, as both will change num from 6 to 7 in this scenario. Understanding dereferencing is crucial, as it allows you to access and modify the values at a specific address dynamically.

Variable Declaration

A variable declaration in C++ is the process of defining a variable with a specific data type and optionally assigning it an initial value. Take the statement:

• int num = 6;

This creates an integer variable named num and assigns it an initial value of 6. The compiler allocates an appropriate amount of memory to store this integer during program execution. Understanding how the memory is assigned and how you can access it is fundamental to effective programming.
To declare a pointer, like int *p = #, you use an asterisk (*) before the pointer variable name to indicate that it is a pointer, not a regular integer. You then assign it the address of another variable using the & operator. This sets up a direct link to the variable num.

Memory Addressing

Memory addressing in C++ refers to working directly with the addresses in which data is stored. Every variable in C++ is stored at a unique memory address, which you can access using pointers. Understanding how to effectively manage these addresses allows for efficient data manipulation. The expression &num gives the memory address of the variable num. Assigning this address to a pointer variable, like p in int *p = #, enables you to work with num through its memory address. This is what makes pointers incredibly versatile and efficient in C++ programming. It helps in dynamic memory allocation, data manipulation, and even in creating complex data structures like linked lists and trees.