Pointer and variable relationship in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a pointer in C++?

A pointer is a variable that stores the memory address of another variable.

beginner

How do you declare a pointer to an integer variable?

You declare it using the syntax: int* ptr; where ptr is the pointer variable.

beginner

What does the * operator do when used with a pointer?

It dereferences the pointer, meaning it accesses the value stored at the memory address the pointer holds.

intermediate

Explain the relationship between a pointer and the variable it points to.

A pointer holds the address of a variable. Changing the value through the pointer changes the original variable's value because they refer to the same memory location.

intermediate

What happens if you change the pointer's value itself?

Changing the pointer's value changes the memory address it points to, so it will point to a different variable or memory location.

What does the following code print?

int x = 10;
int* p = &x;
std::cout << *p;

A Garbage value

B Address of x

C 10

D Compilation error

The pointer p stores the address of x. Using *p accesses the value of x, which is 10.

Which operator is used to get the address of a variable?

A &

B *

C ->

D %

The & operator returns the memory address of a variable.

If int* p; is uninitialized, what happens if you try to dereference it?

A It prints 0

B It causes undefined behavior (likely crash)

C It prints the address stored in p

D It compiles but does nothing

Dereferencing an uninitialized pointer leads to undefined behavior and can crash the program.

What does the pointer store?

A A memory address

B A value

C A variable name

D A data type

Pointers store memory addresses of variables.

How do you change the value of a variable using a pointer?

A You cannot change variable value through pointer

B Change the pointer variable itself

C Use the & operator on the pointer

D Use the * operator to assign a new value

Using *pointer = new_value changes the value at the memory address the pointer points to.

Explain in your own words how a pointer relates to the variable it points to.

Think about how a pointer is like a remote control to a TV.

Describe what happens when you change the pointer's value versus when you change the value it points to.

Consider the pointer as an address label and the variable as the house at that address.

Practice

1. What does a pointer variable in C++ store?

easy

A. The value of another variable

B. The memory address of another variable

C. The size of a variable

D. The type of a variable

Solution

1. Step 1: Understand what a pointer is

   A pointer is a variable that stores the address of another variable, not the value itself.

2. Step 2: Differentiate pointer from normal variables

   Normal variables store values directly, but pointers store where those values are located in memory.

3. Final Answer:

   The memory address of another variable -> Option B

4. Quick Check:

   Pointer = address storage [OK]

Hint: Pointers always hold addresses, not values [OK]

Common Mistakes:

• Confusing pointer value with variable value
• Thinking pointer stores variable type
• Assuming pointer stores variable size

2. Which of the following is the correct way to declare a pointer to an integer in C++?

easy

A. int ptr*;

B. *int ptr;

C. int *ptr;

D. ptr int*;

Solution

1. Step 1: Recall pointer declaration syntax

   In C++, to declare a pointer to an int, you write the type followed by an asterisk and the pointer name.

2. Step 2: Check each option

   Only 'int *ptr;' matches the correct syntax. Others have misplaced asterisks or wrong order.

3. Final Answer:

   int *ptr; -> Option C

4. Quick Check:

   Pointer declaration = type *name [OK]

Hint: Pointer declaration: type *name; [OK]

Common Mistakes:

• Placing * after variable name
• Using * before type keyword
• Mixing order of type and pointer symbol

3. What is the output of the following code?

int x = 10;
int *p = &x;
*p = 20;
std::cout << x << std::endl;

medium

A. 20

B. 10

C. Address of x

D. Compilation error

Solution

1. Step 1: Understand pointer assignment

   Pointer p stores the address of x. Using *p = 20 changes the value at that address, so x becomes 20.

2. Step 2: Output the value of x

   After changing *p, printing x outputs 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   Pointer dereference changes variable value [OK]

Hint: Changing *p changes x's value [OK]

Common Mistakes:

• Thinking *p = 20 changes pointer, not value
• Expecting output to be address
• Confusing pointer and variable values

4. Identify the error in the following code:

int x = 5;
int *p;
*p = x;
std::cout << *p << std::endl;

medium

A. Pointer p is not initialized before dereferencing

B. Variable x is not declared

C. Incorrect pointer declaration syntax

D. Missing semicolon after declaration

Solution

1. Step 1: Check pointer initialization

   Pointer p is declared but not assigned an address before using *p = x; which causes undefined behavior.

2. Step 2: Understand dereferencing uninitialized pointer

   Dereferencing p without initialization leads to runtime error or crash.

3. Final Answer:

   Pointer p is not initialized before dereferencing -> Option A

4. Quick Check:

   Always initialize pointers before dereference [OK]

Hint: Initialize pointers before use to avoid errors [OK]

Common Mistakes:

• Dereferencing uninitialized pointers
• Assuming pointer auto-initializes
• Ignoring runtime errors from bad pointers

5. Given the code below, what will be the output?

int a = 3, b = 4;
int *p = &a;
int *q = &b;
int *temp = p;
p = q;
q = temp;
std::cout << *p << " " << *q << std::endl;

hard

A. 3 4

B. 3 3

C. 4 4

D. 4 3

Solution

1. Step 1: Understand pointer swapping

   Initially, p points to a (3), q points to b (4). temp stores p (address of a).

2. Step 2: Swap pointers

   p = q makes p point to b (4), q = temp makes q point to a (3).

3. Step 3: Print values pointed by p and q

   *p is 4, *q is 3, so output is "4 3".

4. Final Answer:

   4 3 -> Option D

5. Quick Check:

   Pointer swap changes what they point to [OK]

Hint: Swapping pointers swaps pointed values [OK]

Common Mistakes:

• Confusing pointer values with pointed values
• Mixing up which pointer points where after swap
• Assuming values themselves swap