Reference vs pointer in C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a reference to an integer variable.

C++

int x = 10;
int& [1] = x;

Drag options to blanks, or click blank then click option'

A*p

Br

Cref

Dptr

Attempts:

3 left

💡 Hint

Common Mistakes

Using * instead of & for references.

Naming the reference with invalid characters.

In C++, a reference is declared using the & symbol after the type. Here, r is the reference name.

2fill in blank

medium

Complete the code to declare a pointer to an integer variable.

C++

int x = 20;
int [1] p = &x;

Drag options to blanks, or click blank then click option'

A*

B&

Cref

Dptr

Attempts:

3 left

💡 Hint

Common Mistakes

Using & instead of * to declare a pointer.

Confusing pointer declaration with reference declaration.

A pointer is declared using the * symbol before the variable name.

3fill in blank

hard

Fix the error in the code to correctly assign a pointer to an integer variable.

C++

int x = 30;
int* p;
p = [1]x;

Drag options to blanks, or click blank then click option'

A*x

Bx

C&x

Dref x

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning the variable directly instead of its address.

Using * operator incorrectly in assignment.

To assign a pointer to the address of a variable, use the & operator before the variable name.

4fill in blank

hard

Fill both blanks to create a reference and a pointer to the same integer variable.

C++

int value = 40;
int& [1] = value;
int* [2] = &value;

Drag options to blanks, or click blank then click option'

Aref

Bptr

Cr

Dp

Attempts:

3 left

💡 Hint

Common Mistakes

Using invalid variable names.

Mixing up reference and pointer syntax.

Here, r is the reference name and p is the pointer name, both referring to value.

5fill in blank

hard

Fill all three blanks to correctly dereference a pointer and assign its value to a reference.

C++

int num = 50;
int* [1] = #
int& [2] = [3];

Drag options to blanks, or click blank then click option'

Ap

Br

C*p

Dnum

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning the pointer variable itself to the reference instead of the dereferenced value.

Using incorrect variable names.

p is the pointer to num, r is the reference assigned to the value pointed by p using *p.

Practice

1. Which of the following statements correctly describes a reference in C++?

easy

A. A reference is an alias for another variable and cannot be changed to refer to another variable.

B. A reference holds the memory address of a variable and can be reassigned.

C. A reference can be null and does not need to be initialized.

D. A reference is a pointer that requires explicit dereferencing.

Solution

1. Step 1: Understand what a reference is

   A reference acts as another name for an existing variable and must be initialized when declared.

2. Step 2: Compare with pointers

   Unlike pointers, references cannot be changed to refer to another variable or be null.

3. Final Answer:

   A reference is an alias for another variable and cannot be changed to refer to another variable. -> Option A

4. Quick Check:

   Reference = alias, fixed binding [OK]

Hint: References alias variables and never change target [OK]

Common Mistakes:

• Thinking references can be null
• Confusing references with pointers
• Assuming references can be reassigned

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

easy

A. int ptr;

B. int *ptr;

C. int &ptr;

D. int ptr*&;

Solution

1. Step 1: Recall pointer declaration syntax

   In C++, a pointer to an int is declared using an asterisk (*) before the variable name.

2. Step 2: Analyze options

   int *ptr; uses "int *ptr;" which correctly declares a pointer to int. int ptr; declares an int variable, B declares a reference, and D is invalid syntax.

3. Final Answer:

   int *ptr; -> Option B

4. Quick Check:

   Pointer syntax = int *ptr [OK]

Hint: Pointer declaration uses * before variable name [OK]

Common Mistakes:

• Omitting the * for pointer declaration
• Using & which declares a reference
• Combining * and & incorrectly

3. What will be the output of the following C++ code?

int x = 10;
int &ref = x;
int *ptr = &x;
ref = 20;
*ptr = 30;
std::cout << x << std::endl;

medium

A. 30

B. 20

C. 10

D. Compilation error

Solution

1. Step 1: Trace reference assignment

   ref is a reference to x, so ref = 20 sets x to 20.

2. Step 2: Trace pointer assignment

   ptr points to x, so *ptr = 30 sets x to 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Reference and pointer both modify x = 30 [OK]

Hint: Both ref and *ptr change x directly [OK]

Common Mistakes:

• Thinking ref and ptr are independent variables
• Confusing pointer dereference syntax
• Assuming ref can be reassigned

4. Identify the error in the following code snippet:

int a = 5;
int &ref;
ref = a;
std::cout << ref << std::endl;

medium

A. Cannot assign a variable to a reference.

B. Missing * for pointer declaration.

C. Reference must be initialized when declared.

D. Reference cannot be used with std::cout.

Solution

1. Step 1: Check reference declaration rules

   References must be initialized at the time of declaration; they cannot be declared without initialization.

2. Step 2: Analyze the code

   int &ref; is invalid because ref is not initialized. Assigning ref = a; later is not initialization but assignment.

3. Final Answer:

   Reference must be initialized when declared. -> Option C

4. Quick Check:

   Reference declaration requires immediate initialization [OK]

Hint: References need initialization on declaration [OK]

Common Mistakes:

• Thinking references can be declared then assigned
• Confusing assignment with initialization
• Assuming pointers and references behave the same

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

int x = 5;
int y = 10;
int *ptr = &x;
int &ref = y;
ptr = &y;
ref = *ptr + 5;
std::cout << x << ", " << y << std::endl;

hard

A. Compilation error

B. 5, 10

C. 5, 20

D. 5, 15

Solution

1. Step 1: Trace pointer reassignment

   ptr initially points to x, then is reassigned to point to y.

2. Step 2: Trace reference and assignment

   ref is a reference to y. The statement ref = *ptr + 5 means y = y + 5 (since ptr points to y).

3. Step 3: Final values

   x remains 5, y becomes 10 + 5 = 15.

4. Final Answer:

   5, 15 -> Option D

5. Quick Check:

   Pointer changes target; reference updates y = 15 [OK]

Hint: Pointer can change target; reference fixed to y [OK]

Common Mistakes:

• Assuming reference ref points to x
• Forgetting pointer reassignment
• Confusing reference assignment with pointer assignment