Reference vs pointer in C++ - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of reference and pointer modification

What is the output of this C++ code?

C++

#include <iostream>

int main() {
    int x = 10;
    int &ref = x;
    int *ptr = &x;
    ref = 20;
    *ptr = 30;
    std::cout << x << std::endl;
    return 0;
}

A 10

B 20

C Compilation error

D 30

💡 Hint

Remember that both reference and pointer can modify the original variable.

Both the reference and the pointer refer to the same variable x. First, ref = 20 changes x to 20, then *ptr = 30 changes x to 30. So the final output is 30.

❓ Predict Output

intermediate

Pointer vs reference reassignment

What is the output of this C++ code?

C++

#include <iostream>

int main() {
    int a = 5, b = 10;
    int *ptr = &a;
    int &ref = a;
    ptr = &b;
    ref = b;
    std::cout << a << " " << b << std::endl;
    return 0;
}

A 10 10

B 5 5

C 5 10

D Compilation error

💡 Hint

Remember that references cannot be reseated but pointers can.

The pointer ptr is changed to point to b, but the reference ref still refers to a. However, ref = b assigns the value of b to a, so a becomes 10. b remains 10. So output is '10 10'.

🔧 Debug

advanced

Identify the error in pointer and reference usage

Which option will cause a compilation error in this code snippet?

C++

int x = 5;
int &ref = x;
int *ptr = &x;

// Options below show different assignments:

A ptr = &ref;

B int &newRef = ptr;

C ref = 10;

D *ptr = 20;

💡 Hint

Check if the types on both sides match for references.

Option B tries to create a reference to an int pointer (int*), but declares it as int& which is a reference to int. This is a type mismatch causing a compilation error.

🧠 Conceptual

advanced

Difference in memory behavior of references and pointers

Which statement correctly describes the difference between references and pointers in C++?

A Pointers can be reseated to point to different objects, references cannot be reseated after initialization.

B References require explicit dereferencing to access the value, pointers do not.

C References can be null, pointers cannot.

D Pointers are automatically dereferenced when used, references require the * operator.

💡 Hint

Think about whether you can change what a reference or pointer points to after creation.

Pointers can be changed to point to different objects anytime. References must be initialized once and cannot be changed to refer to another object later.

❓ Predict Output

expert

Output of complex pointer and reference manipulation

What is the output of this C++ code?

C++

#include <iostream>

void func(int *&ptrRef, int &ref) {
    *ptrRef = ref + 5;
    ref = *ptrRef + 5;
    ptrRef = &ref;
}

int main() {
    int a = 1, b = 2;
    int *p = &a;
    func(p, b);
    std::cout << a << " " << b << " " << *p << std::endl;
    return 0;
}

A 1 2 1

B 7 7 7

C 7 12 12

D Compilation error

💡 Hint

Track carefully how the pointer reference and reference parameters modify values and pointers.

Initially, p points to a=1, b=2. *ptrRef = ref + 5; sets *p = 2 + 5 = 7, so a=7. ref = *ptrRef + 5; sets b = 7 + 5 = 12. ptrRef = &ref; now p points to b. In main, a=7, b=12, *p = b = 12. Output: '7 12 12'.

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