Reference lifetime in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

C++

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

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

A10

Bx

C&x

D*x

Attempts:

3 left

💡 Hint

Common Mistakes

Using the value 10 instead of the variable name.

Using pointer syntax like &x or *x.

A reference must be initialized with a variable name, not a value or pointer.

2fill in blank

medium

Complete the function to return a reference to the first element of the array.

C++

int& firstElement(int arr[]) {
    return [1];
}

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

Aarr[0]

B*arr

Carr

D&arr[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Returning the array name instead of an element.

Returning a pointer instead of a reference.

Returning arr[0] returns a reference to the first element, matching the return type.

3fill in blank

hard

Fill in the blank with the variable name that matches the return statement.

C++

int& getValue() {
    int [1] = 5;
    return value;
}

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

Avalue

Bref

Cnum

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that does not match the return statement.

Not realizing returning a reference to a local variable causes undefined behavior.

The local variable must match the returned reference name; however, returning a reference to a local variable is unsafe.

4fill in blank

hard

Fill both blanks to create a reference to a string and modify it.

C++

std::string name = "Alice";
std::string& [1] = [2];
[1] = "Bob";
std::cout << name << std::endl;

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

ArefName

Bname

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name for the reference and the original variable.

Not initializing the reference with the original variable.

The reference variable refName refers to the string variable name, so modifying refName changes name.

5fill in blank

hard

Fill all three blanks to create a function returning a reference and use it correctly.

C++

int& [1](int& x) {
    return [2];
}

int main() {
    int a = 10;
    int& ref = [3](a);
    ref = 20;
    std::cout << a << std::endl;
    return 0;
}

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

AgetRef

Bx

Da

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching function name between declaration and call.

Returning a copy instead of a reference.

The function getRef returns a reference to x, and calling getRef(a) returns a reference to a, allowing modification.

Practice

1. What does reference lifetime in C++ mean?

easy

A. The time it takes to create a reference

B. How long a reference remains valid during program execution

C. The size of the reference in memory

D. The number of references to a variable

Solution

1. Step 1: Understand what a reference is

   A reference is an alias for another variable, so it must point to valid data.

2. Step 2: Define lifetime in this context

   Reference lifetime means the duration the reference points to valid memory without becoming invalid.

3. Final Answer:

   How long a reference remains valid during program execution -> Option B

4. Quick Check:

   Reference lifetime = validity duration [OK]

Hint: Reference lifetime = how long it stays valid [OK]

Common Mistakes:

• Confusing lifetime with creation time
• Thinking lifetime is about memory size
• Assuming references count how many times used

2. Which of the following is the correct way to declare a reference to an int variable x?

easy

A. int &ref = x;

B. int ref = &x;

C. int &ref();

D. int *ref = x;

Solution

1. Step 1: Recall reference declaration syntax

   References use & after the type, e.g., int &ref.

2. Step 2: Check each option

   int &ref = x; correctly declares a reference to x. Others either declare pointers or incorrect syntax.

3. Final Answer:

   int &ref = x; -> Option A

4. Quick Check:

   Reference syntax = type &name = variable [OK]

Hint: References use & after type, not * or parentheses [OK]

Common Mistakes:

• Using * instead of & for references
• Adding parentheses after reference name
• Assigning address instead of reference

3. What will be the output of this code?

int &getRef() {
    int x = 10;
    return x;
}

int main() {
    int &r = getRef();
    std::cout << r << std::endl;
    return 0;
}

medium

A. Undefined behavior (likely garbage value)

B. Compilation error

C. 10

D. 0

Solution

1. Step 1: Analyze function return

   The function returns a reference to a local variable x which is destroyed after the function ends.

2. Step 2: Understand consequences

   Using that reference in main leads to undefined behavior because x no longer exists.

3. Final Answer:

   Undefined behavior (likely garbage value) -> Option A

4. Quick Check:

   Returning local reference = undefined behavior [OK]

Hint: Never return references to local variables [OK]

Common Mistakes:

• Assuming local variable reference is valid after return
• Expecting compilation error
• Thinking it prints 0 always

4. Identify the error in this code snippet:

int& createRef() {
    int temp = 5;
    return temp;
}

int main() {
    int &r = createRef();
    std::cout << r << std::endl;
}

medium

A. Reference variable declared without initialization

B. Missing semicolon after return statement

C. Returning reference to a local variable which is destroyed

D. Using & instead of * for pointer

Solution

1. Step 1: Check what is returned

   The function returns a reference to temp, a local variable.

2. Step 2: Understand lifetime issue

   Local variable temp is destroyed when function ends, so reference is invalid.

3. Final Answer:

   Returning reference to a local variable which is destroyed -> Option C

4. Quick Check:

   Returning local reference = lifetime error [OK]

Hint: Local variables die after function ends; don't return their references [OK]

Common Mistakes:

• Thinking compiler allows returning local references safely
• Confusing references with pointers
• Ignoring variable lifetime rules

5. How can you safely return a reference from a function in C++?

hard

A. Return a reference to a temporary object

B. Return a reference to a local variable

C. Return a pointer instead of a reference

D. Return a reference to a static or global variable

Solution

1. Step 1: Understand safe reference return

   References must refer to valid memory after function ends.

2. Step 2: Identify valid memory sources

   Static or global variables live beyond function scope, so returning their reference is safe.

3. Step 3: Evaluate other options

   Local variables and temporaries die after function ends; pointers don't guarantee safety here.

4. Final Answer:

   Return a reference to a static or global variable -> Option D

5. Quick Check:

   Safe reference return = static/global variable [OK]

Hint: Return references only to static/global variables [OK]

Common Mistakes:

• Returning references to local variables
• Confusing pointers with references
• Returning references to temporary objects