Classes and objects in C++ - Time & Space Complexity

Time Complexity: Classes and objects

O(n)

Understanding Time Complexity

When working with classes and objects, it's important to know how the time to run your program changes as you create and use more objects.

We want to see how the program's speed changes when the number of objects grows.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Box {
public:
    int length;
    Box(int l) : length(l) {}
    int getLength() { return length; }
};

void printLengths(Box boxes[], int n) {
    for (int i = 0; i < n; i++) {
        std::cout << boxes[i].getLength() << std::endl;
    }
}
    

This code defines a simple class and prints the length of each Box object in an array.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through each Box object to call getLength()
• How many times: Exactly once for each object in the array (n times)

How Execution Grows With Input

As the number of Box objects increases, the program does more work by calling getLength() for each one.

Input Size (n)	Approx. Operations
10	10 calls to getLength()
100	100 calls to getLength()
1000	1000 calls to getLength()

Pattern observation: The number of operations grows directly with the number of objects.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the code grows in a straight line as you add more objects.

Common Mistake

[X] Wrong: "Creating objects or calling methods inside a loop always takes constant time no matter how many objects there are."

[OK] Correct: Each object and method call adds work, so more objects mean more time spent.

Interview Connect

Understanding how loops over objects affect time helps you explain your code clearly and shows you know how programs scale.

Self-Check

"What if the getLength() method did a complex calculation instead of just returning a value? How would the time complexity change?"

Practice

1. What is the main purpose of a class in C++?

easy

A. To group data and functions that represent a real-world concept

B. To execute a program faster

C. To store only numbers

D. To replace variables with constants

Solution

1. Step 1: Understand what a class represents

   A class groups related data (variables) and functions (methods) to model real-world things.

2. Step 2: Compare options with the definition

   Only To group data and functions that represent a real-world concept correctly describes this purpose; others are unrelated to classes.

3. Final Answer:

   To group data and functions that represent a real-world concept -> Option A

4. Quick Check:

   Class = group data + functions [OK]

Hint: Classes bundle data and actions together [OK]

Common Mistakes:

• Thinking classes only store numbers
• Confusing classes with program speed
• Believing classes replace variables

2. Which of the following is the correct way to declare a class named Car in C++?

easy

A. class Car { };

B. Car class { };

C. class = Car { };

D. class Car();

Solution

1. Step 1: Recall C++ class declaration syntax

   The correct syntax is: class ClassName { }; with a semicolon at the end.

2. Step 2: Check each option

   class Car { }; matches the correct syntax. Options A, C, and D have syntax errors or wrong order.

3. Final Answer:

   class Car { }; -> Option A

4. Quick Check:

   class ClassName { }; syntax [OK]

Hint: Remember: class name follows 'class' keyword, ends with semicolon [OK]

Common Mistakes:

• Writing 'Car class' instead of 'class Car'
• Missing semicolon after class body
• Using parentheses after class name

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

class Box {
public:
  int length;
  void setLength(int len) { length = len; }
  int getLength() { return length; }
};

int main() {
  Box box1;
  box1.setLength(10);
  std::cout << box1.getLength();
  return 0;
}

medium

A. length

B. 0

C. Compilation error

D. 10

Solution

1. Step 1: Understand the class methods

   The method setLength sets the length to 10 for box1. The method getLength returns this value.

2. Step 2: Trace the output statement

   std::cout << box1.getLength(); prints the value 10 stored in length.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   setLength(10) then getLength() = 10 [OK]

Hint: Setter sets value, getter returns it [OK]

Common Mistakes:

• Forgetting to call setLength before getLength
• Expecting default value 0 without initialization
• Confusing method names

4. Identify the error in this class definition:

class Person {
  string name;
  int age;
public:
  void setName(string n) { name = n; }
  void setAge(int a) { age = a; }
};

medium

A. Missing semicolon after class definition

B. Missing public: before data members

C. Missing #include <string> header

D. Incorrect function return types

Solution

1. Step 1: Check class syntax

   The class ends with a semicolon, and functions have correct return types (void). Data members are private by default, which is allowed.

2. Step 2: Check for missing includes

   The code uses string but does not include <string> header, which is required for std::string.

3. Final Answer:

   Missing #include <string> header -> Option C

4. Quick Check:

   Use #include <string> for string type [OK]

Hint: Always include headers for standard types [OK]

Common Mistakes:

• Assuming private members need public keyword
• Forgetting to add semicolon after class
• Confusing function return types

5. Given this class:

class Student {
  int id;
  string name;
public:
  Student(int i, string n) : id(i), name(n) {}
  void display() {
    std::cout << "ID: " << id << ", Name: " << name << std::endl;
  }
};

int main() {
  Student s1(101, "Alice");
  s1.display();
  return 0;
}

What concept does this code demonstrate?

hard

A. Inheritance between classes

B. Constructor initializing object data

C. Function overloading

D. Static class members

Solution

1. Step 1: Identify the special function in the class

   The function Student(int i, string n) : id(i), name(n) {} is a constructor initializing the object's data members.

2. Step 2: Check other options

   There is no inheritance, no function overloading, and no static members shown.

3. Final Answer:

   Constructor initializing object data -> Option B

4. Quick Check:

   Constructor sets initial values [OK]

Hint: Look for function named as class for constructor [OK]

Common Mistakes:

• Confusing constructor with regular method
• Thinking this shows inheritance
• Assuming static members without keyword