Contract inheritance in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Contract inheritance

O(1)

Understanding Time Complexity

When using contract inheritance in blockchain, it's important to know how the execution time changes as contracts grow.

We want to see how adding inherited contracts affects the work the blockchain does.

Scenario Under Consideration

Analyze the time complexity of the following Solidity contract inheritance example.

pragma solidity ^0.8.0;

contract A {
    function foo() public pure returns (uint) {
        return 1;
    }
}

contract B is A {
    function bar() public pure returns (uint) {
        return 2;
    }
}

contract C is B {
    function baz() public pure returns (uint) {
        return 3;
    }
}

This code shows three contracts where each one inherits from the previous, adding new functions.

Identify Repeating Operations

Look for repeated actions that affect execution time.

• Primary operation: Calling functions from inherited contracts.
• How many times: Each function call runs once, and the function to call is determined at compile time, so no runtime chain checking is needed.

How Execution Grows With Input

As more contracts are inherited, the system does not check more places at runtime to find functions because the function calls are resolved at compile time.

Input Size (n)	Approx. Operations
1 contract	1 step to execute function
5 contracts	1 step to execute function
10 contracts	1 step to execute function

Pattern observation: The work to execute a function does not grow with the number of inherited contracts.

Final Time Complexity

Time Complexity: O(1)

This means the time to find and run a function remains constant regardless of how many contracts are inherited.

Common Mistake

[X] Wrong: "Inheritance does not affect execution time because functions are just copied over."

[OK] Correct: Actually, this is correct: function calls are resolved at compile time, so inheritance does not add runtime overhead in finding functions.

Interview Connect

Understanding how contract inheritance affects execution helps you write efficient blockchain code and explain your design choices clearly.

Self-Check

"What if we used multiple inheritance with several contracts at the same level? How would the time complexity change?"

Practice

1. What keyword is used in Solidity to inherit from a parent contract?

easy

A. extends

B. inherits

C. is

D. implements

Solution

1. Step 1: Understand Solidity inheritance syntax

   In Solidity, the keyword is is used to inherit from a parent contract.

2. Step 2: Compare with other options

   Keywords like extends, inherits, and implements are not used in Solidity for inheritance.

3. Final Answer:

   is -> Option C

4. Quick Check:

   Inheritance keyword = is [OK]

Hint: Remember: Solidity uses 'is' for inheritance [OK]

Common Mistakes:

• Confusing Solidity with other languages like Java using 'extends'
• Using 'implements' which is for interfaces
• Trying 'inherits' which is not a Solidity keyword

2. Which of the following is the correct way to declare a function in a parent contract that can be overridden in Solidity?

easy

A. function foo() public virtual {}

B. function foo() public abstract {}

C. function foo() public override {}

D. function foo() public final {}

Solution

1. Step 1: Identify the keyword to allow overriding

   In Solidity, to allow a function to be overridden in child contracts, it must be marked virtual.

2. Step 2: Check other options for correctness

   override is used in child contracts, abstract is for abstract contracts, and final is not a Solidity keyword.

3. Final Answer:

   function foo() public virtual {} -> Option A

4. Quick Check:

   Allow override = virtual [OK]

Hint: Use 'virtual' in parent to allow overrides [OK]

Common Mistakes:

• Using 'override' in parent instead of child
• Confusing 'abstract' with 'virtual'
• Using non-existent 'final' keyword

3. What will be the output of the following Solidity code when calling greet() on contract Child?

contract Parent {
    function greet() public virtual returns (string memory) {
        return "Hello from Parent";
    }
}

contract Child is Parent {
    function greet() public override returns (string memory) {
        return "Hello from Child";
    }
}

medium

A. Runtime error

B. "Hello from Parent"

C. Compilation error due to missing override

D. "Hello from Child"

Solution

1. Step 1: Understand function overriding

   The Child contract overrides the greet() function from Parent using override keyword.

2. Step 2: Determine which function runs on Child

   Calling greet() on Child runs the overridden function returning "Hello from Child".

3. Final Answer:

   "Hello from Child" -> Option D

4. Quick Check:

   Override function output = "Hello from Child" [OK]

Hint: Child's override function runs, not parent's [OK]

Common Mistakes:

• Thinking parent function runs instead of child
• Missing 'override' keyword causing compile error
• Expecting runtime error without cause

4. Identify the error in the following Solidity code snippet:

contract Base {
    function foo() public virtual returns (string memory) {
        return "Base";
    }
}

contract Derived is Base {
    function foo() public returns (string memory) {
        return "Derived";
    }
}

medium

A. Missing 'override' keyword in Derived's foo function

B. Missing 'virtual' keyword in Base's foo function

C. Derived contract should not inherit Base

D. Function foo should be private

Solution

1. Step 1: Check overriding rules in Solidity

   When overriding a function marked virtual in the parent, the child function must use override keyword.

2. Step 2: Identify missing keyword

   The foo() function in Derived lacks the override keyword, causing a compilation error.

3. Final Answer:

   Missing 'override' keyword in Derived's foo function -> Option A

4. Quick Check:

   Override missing = compile error [OK]

Hint: Override functions must use 'override' keyword [OK]

Common Mistakes:

• Forgetting 'override' keyword in child function
• Thinking 'virtual' is needed in child function
• Assuming inheritance itself causes error

5. Given the following contracts, what will be the output of calling getMessage() on contract GrandChild?

contract Parent {
    function getMessage() public virtual returns (string memory) {
        return "Parent";
    }
}

contract Child is Parent {
    function getMessage() public virtual override returns (string memory) {
        return "Child";
    }
}

contract GrandChild is Child {
    function getMessage() public override returns (string memory) {
        return super.getMessage();
    }
}

hard

A. "Parent"

B. "Child"

C. "GrandChild"

D. Compilation error due to missing virtual

Solution

1. Step 1: Understand super keyword in Solidity inheritance

   The GrandChild contract calls super.getMessage(), which refers to the immediate parent Child implementation.

2. Step 2: Identify which function runs

   The Child contract's getMessage() returns "Child", so GrandChild returns "Child" as well.

3. Final Answer:

   "Child" -> Option B

4. Quick Check:

   super calls immediate parent = "Child" [OK]

Hint: super calls immediate parent function [OK]

Common Mistakes:

• Thinking super calls grandparent function
• Expecting 'GrandChild' string without override
• Missing 'virtual' in intermediate contract causing error