Multiple inheritance in Blockchain / Solidity - Time & Space Complexity

Time Complexity: Multiple inheritance

O(n)

Understanding Time Complexity

When using multiple inheritance in blockchain smart contracts, it's important to understand how the execution time grows as contracts combine features from several parents.

We want to know how the cost changes when calling functions inherited from multiple contracts.

Scenario Under Consideration

Analyze the time complexity of the following Solidity code snippet.

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

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

contract C is A, B {
    function combined() public pure returns (uint) {
        return foo() + bar();
    }
}
    

This code shows a contract C inheriting from two contracts A and B, then calling functions from both.

Identify Repeating Operations

Look for repeated actions that affect execution time.

• Primary operation: Calling inherited functions foo() and bar().
• How many times: Each function is called once per combined() call.

How Execution Grows With Input

As the number of inherited contracts grows, the number of function calls inside combined() can increase.

Number of Parents (n)	Approx. Operations
2	2 function calls
5	5 function calls
10	10 function calls

Pattern observation: Execution time grows linearly with the number of inherited contracts called.

Final Time Complexity

Time Complexity: O(n)

This means the execution time increases in a straight line as you add more inherited contracts with functions to call.

Common Mistake

[X] Wrong: "Multiple inheritance makes execution time jump exponentially because of all the parents combined."

[OK] Correct: Each inherited function is called separately, so the time adds up linearly, not exponentially.

Interview Connect

Understanding how multiple inheritance affects execution time helps you write efficient smart contracts and explain your design choices clearly.

Self-Check

"What if the inherited functions called inside combined() themselves call other inherited functions? How would the time complexity change?"

Practice

1.

What is the main purpose of multiple inheritance in blockchain smart contracts?

easy

A. To restrict a contract to inherit from only one contract

B. To allow a contract to use code from more than one contract

C. To make contracts run faster on the blockchain

D. To prevent contracts from sharing any code

Solution

1. Step 1: Understand inheritance in contracts

   Inheritance allows a contract to reuse code from another contract.

2. Step 2: Identify multiple inheritance meaning

   Multiple inheritance means inheriting from more than one contract to combine features.

3. Final Answer:

   To allow a contract to use code from more than one contract -> Option B

4. Quick Check:

   Multiple inheritance = use code from multiple contracts [OK]

Hint: Multiple inheritance means using code from many contracts [OK]

Common Mistakes:

• Thinking multiple inheritance restricts to one contract
• Confusing inheritance with contract speed
• Believing inheritance prevents code sharing

2.

Which of the following is the correct syntax to inherit from two contracts A and B in Solidity?

contract C is ? {}

easy

A. A + B

B. A & B

C. A | B

D. A, B

Solution

1. Step 1: Recall Solidity inheritance syntax

   In Solidity, multiple inheritance uses commas to separate parent contracts.

2. Step 2: Check the options

   Only A, B uses commas correctly: contract C is A, B {}.

3. Final Answer:

   A, B -> Option D

4. Quick Check:

   Use commas to list parents in Solidity inheritance [OK]

Hint: Use commas to list multiple parents in Solidity [OK]

Common Mistakes:

• Using symbols like & or + instead of commas
• Forgetting to separate parent contracts
• Writing inheritance without any separator

3.

Consider the following Solidity contracts:

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

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

contract C is A, B {
    function greet() public pure override(A, B) returns (string memory) {
        return super.greet();
    }
}

What will C().greet() return?

medium

A. "Hello from A"

B. "Hello from C"

C. Compilation error due to ambiguity

D. "Hello from B"

Solution

1. Step 1: Understand method resolution order

   In Solidity, the order of inheritance matters. The first parent contract's function is called by super.

2. Step 2: Check inheritance order and super call

   Contract C inherits A then B. Calling super.greet() calls A's greet due to C3 linearization order C, A, B.

3. Final Answer:

   "Hello from A" -> Option A

4. Quick Check:

   First parent function called by super = "Hello from A" [OK]

Hint: In Solidity, super calls first inherited (leftmost) contract's method [OK]

Common Mistakes:

• Assuming super calls last parent
• Expecting compilation error without override
• Thinking super calls child's own method

4.

Given the contracts below, what is the error in contract C?

contract A {
    function foo() public pure returns (string memory) {
        return "A";
    }
}

contract B {
    function foo() public pure returns (string memory) {
        return "B";
    }
}

contract C is A, B {
    // Missing override keyword
    function foo() public pure returns (string memory) {
        return super.foo();
    }
}

medium

A. Missing override keyword causes compilation error

B. super.foo() calls an undefined function

C. No error, code compiles and runs fine

D. Function foo() must be private

Solution

1. Step 1: Check function overriding rules

   When overriding functions from multiple parents, Solidity requires the override keyword.

2. Step 2: Identify missing override keyword

   Contract C's foo lacks override(A, B), causing a compilation error.

3. Final Answer:

   Missing override keyword causes compilation error -> Option A

4. Quick Check:

   Override keyword required when overriding multiple parents [OK]

Hint: Always add override keyword when overriding multiple parents [OK]

Common Mistakes:

• Forgetting override keyword in child function
• Assuming super.foo() is undefined
• Thinking function visibility must be private

5.

You have two contracts:

contract Token {
    function transfer() public pure virtual returns (string memory) {
        return "Token transfer";
    }
}

contract Mintable {
    function transfer() public pure virtual returns (string memory) {
        return "Mintable transfer";
    }
}

contract MyToken is Token, Mintable {
    function transfer() public pure override(Token, Mintable) returns (string memory) {
        return string.concat(Token.transfer(), " & ", Mintable.transfer());
    }
}

What will MyToken().transfer() return?

hard

A. "Mintable transfer & Token transfer"

B. Compilation error due to ambiguous override

C. "Token transfer & Mintable transfer"

D. "Token transfer"

Solution

1. Step 1: Understand explicit calls to parent functions

   MyToken's transfer calls both Token.transfer() and Mintable.transfer() explicitly.

2. Step 2: Check string concatenation order

   The returned string concatenates Token.transfer() first, then Mintable.transfer() with " & " in between.

3. Final Answer:

   "Token transfer & Mintable transfer" -> Option C

4. Quick Check:

   Explicit calls combine both parents' transfer strings [OK]

Hint: Explicitly call each parent to combine their outputs [OK]

Common Mistakes:

• Assuming super calls only one parent
• Expecting compilation error without override
• Mixing order of concatenation