Multiple inheritance in Blockchain / Solidity - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a contract that inherits from two contracts.

Blockchain / Solidity

contract MyContract is [1], ContractB {}

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

AContractC

BMyContract

CBaseContract

DContractA

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to list the first parent contract.

Using the child contract name instead of a parent contract.

The contract MyContract inherits from ContractA and ContractB. You must list the parent contracts after is.

2fill in blank

medium

Complete the code to call the constructor of the first parent contract.

Blockchain / Solidity

contract MyContract is ContractA, ContractB {
    constructor() [1]() {}
}

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

AContractA

Boverride

Csuper

Dpublic

Attempts:

3 left

💡 Hint

Common Mistakes

Using super instead of the parent contract's name.

Omitting the parentheses after the contract name.

To call a parent contract's constructor explicitly, write its name followed by parentheses.

3fill in blank

hard

Fix the error in the function override declaration for multiple inheritance.

Blockchain / Solidity

function foo() public view virtual override([1]) returns (string memory) {
    return "Hello";
}

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AContractB

BContractA, ContractB

CContractA

DMyContract

Attempts:

3 left

💡 Hint

Common Mistakes

Listing only one parent contract in the override.

Using the child contract name in the override list.

When overriding a function from multiple parent contracts, list all parent contracts in the override keyword separated by commas.

4fill in blank

hard

Fill both blanks to define a contract that inherits and overrides a function from two parents.

Blockchain / Solidity

contract MyContract is [1], [2] {
    function foo() public view virtual override(ContractA, ContractB) returns (string memory) {
        return "Hello from MyContract";
    }
}

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

AContractA

BMyContract

CContractB

DBaseContract

Attempts:

3 left

💡 Hint

Common Mistakes

Using the child contract name in the inheritance list.

Forgetting to list both parent contracts.

The contract inherits from ContractA and ContractB. The function foo overrides the versions from both parents.

5fill in blank

hard

Fill all three blanks to create a mapping with keys from one contract and values from another.

Blockchain / Solidity

mapping([1] => [2]) public data;

function setData([3] key, uint value) public {
    data[key] = value;
}

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

Aaddress

Buint

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using mismatched types for mapping keys and function parameters.

Using string instead of address for keys.

The mapping uses address as keys and uint as values. The function parameter key is of type address.

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