Multiple inheritance in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of multiple inheritance method call

What is the output of this Solidity contract when calling getValue() on Child?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract ParentA {
    function getValue() public pure virtual returns (string memory) {
        return "A";
    }
}

contract ParentB {
    function getValue() public pure virtual returns (string memory) {
        return "B";
    }
}

contract Child is ParentA, ParentB {
    function getValue() public pure override(ParentA, ParentB) returns (string memory) {
        return super.getValue();
    }
}

A Compilation error due to ambiguous super call

B "B"

C "A"

D "AB"

💡 Hint

In Solidity, the order of inheritance matters for super calls.

In Solidity, when multiple inheritance is used, the order of base contracts in the inheritance list determines which parent contract's method is called by super. Here, ParentA is listed first, so super.getValue() calls ParentA's version, returning "A".

❓ Predict Output

intermediate

Diamond problem resolution in Solidity

Given these contracts, what will Child.getValue() return?

Blockchain / Solidity

pragma solidity ^0.8.0;

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

contract B is A {
    function getValue() public pure virtual override returns (string memory) {
        return string(abi.encodePacked("B", super.getValue()));
    }
}

contract C is A {
    function getValue() public pure virtual override returns (string memory) {
        return string(abi.encodePacked("C", super.getValue()));
    }
}

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

A "BC"

B "BCA"

C Compilation error due to inheritance order

D "CBA"

💡 Hint

Remember Solidity uses C3 linearization for inheritance order.

Solidity uses C3 linearization to resolve inheritance. The order is Child -> B -> C -> A. Calling super.getValue() in Child calls B's getValue(), which calls C's getValue(), which calls A's getValue(). The concatenation results in "BCA".

🔧 Debug

advanced

Identify the error in multiple inheritance override

What error will this Solidity code produce when compiled?

Blockchain / Solidity

pragma solidity ^0.8.0;

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

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

contract Z is X, Y {
    function foo() public pure override returns (string memory) {
        return super.foo();
    }
}

A Compilation error: function foo must override both X and Y

B Runtime error: super.foo() is ambiguous

C Compilation error: override specifier missing base contracts

D No error, outputs "X"

💡 Hint

Check the override keyword and which base contracts are specified.

In Solidity, when overriding a function from multiple base contracts, you must specify all base contracts in the override keyword. Here, override() is missing the base contracts X and Y, causing a compilation error.

📝 Syntax

advanced

Correct syntax for multiple inheritance constructor call

Which option correctly calls constructors of both parents in Solidity?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract A {
    uint public a;
    constructor(uint _a) {
        a = _a;
    }
}

contract B {
    uint public b;
    constructor(uint _b) {
        b = _b;
    }
}

contract Child is A, B {
    constructor(uint _a, uint _b) A(_a) B(_b) {
    }
}

A constructor(uint _a, uint _b) A(_a) { B(_b); }

B constructor(uint _a, uint _b) { A(_a); B(_b); }

C constructor(uint _a, uint _b) { super(_a, _b); }

D constructor(uint _a, uint _b) A(_a) B(_b) {}

💡 Hint

Parent constructors are called in the inheritance list after the constructor signature.

In Solidity, parent constructors are called in the constructor header using the syntax ParentName(args). Option D correctly calls both A and B constructors. Options B and D try to call constructors inside the body, which is invalid. Option D tries to call super with two arguments, which is invalid.

🚀 Application

expert

Determine final state after multiple inheritance function calls

Given these contracts, what is the value of counter after calling Child.increment() once?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract CounterA {
    uint public counter;
    function increment() public virtual {
        counter += 1;
    }
}

contract CounterB {
    uint public counter;
    function increment() public virtual {
        counter += 10;
    }
}

contract Child is CounterA, CounterB {
    function increment() public override(CounterA, CounterB) {
        CounterA.increment();
        CounterB.increment();
    }
}

A Compilation error due to duplicate state variables

B 11

C 1

D 10

💡 Hint

Check how Solidity handles state variables with the same name in multiple inheritance.

Solidity does not allow multiple base contracts to have state variables with the same name because it causes storage layout conflicts. This code will fail to compile due to duplicate 'counter' variables in CounterA and CounterB.