Constructor function in Blockchain / Solidity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Solidity contract's constructor?

Consider the following Solidity contract. What will be the value of owner after deployment?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract MyContract {
    address public owner;

    constructor() {
        owner = msg.sender;
    }
}

A The <code>owner</code> will be a random address generated by the constructor.

B The <code>owner</code> will be the zero address (0x0000000000000000000000000000000000000000).

C The <code>owner</code> will be the address of the contract itself.

D The <code>owner</code> will be the address of the account that deployed the contract.

💡 Hint

Remember that msg.sender in the constructor is the account deploying the contract.

In Solidity, the constructor runs once when the contract is deployed. The msg.sender during deployment is the address that creates the contract, so owner is set to that address.

❓ Predict Output

intermediate

What happens if a Solidity contract has multiple constructors?

Examine this Solidity contract snippet. What will happen when compiling?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Test {
    constructor() {
        // First constructor
    }

    constructor(uint x) {
        // Second constructor
    }
}

A The contract will compile and use the constructor with parameters by default.

B The contract will compile and use the constructor with no parameters by default.

C Compilation error: Solidity does not allow multiple constructors.

D The contract will compile and randomly pick one constructor at deployment.

💡 Hint

Solidity only supports one constructor per contract.

Solidity contracts can have only one constructor. Defining multiple constructors causes a compilation error.

🔧 Debug

advanced

Why does this Solidity contract fail to set the owner in the constructor?

Look at this contract. Why is owner still zero after deployment?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Broken {
    address public owner;

    function Broken() public {
        owner = msg.sender;
    }
}

A The function named <code>Broken</code> is not a constructor in Solidity 0.8.0, so it is a normal function and never called automatically.

B The <code>msg.sender</code> is not available in constructors.

C The <code>owner</code> variable is private and cannot be set.

D The contract has a syntax error because the constructor must be external.

💡 Hint

In Solidity versions 0.7.0 and later, constructors use the constructor keyword.

Before Solidity 0.7.0, constructors were functions named after the contract. From 0.7.0 onwards, the constructor keyword must be used. Here, Broken() is a normal function, not a constructor, so owner remains unset.

🧠 Conceptual

advanced

What is the purpose of a constructor in a smart contract?

Choose the best description of what a constructor does in a smart contract.

A It initializes the contract's state variables once when the contract is deployed.

B It runs every time a function is called in the contract.

C It automatically destroys the contract after deployment.

D It allows users to send Ether to the contract after deployment.

💡 Hint

Think about what happens only once when a contract is created.

A constructor runs only once during deployment to set initial values or perform setup tasks.

❓ Predict Output

expert

What is the output of this Solidity contract with inheritance and constructors?

Given these contracts, what will be the value of value in Child after deployment?

Blockchain / Solidity

pragma solidity ^0.8.0;

contract Parent {
    uint public value;

    constructor(uint _value) {
        value = _value;
    }
}

contract Child is Parent {
    constructor() Parent(42) {
        value = value + 1;
    }
}

A 42

B 43

C 0

D Compilation error due to constructor syntax

💡 Hint

The child constructor calls the parent constructor with 42, then adds 1.

The Parent constructor sets value to 42. Then the Child constructor adds 1, so final value is 43.