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Understanding Constructor in Solidity

Understanding Constructors in Solidity

Constructor in Solidity plays a crucial role, similar to how a construction crew sets the foundation for a building. It’s a special function that gets called automatically only once – during the creation, or deployment, of your smart contract on the blockchain.

Think of it this way: when you design a blueprint for a house, you specify the number of rooms, their sizes, and maybe even the placement of certain features. A constructor in Solidity acts like that initial setup phase for your smart contract. It allows you to define the starting state of your contract by initializing its essential variables.

Here’s a breakdown of the key aspects of the constructor in Solidity:

The constructor keyword

The magic word that brings a constructor function to life is constructor. It’s placed right before the function declaration, indicating its special role in the contract’s creation process.

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Constructor in Solidity: Optional but essential

While not strictly mandatory, constructors are highly recommended. If you don’t define one explicitly, Solidity provides a default constructor that simply does nothing. However, for most practical purposes, you’ll want a custom constructor to ensure your contract starts in a well-defined state.

Constructor in Solidity: One and only

A contract can only have one constructor. This makes sense because the construction phase happens just once during deployment. So, you can’t have multiple constructors with different functionalities.

Initializing state variables

The primary purpose of a constructor is to assign initial values to your contract’s state variables. These variables hold the data that defines the current state of your smart contract. By initializing them in the constructor, you ensure they have meaningful values from the get-go.

For example, imagine you’re creating a smart contract for a simple crowdfunding campaign. You might have state variables like targetAmount (the goal amount to raise) and balance (the current amount collected). The constructor would be the perfect place to set the targetAmount and initialize balance to zero.

Constructor in Solidity: Function body and syntax

The body of a constructor function looks similar to other functions in Solidity. You can include logic, calculations, and even calls to other functions within the constructor. Here’s a basic syntax example:

constructor() public {
  // Initialization code for state variables
  targetAmount = 100 ether;
  balance = 0;

In this example, the constructor is public (we’ll discuss visibility later) and takes no arguments. Inside the curly braces, we have code to assign values to the state variables targetAmount and balance.

Taking it up a notch: Constructors with arguments

Constructors can also be defined to accept arguments. This allows you to provide custom initial values during deployment. Imagine the crowdfunding example again. What if you want to allow the campaign creator to specify the targetAmount during deployment? Here’s how you can achieve that:

constructor(uint target) {
  targetAmount = target;
  balance = 0;

In this case, the constructor takes a single argument of type uint (unsigned integer) named target. When someone deploys the contract, they can specify the desired targetAmount as a value during deployment.

Remember: When using arguments in the constructor, make sure to adjust the initialization code within the function body to use the provided values.

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Visibility Matters: Public, internal, or private?

Constructors can have different visibility modifiers, just like other functions in Solidity. Let’s see what they are and how they works using coding examples.

Public constructor

This is the most widely used type of constructor. It allows anyone to deploy the contract using this specific constructor.


contract SimpleStore {
  uint public value;

  constructor() public {
    value = 10;

In this example:

  • The contract is named SimpleStore.
  • It has a state variable named value of type uint (unsigned integer) with public visibility.
  • The constructor is also public, allowing anyone to deploy the contract using this constructor.
  • Inside the constructor, the value variable is initialized to 10.

Internal constructor

An internal constructor restricts deployment using this constructor to contracts within the same inheritance hierarchy. This is useful for creating base (parent) contracts that can’t be directly deployed but serve as a foundation for child contracts.


contract BaseContract {
  uint internal someData;

  constructor(uint data) internal {
    someData = data;

contract DerivedContract is BaseContract {
  constructor(uint data) public BaseContract(data) {}

Here’s a breakdown:

  • The BaseContract has an internal constructor that takes an argument of type uint named data.
  • Since it’s internal, this constructor can only be called by derived contracts within the same inheritance hierarchy.
  • The DerivedContract inherits from BaseContract.
  • Its constructor is public, allowing anyone to deploy the derived contract.
  • It explicitly calls the internal constructor of the parent class (BaseContract) using super(data) and passes the received argument data. This ensures proper initialization of the inherited variable someData.

Private constructor:

A private constructor makes the constructor unusable for deployment from any external contract. It’s rarely used for constructors but can be helpful in specific scenarios like creating helper contracts that are meant to be used internally by other contracts within the same project.


contract Factory {
  // Helper function to deploy another contract
  function deployContract() public {
    new PrivateContract(); // This can only be called within Factory

contract PrivateContract {
  constructor() private {}


  • The Factory contract has a public function deployContract.
  • Inside this function, it attempts to deploy a new instance of the PrivateContract using new PrivateContract().
  • However, the constructor of PrivateContract is marked as private. This means it can only be called from within the Factory contract itself, not from any external source during deployment.

By using different visibility specifiers for constructors, you can control who can deploy your contract and how. Remember to choose the appropriate visibility based on your specific use case and the desired level of access.

Beyond the basics: Advanced constructor concepts

While the core concepts are covered, there’s more to explore in the world of Solidity constructors. Let’s understand them in detail using the coding examples.

Constructor chaining

In inheritance scenarios, constructor chaining allows you to call the constructor of the parent contract from the child contract’s constructor. This ensures that the parent contract’s initialization logic is executed first, followed by the child contract’s specific initialization.


contract Ownable {
  address public owner;

  constructor() public {
    owner = msg.sender;

contract MyContract is Ownable {
  uint public value;

  constructor(uint val) public Ownable() {
    value = val;


  • We have two contracts: Ownable (parent) and MyContract (child).
  • Ownable has a state variable owner to store the address of the contract owner and a public constructor that sets the owner to the message sender (the person deploying the contract).
  • MyContract inherits from Ownable.
  • Its constructor takes an argument val of type uint.
  • Inside the MyContract constructor, we use super() to call the constructor of the parent contract (Ownable). This ensures that the owner variable gets initialized first in the inheritance hierarchy.
  • Then, the child contract’s specific initialization happens by assigning the value of val to its state variable value.

Payable constructor in Solidity

By marking a constructor as payable, you allow users to send Ether along with the deployment transaction. This can be useful for scenarios where the contract needs some initial funding upon deployment (e.g., for storage fees on the blockchain).


contract DonationPool {
  address public owner;
  uint public totalDonations;

  constructor() public payable {
    owner = msg.sender;
    totalDonations = msg.value;


  • This contract represents a donation pool.
  • It has state variables to track the owner (owner) and the total amount of donations received (totalDonations).
  • The constructor is marked as payable, allowing users to send Ether during deployment.
  • Inside the constructor, it sets the owner and then assigns the value of msg.value (the amount of Ether sent with the deployment transaction) to totalDonations. This ensures that the initial donation amount is captured correctly.

Important Note:

While payable constructors allow receiving funds, remember to include proper checks and logic within your contract to handle the received Ether securely and transparently.

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Constructors in Solidity are fundamental building blocks. By understanding how to use them effectively, you can ensure your smart contracts start in a well-defined state and are ready to function as intended.

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