Understand the language basics

  • 8 minutes

All Solidity contracts typically include:

  • Pragma directives
  • State variables
  • Functions
  • Events

While there's more you'll need to know to program production level smart contracts, these things should get you off on the right foot.

If you have an understanding of these concepts, you can get started writing smart contracts for a variety of use cases right away!

Pragma directives

Pragma is the keyword that you use to ask the compiler to check whether its version of Solidity matches the one required. A match means your source file can run successfully. If it doesn't match, the compiler will give an error.

Always make sure to include the latest version of Solidity in your contract definition. To find the current version of Solidity, visit the Solidity website. Use the most recent version in your source file.

A version pragma directive looks like:

pragma solidity ^0.7.0;

This line means that the source file will compile with a compiler greater than the version 0.7.0, up to 0.7.9. Starting in version 0.8.0 there will likely be breaking changes introduced that the source file can't compile successfully.

State variables

State variables are key to any Solidity source file. State variable values are stored in contract storage permanently.

pragma solidity >0.7.0 <0.8.0;

contract Marketplace {
    uint price; // State variable

Note

Contract source files always start with the definition contract ContractName.

In this example, the state variable is named price with type uint. The integer type uint indicates that this variable is an unsigned integer with 256 bits. That means it can store positive numbers in the range of 0 to 2256 -1.

For all variable definitions, you must specify the type and the variable name.

Additionally, you can specify the visibility of a state variable as:

  • public: part of the contract interface and can be accessed from other contracts.
  • internal: only accessed internally from the current contract.
  • private: only visible for the contract it's defined in.

Functions

Within a contract, executable units of code are known as functions. Functions describe a single action for achieving one task. They're reusable and can also be called from other source files like libraries. Functions in Solidity behave similarly to functions in other programming languages.

Here's a basic example of defining a function:

pragma solidity >0.7.0 <0.8.0;

contract Marketplace {
    function buy() public {
        // ...
    }
}

This code shows a function with the name buy that has a public visibility, meaning that it can be accessed by other contracts. Functions can use one of the following visibility specifiers: public, private, internal, and external.

A function can be called internally or externally from another contract. Functions can accept parameters and return variables to pass parameters and values between them.

Here's an example of a function that accepts a parameter - an integer called price - and returns an integer:

pragma solidity >0.7.0 <0.8.0;

contract Marketplace {
    function buy(uint price) public returns (uint) {
        // ...
    }
}

Function modifiers

Function modifiers can be used to change the behavior of functions. They work by checking a condition before the function executes. For example, a function could check that only a user designated as a seller can list an item for sale.

pragma solidity >0.7.0 <0.8.0;

contract Marketplace {
    address public seller;

    modifier onlySeller() {
        require(
            msg.sender == seller,
            "Only seller can put an item up for sale."
        );
        _;
    }

    function listItem() public view onlySeller {
        // ...
    }
}

This example introduces the following items:

  • A variable with a type address that stores the 20-byte Ethereum address of the seller user. You'll learn more about these variables later in this module.
  • A modifier called onlySeller that describes that only a seller can list an item.
  • A special symbol _; to indicate where the function body gets inserted.
  • A function definition that uses the modifier onlySeller.

Other function modifiers that can be used in the function definition are:

  • pure to describe functions that don't allow modifications or access of state.
  • view to describe functions that don't allow modifications of state.
  • payable to describe functions that can receive Ether.

Events

Events describe actions that are taken in the contract. Similar to functions, events have parameters that need to be specified when the event is called.

To call an event, you must use the keyword emit with the event name and its parameters.

pragma solidity >0.7.0 <0.8.0;

contract Marketplace {
    event PurchasedItem(address buyer, uint price);

    function buy() public {
        // ...
        emit PurchasedItem(msg.sender, msg.value);
    }
}

When you call an event, the event is captured as a transaction in the transaction log, which is a special data structure in the blockchain. These logs are associated with the address of the contract, are incorporated into the blockchain, and stay there forever. The log and its event data aren't accessible from within contracts, and it can’t be modified.