Building a Smart Contract with Solidity

Solidity was created by Dr. Gavin Wood (coauthor of this book) as a language explicitly for writing smart contracts with features to directly support execution in the decentralized environment of the Ethereum world computer. The resulting attributes are quite general, and so it has ended up being used for coding smart contracts on several other blockchain platforms. It was developed by Christian Reitiwessner and then also by Alex Beregszaszi, Liana Husikyan, Yoichi Hirai, and several former Ethereum core contributors. Solidity is now developed and maintained as an independent project on GitHub.

The main “product” of the Solidity project is the Solidity compiler, solc, which converts programs written in the Solidity language to EVM bytecode. The project also manages the important application binary interface (ABI) standard for Ethereum smart contracts, which we will explore in detail in this chapter. Each version of the Solidity compiler corresponds to and compiles a specific version of the Solidity language.

To get started, we will download a binary executable of the Solidity compiler. Then we will develop and compile a simple contract, following on from the example we started with in [intro_chapter].

Selecting a Version of Solidity

Solidity follows a versioning model called semantic versioning, which specifies version numbers structured as three numbers separated by dots: MAJOR.MINOR.PATCH. The “major” number is incremented for major and backward-incompatible changes, the “minor” number is incremented as backward-compatible features are added in between major releases, and the “patch” number is incremented for backward-compatible bug fixes.

At the time of writing, Solidity is at version 0.4.24. The rules for major version 0, which is for initial development of a project, are different: anything may change at any time. In practice, Solidity treats the “minor” number as if it were the major version and the “patch” number as if it were the minor version. Therefore, in 0.4.24, 4 is considered to be the major version and 24 the minor version.

The 0.5 major version release of Solidity is anticipated imminently.

As you saw in [intro_chapter], your Solidity programs can contain a pragma directive that specifies the minimum and maximum versions of Solidity that it is compatible with, and can be used to compile your contract.

Since Solidity is rapidly evolving, it is often better to install the latest release.

Download and Install

There are a number of methods you can use to download and install Solidity, either as a binary release or by compiling from source code. You can find detailed instructions in the Solidity documentation.

Here’s how to install the latest binary release of Solidity on an Ubuntu/Debian operating system, using the apt package manager:

$ sudo add-apt-repository ppa:ethereum/ethereum
$ sudo apt update
$ sudo apt install solc

Once you have solc installed, check the version by running:

$ solc --version
solc, the solidity compiler commandline interface
Version: 0.4.24+commit.e67f0147.Linux.g++

There are a number of other ways to install Solidity, depending on your operating system and requirements, including compiling from the source code directly. For more information see https://github.com/ethereum/solidity.

Development Environment

To develop in Solidity, you can use any text editor and solc on the command line. However, you might find that some text editors designed for development, such as Emacs, Vim, and Atom, offer additional features such as syntax highlighting and macros that make Solidity development easier.

There are also web-based development environments, such as Remix IDE and EthFiddle.

Use the tools that make you productive. In the end, Solidity programs are just plain text files. While fancy editors and development environments can make things easier, you don’t need anything more than a simple text editor, such as nano (Linux/Unix), TextEdit (macOS), or even NotePad (Windows). Simply save your program source code with a .sol extension and it will be recognized by the Solidity compiler as a Solidity program.

Writing a Simple Solidity Program

In [intro_chapter], we wrote our first Solidity program. When we first built the Faucet contract, we used the Remix IDE to compile and deploy the contract. In this section, we will revisit, improve, and embellish Faucet.

Our first attempt looked like Faucet.sol: A Solidity contract implementing a faucet.

Example 1. Faucet.sol: A Solidity contract implementing a faucet

link:code/Solidity/Faucet.sol[]

Compiling with the Solidity Compiler (solc)

Now, we will use the Solidity compiler on the command line to compile our contract directly. The Solidity compiler solc offers a variety of options, which you can see by passing the —help argument.

We use the —bin and —optimize arguments of solc to produce an optimized binary of our example contract:

$ solc --optimize --bin Faucet.sol
======= Faucet.sol:Faucet =======
Binary:
6060604052341561000f57600080fd5b60cf8061001d6000396000f300606060405260043610603e5
763ffffffff7c01000000000000000000000000000000000000000000000000000000006000350416
632e1a7d4d81146040575b005b3415604a57600080fd5b603e60043567016345785d8a00008111156
06357600080fd5b73ffffffffffffffffffffffffffffffffffffffff331681156108fc0282604051
600060405180830381858888f19350505050151560a057600080fd5b505600a165627a7a723058203
556d79355f2da19e773a9551e95f1ca7457f2b5fbbf4eacf7748ab59d2532130029

The result that solc produces is a hex-serialized binary that can be submitted to the Ethereum blockchain.