Layout of a Solidity Source File

Source files can contain an arbitrary number of contract definitions, include directives and pragma directives.

Version Pragma

Source files can (and should) be annotated with a so-called version pragma to reject being compiled with future compiler versions that might introduce incompatible changes. We try to keep such changes to an absolute minimum and especially introduce changes in a way that changes in semantics will also require changes in the syntax, but this is of course not always possible. Because of that, it is always a good idea to read through the changelog at least for releases that contain breaking changes, those releases will always have versions of the form 0.x.0 or x.0.0.

The version pragma is used as follows:

pragma solidity ^0.4.0;

Such a source file will not compile with a compiler earlier than version 0.4.0 and it will also not work on a compiler starting form version 0.5.0 (this second condition is added by using ^). The idea behind this is that there will be no breaking changes until version 0.5.0, so we can always be sure that our code will compile the way we intended it to. We do not fix the exact version of the compiler, so that bugfix releases are still possible.

It is possible to specify much more complex rules for the compiler version, the expression follows those used by npm.

Importing other Source Files

Syntax and Semantics

Solidity supports import statements that are very similar to those available in JavaScript (from ES6 on), although Solidity does not know the concept of a “default export”.

At a global level, you can use import statements of the following form:

import "filename";

This statement imports all global symbols from “filename” (and symbols imported there) into the current global scope (different than in ES6 but backwards-compatible for Solidity).

import * as symbolName from "filename";

...creates a new global symbol symbolName whose members are all the global symbols from "filename".

import {symbol1 as alias, symbol2} from "filename";

...creates new global symbols alias and symbol2 which reference symbol1 and symbol2 from "filename", respectively.

Another syntax is not part of ES6, but probably convenient:

import "filename" as symbolName;

which is equivalent to import * as symbolName from "filename";.

Paths

In the above, filename is always treated as a path with / as directory separator, . as the current and .. as the parent directory. Path names that do not start with . are treated as absolute paths.

To import a file x from the same directory as the current file, use import "./x" as x;. If you use import "x" as x; instead, a different file could be referenced (in a global “include directory”).

It depends on the compiler (see below) how to actually resolve the paths. In general, the directory hierarchy does not need to strictly map onto your local filesystem, it can also map to resources discovered via e.g. ipfs, http or git.

Use in Actual Compilers

When the compiler is invoked, it is not only possible to specify how to discover the first element of a path, but it is possible to specify path prefix remappings so that e.g. github.com/ethereum/dapp-bin/library is remapped to /usr/local/dapp-bin/library and the compiler will read the files from there. If remapping keys are prefixes of each other, the longest is tried first. This allows for a “fallback-remapping” with e.g. "" maps to "/usr/local/include/solidity". Furthermore, these remappings can depend on the context, which allows you to configure packages to import e.g. different versions of a library of the same name.

solc:

For solc (the commandline compiler), these remappings are provided as context:prefix=target arguments, where both the context: and the =target parts are optional (where target defaults to prefix in that case). All remapping values that are regular files are compiled (including their dependencies). This mechanism is completely backwards-compatible (as long as no filename contains = or :) and thus not a breaking change. All imports in files in or below the directory context that import a file that starts with prefix are redirected by replacing prefix by target.

So as an example, if you clone github.com/ethereum/dapp-bin/ locally to /usr/local/dapp-bin, you can use the following in your source file:

import "github.com/ethereum/dapp-bin/library/iterable_mapping.sol" as it_mapping;

and then run the compiler as

solc github.com/ethereum/dapp-bin/=/usr/local/dapp-bin/ source.sol

As a more complex example, suppose you rely on some module that uses a very old version of dapp-bin. That old version of dapp-bin is checked out at /usr/local/dapp-bin_old, then you can use

solc module1:github.com/ethereum/dapp-bin/=/usr/local/dapp-bin/ \
     module2:github.com/ethereum/dapp-bin/=/usr/local/dapp-bin_old/ \
     source.sol

so that all imports in module2 point to the old version but imports in module1 get the new version.

Note that solc only allows you to include files from certain directories: They have to be in the directory (or subdirectory) of one of the explicitly specified source files or in the directory (or subdirectory) of a remapping target. If you want to allow direct absolute includes, just add the remapping =/.

If there are multiple remappings that lead to a valid file, the remapping with the longest common prefix is chosen.

browser-solidity:

The browser-based compiler provides an automatic remapping for github and will also automatically retrieve the file over the network: You can import the iterable mapping by e.g. import "github.com/ethereum/dapp-bin/library/iterable_mapping.sol" as it_mapping;.

Other source code providers may be added in the future.

Comments

Single-line comments (//) and multi-line comments (/*...*/) are possible.

// This is a single-line comment.

/*
This is a
multi-line comment.
*/

Additionally, there is another type of comment called a natspec comment, for which the documentation is not yet written. They are written with a triple slash (///) or a double asterisk block(/** ... */) and they should be used directly above function declarations or statements. You can use Doxygen-style tags inside these comments to document functions, annotate conditions for formal verification, and provide a confirmation text which is shown to users when they attempt to invoke a function.

In the following example we document the title of the contract, the explanation for the two input parameters and two returned values.

   pragma solidity ^0.4.0;

/** @title Shape calculator.*/
contract shapeCalculator{
    /**@dev Calculates a rectangle's surface and perimeter.
     * @param w Width of the rectangle.
     * @param h Height of the rectangle.
     * @return s The calculated surface.
     * @return p The calculated perimeter.
     */
    function rectangle(uint w, uint h) returns (uint s, uint p) {
        s = w * h;
        p = 2 * (w + h);
    }
}