Layout of a Solidity Source File

Source files can contain an arbitrary number of contract definitions, import directives, pragma directives and struct, enum, function and constant variable definitions.

SPDX License Identifier

Trust in smart contract can be better established if their source code is available. Since making source code available always touches on legal problems with regards to copyright, the Solidity compiler encourages the use of machine-readable SPDX license identifiers. Every source file should start with a comment indicating its license:

// SPDX-License-Identifier: MIT

The compiler does not validate that the license is part of the list allowed by SPDX, but it does include the supplied string in the bytecode metadata.

If you do not want to specify a license or if the source code is not open-source, please use the special value UNLICENSED.

Supplying this comment of course does not free you from other obligations related to licensing like having to mention a specific license header in each source file or the original copyright holder.

The comment is recognized by the compiler anywhere in the file at the file level, but it is recommended to put it at the top of the file.

More information about how to use SPDX license identifiers can be found at the SPDX website.


The pragma keyword is used to enable certain compiler features or checks. A pragma directive is always local to a source file, so you have to add the pragma to all your files if you want enable it in your whole project. If you import another file, the pragma from that file does not automatically apply to the importing file.

Version Pragma

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

The version pragma is used as follows: pragma solidity ^0.5.2;

A source file with the line above does not compile with a compiler earlier than version 0.5.2, and it also does not work on a compiler starting from version 0.6.0 (this second condition is added by using ^). Because there will be no breaking changes until version 0.6.0, you can be sure that your code compiles the way you intended. The exact version of the compiler is not fixed, so that bugfix releases are still possible.

It is possible to specify more complex rules for the compiler version, these follow the same syntax used by npm.


Using the version pragma does not change the version of the compiler. It also does not enable or disable features of the compiler. It just instructs the compiler to check whether its version matches the one required by the pragma. If it does not match, the compiler issues an error.

Experimental Pragma

The second pragma is the experimental pragma. It can be used to enable features of the compiler or language that are not yet enabled by default. The following experimental pragmas are currently supported:


The new ABI encoder is able to encode and decode arbitrarily nested arrays and structs. It might produce less optimal code and has not received as much testing as the old encoder, but is considered non-experimental as of Solidity 0.6.0. You still have to explicitly activate it using pragma experimental ABIEncoderV2; - we kept the same pragma, even though it is not considered experimental anymore.

The set of types supported by the new encoder is a strict superset of the ones supported by the old one. Contracts that use it can interact with ones that do not without limitations. The reverse is possible only as long as the non-ABIEncoderV2 contract does not try to make calls that would require decoding types only supported by the new encoder. The compiler can detect this and will issue an error. Simply enabling ABIEncoderV2 for your contract is enough to make the error go away.


This pragma applies to all the code defined in the file where it is activated, regardless of where that code ends up eventually. This means that a contract without the ABIEncoderV2 pragma can still contain code that uses the new encoder by inheriting it from another contract. This is allowed if the new types are only used internally and not in external function signatures.


This component has to be enabled when the Solidity compiler is built and therefore it is not available in all Solidity binaries. The build instructions explain how to activate this option. It is activated for the Ubuntu PPA releases in most versions, but not for the Docker images, Windows binaries or the statically-built Linux binaries. It can be activated for solc-js via the smtCallback if you have an SMT solver installed locally and run solc-js via node (not via the browser).

If you use pragma experimental SMTChecker;, then you get additional safety warnings which are obtained by querying an SMT solver. The component does not yet support all features of the Solidity language and likely outputs many warnings. In case it reports unsupported features, the analysis may not be fully sound.

Importing other Source Files

Syntax and Semantics

Solidity supports import statements to help modularise your code that are similar to those available in JavaScript (from ES6 on). However, Solidity does not support 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). This form is not recommended for use, because it unpredictably pollutes the namespace. If you add new top-level items inside “filename”, they automatically appear in all files that import like this from “filename”. It is better to import specific symbols explicitly.

The following example creates a new global symbol symbolName whose members are all the global symbols from "filename":

import * as symbolName from "filename";

which results in all global symbols being available in the format symbolName.symbol.

A variant of this syntax that is not part of ES6, but possibly useful is:

import "filename" as symbolName;

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

If there is a naming collision, you can rename symbols while importing. For example, the code below creates new global symbols alias and symbol2 which reference symbol1 and symbol2 from inside "filename", respectively.

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


In the above, filename is always treated as a path with / as directory separator, and . as the current and .. as the parent directory. When . or .. is followed by a character except /, it is not considered as the current or the parent directory. All path names are treated as absolute paths unless they start with the current . or the parent directory ...

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

It depends on the compiler (see Use in Actual Compilers) how to actually resolve the paths. In general, the directory hierarchy does not need to strictly map onto your local filesystem, and the path can also map to resources such as ipfs, http or git.


Always use relative imports like import "./filename.sol"; and avoid using .. in path specifiers. In the latter case, it is probably better to use global paths and set up remappings as explained below.

Use in Actual Compilers

When invoking the compiler, you can specify how to discover the first element of a path, and also path prefix remappings. For example you can setup a remapping so that everything imported from the virtual directory would actually be read from your local directory /usr/local/dapp-bin/library. If multiple remappings apply, the one with the longest key is tried first. An empty prefix is not allowed. The remappings can depend on a context, which allows you to configure packages to import e.g., different versions of a library of the same name.


For solc (the commandline compiler), you provide these path remappings as context:prefix=target arguments, where both the context: and the =target parts are optional (target defaults to prefix in this case). All remapping values that are regular files are compiled (including their dependencies).

This mechanism is backwards-compatible (as long as no filename contains = or :) and thus not a breaking change. All files in or below the context directory that import a file that starts with prefix are redirected by replacing prefix by target.

For example, if you clone locally to /usr/local/dapp-bin, you can use the following in your source file:

import "" as it_mapping;

Then run the compiler:

solc source.sol

As a more complex example, suppose you rely on a module that uses an old version of dapp-bin that you checked out to /usr/local/dapp-bin_old, then you can run:

solc \ \

This means that all imports in module2 point to the old version but imports in module1 point to the new version.


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, add the remapping /=/.

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


Remix provides an automatic remapping for GitHub and automatically retrieves the file over the network. You can import the iterable mapping as above, e.g.

import "" as it_mapping;

Remix may add other source code providers in the future.


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

// This is a single-line comment.

This is a
multi-line comment.


A single-line comment is terminated by any unicode line terminator (LF, VF, FF, CR, NEL, LS or PS) in utf8 encoding. The terminator is still part of the source code after the comment, so if it is not an ascii symbol (these are NEL, LS and PS), it will lead to a parser error.

Additionally, there is another type of comment called a natspec comment, which is detailed in the style guide. 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 function parameters and two return variables.

// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.21 <0.8.0;

/** @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) public pure returns (uint s, uint p) {
        s = w * h;
        p = 2 * (w + h);