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# Boost Library Requirements and Guidelines

Introduction
Requirements
License requirements
Portability requirements
Ownership
Guidelines
Design and programming
Directory structure and filenames
Naming consistency
Documentation
Rationale
Exception-specification rationale
Naming conventions rationale
Source code fonts rationale
Tabs rationale
ECMAScript/JavaScript rationale
Rationale rationale
Acknowledgements rationale

## Introduction

This page describes requirements and guidelines for the content of a library submitted to Boost.

See the Boost Library Submission Process page for a description of the process involved.

## Requirements

To avoid the frustration and wasted time of a proposed library being rejected, it must meets these requirements:

- The license must meet the license requirements below. Restricted licenses like the GPL and LGPL are not acceptable.
- The copyright ownership must be clear.
- The library must be generally useful and not restricted to a narrow problem domain.
- The library must meet the portability requirements below.
- The library must come reasonably close to meeting the Guidelines below.
  - Design and Programming
  - Directory Structure
  - Documentation
- The author must be willing to participate in discussions on the mailing list, and to refine the library accordingly.

There's no requirement that an author read the mailing list for a time before making a submission. It has been noted, however, that submissions which begin "I just started to read this mailing list ..." seem to fail, often embarrassingly.

### License requirements

- Must be simple to read and understand.
- Must grant permission without fee to copy, use and modify the software for any use (commercial and non-commercial).
- Must require that the license appear on all copies of the software source code.
- Must not require that the license appear with executables or other binary uses of the library.
- Must not require that the source code be available for execution or other binary uses of the library.
- May restrict the use of the name and description of the library to the standard version found on the Boost web site.

### Portability requirements

- A library's interface must portable and not restricted to a particular compiler or operating system.

- A library's implementation must if possible be portable and not restricted to a particular compiler or operating system. If a portable implementation is not possible, non-portable constructions are acceptable if reasonably easy to port to other environments, and implementations are provided for at least two popular operating systems (such as UNIX and Windows).

- There is no requirement that a library run on C++ compilers which do not conform to the ISO standard.

- There is no requirement that a library run on any particular C++ compiler. Boost contributors often try to ensure their libraries work with popular compilers. The boost/config.hpp configuration header is the preferred mechanism for working around compiler deficiencies.

Since there is no absolute way to prove portability, many boost submissions demonstrate practical portability by compiling and executing correctly with two different C++ compilers, often under different operating systems. Otherwise reviewers may disbelieve that porting is in fact practical.

### Ownership

Are you sure you own the library you are thinking of submitting? "How to Copyright Software" by MJ Salone, Nolo Press, 1990 says:

> Doing work on your own time that is very similar to programming you do for your employer on company time can raise nasty legal problems. In this situation, it's best to get a written release from your employer in advance.

Place a copyright notice in all the important files you submit. Boost won't accept libraries without clear copyright information.

## Guidelines

Please use these guidelines as a checklist for preparing the content a library submission. Not every guideline applies to every library, but a reasonable effort to comply is expected.

### Design and Programming

- Aim first for clarity and correctness; optimization should be only a secondary concern in most Boost libraries.

- Aim for ISO Standard C++. Than means making effective use of the standard features of the language, and avoiding non-standard compiler extensions. It also means using the C++ Standard Library where applicable.

- Headers should be good neighbors. See the header policy. See Naming consistency.

- Follow quality programming practices. See, for example, "Effective C++" 2nd Edition, and "More Effective C++", both by Scott Meyers, published by Addison Wesley.

- Use the C++ Standard Library or other Boost libraries, but only when the benefits outweigh the costs. Do not use libraries other than the C++ Standard Library or Boost. See Library reuse.

- Read Implementation Variation to see how to supply performance, platform, or other implementation variations.

- Use the naming conventions of the C++ Standard Library (See Naming conventions rationale):
  - Names (except as noted below) should be all lowercase, with words separated by underscores.
  - Acronyms should be treated as ordinary names (e.g. `xml_parser` instead of `XML_parser`).
  - Template parameter names begin with an uppercase letter.
  - Macro (gasp!) names all uppercase and begin with BOOST_.

- Choose meaningful names - explicit is better than implicit, and readability counts. There is a strong preference for clear and descriptive names, even if lengthy.

- Use exceptions to report errors where appropriate, and write code that is safe in the face of exceptions.

- Avoid exception-specifications. See exception-specification rationale.

- Provide sample programs or confidence tests so potential users can see how to use your library.

- Provide a regression test program or programs which follow the Test Policies and Protocols.

- Although some boost members use proportional fonts, tabs, and unrestricted line lengths in their own code, boost's widely distributed source code should follow more conservative guidelines:
  - Use fixed-width fonts. See fonts rationale.
  - Use spaces rather than tabs. See tabs rationale.
  - Limit line lengths to 80 characters.

- Begin all source files with:
  - A comment line describing the contents of the file.
  - Comments describing copyright and licensing.
  - A comment line referencing your library on the Boost web site:
    `// See http://www.boost.org/libs/foo for documentation.`
    where `foo` is the directory name (see below) for your library. As well as aiding users who come across a Boost file detached from its documentation, some of Boost's automatic tools depend on this comment to identify which library header files belong to.

### Directory Structure and Filenames

- File and directory names must contain only lowercase ASCII letters , numbers, underscores, and a period. Leading character must be alphabetic. Maximum length 31. Only a single period is permitted. These requirements ensure file and directory names are relatively portable.
- All libraries have at their highest level a primary directory named for the particular library. See Naming consistency. The primary directory may have sub-directories.
- For very simple libraries implemented entirely within the library header, all files go in the primary directory (except headers, which go in the boost header directory).

> Boost standard sub-directory names

> Sub-directory | Contents | Required
> `build` | Library build files such as a Jamfile. | If any build files.
> doc | Documentation (HTML) files. | If several doc files.
> `example` | Sample program files. | If several sample files.
> `src` | Source files which must be compiled to build the library. | If any source files.
> `test` | Regression or other test programs or scripts. | If several test files.

#### Redirection

The primary directory should always contain a file named index.html (or index.htm). Authors have requested this so that they can publish URL's in the form http://www.boost.org/libs/lib-name with the assurance a documentation reorganization won't invalidate the URL. Boost's internal tools are also simplified by knowing that a library's documentation is always reachable via the simplified URL.

If the documentation is in a doc sub-directory, the primary directory index.html file should just do an automatic redirection to the doc subdirectory:

>     <html>
>     <head>
>     <meta http-equiv="refresh" content="0; URL=doc/index.html">
>     </head>
>     <body>
>     Automatic redirection failed, please go to
>     <a href="doc/index.html">doc/index.html</a>
>     </body>
>     </html>

### Naming consistency

As library developers and users have gained experience with Boost, the following consistent naming approach has come to be viewed as very helpful, particularly for larger libraries which need their own header subdirectories and namespaces.

Here is how it works. The library is given a name which describes the contents of the library. Cryptic abbreviations are not acceptable. Following the practice of the C++ Standard Library, names are usually singular rather than plural. For example, a library dealing with file systems might chose the name "filesystem", but not "filesystems", "fs" or "nicecode".

- The library's primary directory (in parent boost-root/libs) is given that same name. For example, boost-root/libs/filesystem.
- The library's primary header directory (in parent boost-root/boost) is given that same name. For example, boost-root/boost/filesystem.
- The library's primary namespace (in parent ::boost) is given that same name. For example, ::boost::filesystem.

### Documentation

Even the simplest library needs some documentation; the amount should be proportional to the need. The documentation should assume the readers have a basic knowledge of C++, but are not necessarily experts.

The format for documentation should be HTML, and should not require an advanced browser or server-side extensions. Style sheets are acceptable. ECMAScript/JavaScript is not acceptable. The documentation entry point should always be a file named index.html or index.htm; see Redirection.

There is no single right way to do documentation. HTML documentation is often organized quite differently from traditional printed documents. Task-oriented styles differ from reference oriented styles. In the end, it comes down to the question: Is the documentation sufficient for the mythical "average" C++ programmer to use the library successfully?

Appropriate topics for documentation often include:

- General introduction to the library.
- Description of each class.
- Relationship between classes.
- For each function, as applicable, description, requirements (preconditions), effects, post-conditions, returns, and throws.
- Discussion of error detection and recovery strategy.
- How to use including description of typical uses.
- How to compile and link.
- How to test.
- Version or revision history.
- Rationale for design decisions. See Rationale rationale.
- Acknowledgements. See Acknowledgments rationale.

If you need more help with how to write documentation you can check out the article on Writing Documentation for Boost.

## Rationale

Rationale for some of the requirements and guidelines follows.

### Exception-specification rationale

Exception specifications [ISO 15.4] are sometimes coded to indicate what exceptions may be thrown, or because the programmer hopes they will improved performance. But consider the following member from a smart pointer:

        T& operator*() const throw()  { return *ptr; }

This function calls no other functions; it only manipulates fundamental data types like pointers Therefore, no runtime behavior of the exception-specification can ever be invoked. The function is completely exposed to the compiler; indeed it is declared inline Therefore, a smart compiler can easily deduce that the functions are incapable of throwing exceptions, and make the same optimizations it would have made based on the empty exception-specification. A "dumb" compiler, however, may make all kinds of pessimizations.

For example, some compilers turn off inlining if there is an exception-specification. Some compilers add try/catch blocks. Such pessimizations can be a performance disaster which makes the code unusable in practical applications.

Although initially appealing, an exception-specification tends to have consequences that require very careful thought to understand. The biggest problem with exception-specifications is that programmers use them as though they have the effect the programmer would like, instead of the effect they actually have.

A non-inline function is the one place a "throws nothing" exception-specification may have some benefit with some compilers.

### Naming conventions rationale

The C++ standard committee's Library Working Group discussed this issue in detail, and over a long period of time. The discussion was repeated again in early boost postings. A short summary:

- Naming conventions are contentious, and although several are widely used, no one style predominates.
- Given the intent to propose portions of boost for the next revision of the C++ standard library, boost decided to follow the standard library's conventions.
- Once a library settles on a particular convention, a vast majority of stakeholders want that style to be consistently used.

### Source code fonts rationale

Dave Abrahams comments: An important purpose (I daresay the primary purpose) of source code is communication: the documentation of intent. This is a doubly important goal for boost, I think. Using a fixed-width font allows us to communicate with more people, in more ways (diagrams are possible) right there in the source. Code written for fixed-width fonts using spaces will read reasonably well when viewed with a variable-width font, and as far as I can tell every editor supporting variable-width fonts also supports fixed width. I don't think the converse is true.

### Tabs rationale

Tabs are banned because of the practical problems caused by tabs in multi-developer projects like Boost, rather than any dislike in principle. See mailing list archives. Problems include maintenance of a single source file by programmers using tabs and programmers using spaces, and the difficulty of enforcing a consistent tab policy other than just "no tabs". Discussions concluded that Boost files should either all use tabs, or all use spaces, and thus the decision to stick with spaces.

### ECMAScript/JavaScript rationale

Before the 1.29.0 release, two Boost libraries added ECMAScript/JavaScript documentation. Controversy followed (see mailing list archives), and the developers were asked to remove the ECMAScript/JavaScript. Reasons given for banning included:

- Incompatible with some older browsers and some text based browsers.
- Makes printing docs pages difficult.
- Often results in really bad user interface design.
- "It's just annoying in general."
- Would require Boost to test web pages for ECMAScript/JavaScript compliance.
- Makes docs maintenance by other than the original developer more difficult.

### Rationale rationale

Rationale is defined as "The fundamental reasons for something; basis" by the American Heritage Dictionary.

Beman Dawes comments: Failure to supply contemporaneous rationale for design decisions is a major defect in many software projects. Lack of accurate rationale causes issues to be revisited endlessly, causes maintenance bugs when a maintainer changes something without realizing it was done a certain way for some purpose, and shortens the useful lifetime of software.

Rationale is fairly easy to provide at the time decisions are made, but very hard to accurately recover even a short time later.

### Acknowledgements rationale

As a library matures, it almost always accumulates improvements suggested to the authors by other boost members. It is a part of the culture of boost.org to acknowledge such contributions, identifying the person making the suggestion. Major contributions are usually acknowledged in the documentation, while minor fixes are often mentioned in comments within the code itself.

Revised 26 February, 2003