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# Writing Documentation for Boost

## Documentation Structure

Introduction
Standards Conforming Documentation
  Document elements
    Summary
    Requirements
    Detailed specifications
    References to the Standard C++ library
    References to the Standard C library
  Other conventions
    Type descriptions
More Information
  Function semantic element explanations
    Requires
    Effects
    Postconditions
    Returns
    Throws
    Complexity
    Rationale
Footnotes

## Introduction

Boost itself does not require any specific documentation structure. The C++ Standard, however, has very explicit requirements for the description of library components (Section 17.3). So for Boost libraries likely to be proposed for inclusion in the standard, it is highly desirable to structure documentation in a way that meets the requirements of the the standard. Doing so eliminates the need to rewrite the documentation for standardization.

Library developers should remember that for a library to be accepted as part of the C++ Standard Library, the proposal must include full wording. The committee will not do that work for you.

Beyond that, the documentation structure required for the standard is an effective way to communicate the technical specifications for a library. Although terse, it is already familiar to many Boost users, and is far more precise than most ad hoc documentation structures.

The following description is for the structure of documentation required by the standard. Boost libraries should also provided additional documentation, such as introductory, tutorial, example, and rationale material.

## Standards Conforming Documentation

### Document elements

Each document contains the following elements, as applicable(1):

- Summary
- Requirements
- Detailed specifications
- References to the Standard C++ library
- References to the Standard C library

#### Summary

The Summary provides a synopsis of the category, and introduces the first-level subclauses. Each subclause also provides a summary, listing the headers specified in the subclause and the library entities provided in each header.

Paragraphs labeled "Note(s):" or "Example(s):" are informative, other paragraphs are normative.

The summary and the detailed specifications are presented in the order:

- Macros
- Values
- Types
- Classes
- Functions
- Objects

#### Requirements

The library can be extended by a C++ program. Each clause, as applicable, describes the requirements that such extensions must meet. Such extensions are generally one of the following:

- Template arguments
- Derived classes
- Containers, iterators, and/or algorithms that meet an interface convention

Interface convention requirements are stated as generally as possible. Instead of stating "`class X` has to define a member function `operator++()`," the interface requires "for any object `x` of `class X`, `++x` is defined." That is, whether the operator is a member is unspecified.

Requirements are stated in terms of well-defined expressions, which define valid terms of the types that satisfy the requirements. For every set of requirements there is a table that specifies an initial set of the valid expressions and their semantics. Any generic algorithm that uses the requirements is described in terms of the valid expressions for its formal type parameters.

Template argument requirements are sometimes referenced by name.

In some cases the semantic requirements are presented as C++ code. Such code is intended as a specification of equivalance of a construct to another construct, not necessarily as the way the construct must be implemented.(2)

#### Detailed specification

The detailed specifications each contain the following elements:

- Name and brief description
- Synopsis (class definition or function prototype, as appropriate)
- Restrictions on template arguments, if any
- Description of class invariants
- Description of function semantics

Descriptions of class member functions follow the order (as appropriate)(3):

- Constructor(s) and destructor
- Copying and assignment functions
- Comparison functions
- Modifier functions
- Observer functions
- Operators and other non-member functions

Descriptions of function semantics contain the following elements (as appropriate)(4):

Requires: the preconditions for calling the function
Effects: the actions performed by the function
Postconditions: the observable results established by the function
Returns: a description of the value(s) returned by the function
Throws: any exceptions thrown by the function, and the conditions that would cause the exception
Complexity: the time and/or space complexity of the function
Rationale: the rationale for the function's design or existence

Complexity requirements specified in the library clauses are upper bounds, and implementations that provide better complexity guarantees satisfy the requirements.

#### References to the C++ Standard library

#### References to the C Standard library

### Other conventions

These conventions are for describing implementation-defined types, and member functions.

#### Type descriptions

The Requirements subclauses may describe names that are used to specify constraints on template arguments.

## More Information

### Function semantic element explanations

The function semantic element description above is taken directly from the C++ standard, and is quite terse. Here is a more detailed explanation of each of the elements.

Note the use of the `<code> ... </code>` font tag to distinguish actual C++ usage from English prose.

#### Requires

Preconditions for calling the function, typically expressed as predicates. The most common preconditions are requirements on the value of arguments, often in the form of C++ expressions. For example,

    void limit( int * p, int min, int max );

Requires: `p != 0 && min <= max`

Requirements already enforced by the C++ language rules (such as the type of arguments) are not repeated in Requires paragraphs.

#### Effects

The actions performed by the function, described either in prose or in C++. A description in prose is often less limiting on implementors, but is often less precise than C++ code.

If an effect is specified in one of the other elements, particularly postconditions, returns, or throws, it is not also described in the effects paragraph. Having only a single description ensures that there is one and only one specification, and thus eliminates the risk of divergence.

#### Postconditions

The observable results of the function, such as the value of variables. Postconditions are often expressed as predicates that are true after the function completes, in the form of C++ expressions. For example:

    void make_zero_if_negative( int & x );

Postcondition: `x >= 0`

#### Returns

The value returned by the function, usually in the form of a C++ expression. For example:

    int sum( int x, int y );

Returns: `x + y`

Only specify the return value; the type is already dictated by C++ language rules.

#### Throws

Specify both the type of exception thrown, and the condition that causes the exception to be thrown. For example, the `std::basic_string` class specifies:

    void resize(size_type n, charT c);

Throws: `length_error` if `n > max_size()`.

#### Complexity

Specifying the time and/or space complexity of a function is often not desirable because it over-constrains implementors and is hard to specify correctly. Complexity is thus often best left as a quality of implementation issue.

A library component, however, can become effectively non-portable if there is wide variation in performance between conforming implementations. Containers are a prime example. In these cases it becomes worthwhile to specify complexity.

Complexity is often specified in generalized "Big-O" notation.

#### Rationale

Specifying the rationale for a function's design or existence can often give users a lot of insight into why a library is designed the way it is. More importantly, it can help prevent "fixing" something that wasn't really broken as the library matures.

## Footnotes

(1) To save space, items that do not apply to a clause are omitted. For example, if a clause does not specify any requirements, there will be no "Requirements" subclause.
(2) Although in some cases the code is unambiguously the optimum implementation.
(3) To save space, items that do not apply to a class are omitted. For example, if a class does not specify any comparison functions, there will be no "Comparison functions" subclause.
(4) To save space, items that do not apply to a function are omitted. For example, if a function does not specify any precondition, there will be no "Requires" paragraph.

Revised 04 December, 2006

Copyright © 2001 William E. Kempf

Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)