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= Documentation Components
:navtitle: Documentation Components

== Introduction

Boost does not require any specific documentation structure. However,
there are some important considerations that influence content and
structure. For example, many Boost libraries wind up being proposed for
inclusion in the pass:[C++] Standard, so writing them initially with text
suitable for inclusion in the Standard may be helpful. 

Also, Boost library documentation is often accessed via the World Wide Web, including via search engines, so context is often important for every page. 

Finally, Boost libraries should provide additional documentation,
such as introductory, tutorial, example, and rationale content. With
those things in mind, we suggest the following guidelines for Boost
library documentation.

== Standards Conforming Documentation

The documentation structure required for the pass:[C++] Standard is an
effective way to describe the technical specifications for a library.
Although terse, that format is familiar to many Boost users and is far
more precise than most ad hoc formats. The following description is
based upon §17.3 of the Standard. (Note that while final Standard
proposals must include full standard-ese wording, which the committee
will not do for you, that level of detail is not expected of Boost
library documentation.)

=== Document Elements

[#footnote1-location]
Each document contains the following elements, as
applicable. link:#footnote1[(1)]:

[disc]
* <<Summary>>
* <<Requirements>>
* <<Detailed specifications>>
* <<References to the C++ Standard Library>>
* <<References to the C Standard 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:

[disc]
* Macros
* Values
* Types
* Classes
* Functions
* Objects

==== Requirements

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

[disc]
* 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.

[#footnote2-location]
In some cases the semantic requirements are presented as pass:[C++] code. Such
code is intended as a specification of equivalence of a construct to
another construct, not necessarily as the way the construct must be
implemented.link:#footnote2[(2)]

==== Detailed specifications

The detailed specifications each contain the following elements:

[disc]
* 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

[#footnote3-location]
Descriptions of class member functions follow the order (as
appropriate) link:#footnote3[(3)]:

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

[#footnote4-location]
Descriptions of function semantics contain the following
elements (as appropriate) link:#footnote4[(4):]

[disc]
* *<<Requires>>*: the preconditions for calling the function

* *<<Effects>>*: the actions performed by the function

* *<<Post-conditions>>"*: 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 pass:[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 pass:[C++] expressions. For example,

[source,cpp]
----
 
void limit( int * p, int min, int max );
----

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

Requirements already enforced by the pass:[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
pass:[C++]. A description in prose is often less limiting on implementors, but
is often less precise than pass:[C++] code.

If an effect is specified in one of the other elements, particularly
_post-conditions_, _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.

==== Post-conditions

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

[source,cpp]
----
 
void make_zero_if_negative( int & x );
----

*Post-condition:* `x >= 0`

==== Returns

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

[source,cpp]
----
int sum( int x, int y );
----

*Returns*: `x + y`

Only specify the return value; the type is already dictated by pass:[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:

[source,cpp]
----
 
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
https://web.mit.edu/16.070/www/lecture/big_o.pdf["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.

== Web Reference Documentation

Boost library documentation is often accessed via the World Web. Using
search engines, a page deep in the reference content could be viewed
without any further context. Therefore, it is helpful to add extra
context, such as the following, to each page:

[disc]
* Describe the enclosing namespace or use fully scoped identifiers.
* Document required headers for each type or function.
* Link to relevant tutorial information.
* Link to related example code.
* Include the library name.
* Include navigation elements to the beginning of the documentation.

It is also useful to consider the effectiveness of a description in
search engines. Terse or cryptic descriptions are less likely to help
the curious find a relevant function or type.

== Footnotes

[#footnote1]
link:#footnote1-location[(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.

[#footnote2]
link:#footnote2-location[(2)] Although in some cases the code is
unambiguously the optimum implementation.

[#footnote3]
link:#footnote3-location[(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.

[#footnote4]
link:#footnote4-location[(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.

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_Revised April, 2023_

_Distributed under the Boost Software License, Version 1.0. Refer to http://www.boost.org/LICENSE_1_0.txt_.