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 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.
Conforming to Standards
The documentation structure required for the 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 ยง17.3 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
Each document contains the following elements, as applicable. (1):
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:
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Macros
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Values
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Types
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Classes
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Functions
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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:
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Template arguments
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Derived classes
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Containers, iterators, and/or algorithms that meet an interface convention
Interface convention requirements are stated as generally as possible.
For example, instead of stating " class X has to define a member function operator++() ", say " the interface requires for any object x of class X, ++x is defined" (noting that whether the operator is a member or not 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 equivalence of a construct to another construct, not necessarily as the way the construct must be implemented.(2)
Detailed Specifications
The detailed specifications each contain the following elements:
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Name and brief description
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Synopsis (class definition or function prototype, as appropriate)
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Restrictions on template arguments, if any
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Description of class invariants
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Description of function semantics
Descriptions of class member functions follow the order (as appropriate) (3):
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Constructor(s) and destructor
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Copying and assignment functions
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Comparison functions
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Modifier functions
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Observer functions
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Operators and other non-member functions
Descriptions of function semantics contain the following elements (as appropriate) (4):
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Requires: the preconditions for calling the function
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Effects: the actions performed by the function
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Post-conditions": the observable results established by the function
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Returns: a description of the value(s) returned by the function
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Throws: any exceptions thrown by the function, and the conditions that would cause the exception
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Complexity: the time and/or space complexity of the function
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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.
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 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 C++ expressions. For example:
void make_zero_if_negative( int & x );Post-condition: 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.
Web References
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:
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Describe the enclosing namespace or use fully scoped identifiers.
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Document required headers for each type or function.
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Link to relevant tutorial information.
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Link to related example code.
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Include the library name.
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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
(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 April, 2023
Distributed under the Boost Software License, Version 1.0. Refer to http://www.boost.org/LICENSE_1_0.txt.