Squashed 'third_party/boostorg/utility/' content from commit ebe4429
Change-Id: I8e6ee78273db31df18f99d29034f855ccc064551
git-subtree-dir: third_party/boostorg/utility
git-subtree-split: ebe44296ca698e333a09e8268ea8ccedb3886c4d
diff --git a/doc/base_from_member.qbk b/doc/base_from_member.qbk
new file mode 100644
index 0000000..9afd949
--- /dev/null
+++ b/doc/base_from_member.qbk
@@ -0,0 +1,363 @@
+[/
+ Copyright 2001, 2003, 2004, 2012 Daryle Walker.
+
+ Distributed under the Boost Software License, Version 1.0.
+
+ See accompanying file LICENSE_1_0.txt
+ or copy at http://boost.org/LICENSE_1_0.txt
+]
+
+[article Base_From_Member
+ [quickbook 1.5]
+ [authors [Walker, Daryle]]
+ [copyright 2001, 2003, 2004, 2012 Daryle Walker]
+ [license
+ 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])
+ ]
+]
+
+[section Rationale]
+
+When developing a class, sometimes a base class needs to be initialized
+with a member of the current class. As a na\u00EFve example:
+
+ #include <streambuf> /* for std::streambuf */
+ #include <ostream> /* for std::ostream */
+
+ class fdoutbuf
+ : public std::streambuf
+ {
+ public:
+ explicit fdoutbuf( int fd );
+ //...
+ };
+
+ class fdostream
+ : public std::ostream
+ {
+ protected:
+ fdoutbuf buf;
+ public:
+ explicit fdostream( int fd )
+ : buf( fd ), std::ostream( &buf ) {}
+ //...
+ };
+
+This is undefined because C++'s initialization order mandates that the base
+class is initialized before the member it uses. [@http://www.moocat.org R.
+Samuel Klatchko] developed a way around this by using the initialization
+order in his favor. Base classes are intialized in order of declaration, so
+moving the desired member to another base class, that is initialized before
+the desired base class, can ensure proper initialization.
+
+A custom base class can be made for this idiom:
+
+ #include <streambuf> /* for std::streambuf */
+ #include <ostream> /* for std::ostream */
+
+ class fdoutbuf
+ : public std::streambuf
+ {
+ public:
+ explicit fdoutbuf( int fd );
+ //...
+ };
+
+ struct fdostream_pbase
+ {
+ fdoutbuf sbuffer;
+
+ explicit fdostream_pbase( int fd )
+ : sbuffer( fd ) {}
+ };
+
+ class fdostream
+ : private fdostream_pbase
+ , public std::ostream
+ {
+ typedef fdostream_pbase pbase_type;
+ typedef std::ostream base_type;
+
+ public:
+ explicit fdostream( int fd )
+ : pbase_type( fd ), base_type( &sbuffer ) {}
+ //...
+ };
+
+Other projects can use similar custom base classes. The technique is basic
+enough to make a template, with a sample template class in this library.
+The main template parameter is the type of the enclosed member. The
+template class has several (explicit) constructor member templates, which
+implicitly type the constructor arguments and pass them to the member. The
+template class uses implicit copy construction and assignment, cancelling
+them if the enclosed member is non-copyable.
+
+Manually coding a base class may be better if the construction and/or
+copying needs are too complex for the supplied template class, or if the
+compiler is not advanced enough to use it.
+
+Since base classes are unnamed, a class cannot have multiple (direct) base
+classes of the same type. The supplied template class has an extra template
+parameter, an integer, that exists solely to provide type differentiation.
+This parameter has a default value so a single use of a particular member
+type does not need to concern itself with the integer.
+
+[endsect]
+
+[section Synopsis]
+
+ #include <type_traits> /* exposition only */
+
+ #ifndef BOOST_BASE_FROM_MEMBER_MAX_ARITY
+ #define BOOST_BASE_FROM_MEMBER_MAX_ARITY 10
+ #endif
+
+ template < typename MemberType, int UniqueID = 0 >
+ class boost::base_from_member
+ {
+ protected:
+ MemberType member;
+
+ #if ``['C++11 is in use]``
+ template< typename ...T >
+ explicit constexpr base_from_member( T&& ...x )
+ noexcept( std::is_nothrow_constructible<MemberType, T...>::value );
+ #else
+ base_from_member();
+
+ template< typename T1 >
+ explicit base_from_member( T1 x1 );
+
+ template< typename T1, typename T2 >
+ base_from_member( T1 x1, T2 x2 );
+
+ //...
+
+ template< typename T1, typename T2, typename T3, typename T4,
+ typename T5, typename T6, typename T7, typename T8, typename T9,
+ typename T10 >
+ base_from_member( T1 x1, T2 x2, T3 x3, T4 x4, T5 x5, T6 x6, T7 x7,
+ T8 x8, T9 x9, T10 x10 );
+ #endif
+ };
+
+ template < typename MemberType, int UniqueID >
+ class base_from_member<MemberType&, UniqueID>
+ {
+ protected:
+ MemberType& member;
+
+ explicit constexpr base_from_member( MemberType& x )
+ noexcept;
+ };
+
+The class template has a first template parameter `MemberType` representing
+the type of the based-member. It has a last template parameter `UniqueID`,
+that is an `int`, to differentiate between multiple base classes that use
+the same based-member type. The last template parameter has a default value
+of zero if it is omitted. The class template has a protected data member
+called `member` that the derived class can use for later base classes (or
+itself).
+
+If the appropriate features of C++11 are present, there will be a single
+constructor template. It implements ['perfect forwarding] to the best
+constructor call of `member` (if any). The constructor template is marked
+both `constexpr` and `explicit`. The former will be ignored if the
+corresponding inner constructor call (of `member`) does not have the marker.
+The latter binds the other way; always taking effect, even when the inner
+constructor call does not have the marker. The constructor template
+propagates the `noexcept` status of the inner constructor call. (The
+constructor template has a trailing parameter with a default value that
+disables the template when its signature is too close to the signatures of
+the automatically-defined non-template copy- and/or move-constructors of
+`base_from_member`.)
+
+On earlier-standard compilers, there is a default constructor and several
+constructor member templates. These constructor templates can take as many
+arguments (currently up to ten) as possible and pass them to a constructor
+of the data member.
+
+A specialization for member references offers a single constructor taking
+a `MemberType&`, which is the only way to initialize a reference.
+
+Since C++ does not allow any way to explicitly state the template parameters
+of a templated constructor, make sure that the arguments are already close
+as possible to the actual type used in the data member's desired constructor.
+Explicit conversions may be necessary.
+
+The `BOOST_BASE_FROM_MEMBER_MAX_ARITY` macro constant specifies the maximum
+argument length for the constructor templates. The constant may be overridden
+if more (or less) argument configurations are needed. The constant may be
+read for code that is expandable like the class template and needs to
+maintain the same maximum size. (Example code would be a class that uses
+this class template as a base class for a member with a flexible set of
+constructors.) This constant is ignored when C++11 features are present.
+
+[endsect]
+
+[section Usage]
+
+With the starting example, the `fdoutbuf` sub-object needs to be
+encapsulated in a base class that is inheirited before `std::ostream`.
+
+ #include <boost/utility/base_from_member.hpp>
+
+ #include <streambuf> // for std::streambuf
+ #include <ostream> // for std::ostream
+
+ class fdoutbuf
+ : public std::streambuf
+ {
+ public:
+ explicit fdoutbuf( int fd );
+ //...
+ };
+
+ class fdostream
+ : private boost::base_from_member<fdoutbuf>
+ , public std::ostream
+ {
+ // Helper typedef's
+ typedef boost::base_from_member<fdoutbuf> pbase_type;
+ typedef std::ostream base_type;
+
+ public:
+ explicit fdostream( int fd )
+ : pbase_type( fd ), base_type( &member ){}
+ //...
+ };
+
+The base-from-member idiom is an implementation detail, so it should not
+be visible to the clients (or any derived classes) of `fdostream`. Due to
+the initialization order, the `fdoutbuf` sub-object will get initialized
+before the `std::ostream` sub-object does, making the former sub-object
+safe to use in the latter sub-object's construction. Since the `fdoutbuf`
+sub-object of the final type is the only sub-object with the name `member`
+that name can be used unqualified within the final class.
+
+[endsect]
+
+[section Example]
+
+The base-from-member class templates should commonly involve only one
+base-from-member sub-object, usually for attaching a stream-buffer to an
+I/O stream. The next example demonstrates how to use multiple
+base-from-member sub-objects and the resulting qualification issues.
+
+ #include <boost/utility/base_from_member.hpp>
+
+ #include <cstddef> /* for NULL */
+
+ struct an_int
+ {
+ int y;
+
+ an_int( float yf );
+ };
+
+ class switcher
+ {
+ public:
+ switcher();
+ switcher( double, int * );
+ //...
+ };
+
+ class flow_regulator
+ {
+ public:
+ flow_regulator( switcher &, switcher & );
+ //...
+ };
+
+ template < unsigned Size >
+ class fan
+ {
+ public:
+ explicit fan( switcher );
+ //...
+ };
+
+ class system
+ : private boost::base_from_member<an_int>
+ , private boost::base_from_member<switcher>
+ , private boost::base_from_member<switcher, 1>
+ , private boost::base_from_member<switcher, 2>
+ , protected flow_regulator
+ , public fan<6>
+ {
+ // Helper typedef's
+ typedef boost::base_from_member<an_int> pbase0_type;
+ typedef boost::base_from_member<switcher> pbase1_type;
+ typedef boost::base_from_member<switcher, 1> pbase2_type;
+ typedef boost::base_from_member<switcher, 2> pbase3_type;
+
+ typedef flow_regulator base1_type;
+ typedef fan<6> base2_type;
+
+ public:
+ system( double x );
+ //...
+ };
+
+ system::system( double x )
+ : pbase0_type( 0.2 )
+ , pbase1_type()
+ , pbase2_type( -16, &this->pbase0_type::member.y )
+ , pbase3_type( x, static_cast<int *>(NULL) )
+ , base1_type( pbase3_type::member, pbase1_type::member )
+ , base2_type( pbase2_type::member )
+ {
+ //...
+ }
+
+The final class has multiple sub-objects with the name `member`, so any
+use of that name needs qualification by a name of the appropriate base
+type. (Using `typedef`s ease mentioning the base types.) However, the fix
+introduces a new problem when a pointer is needed. Using the address
+operator with a sub-object qualified with its class's name results in a
+pointer-to-member (here, having a type of `an_int boost::base_from_member<
+an_int, 0> :: *`) instead of a pointer to the member (having a type of
+`an_int *`). The new problem is fixed by qualifying the sub-object with
+`this->` and is needed just for pointers, and not for references or values.
+
+There are some argument conversions in the initialization. The constructor
+argument for `pbase0_type` is converted from `double` to `float`. The first
+constructor argument for `pbase2_type` is converted from `int` to `double`.
+The second constructor argument for `pbase3_type` is a special case of
+necessary conversion; all forms of the null-pointer literal in C++ (except
+`nullptr` from C++11) also look like compile-time integral expressions, so
+C++ always interprets such code as an integer when it has overloads that can
+take either an integer or a pointer. The last conversion is necessary for the
+compiler to call a constructor form with the exact pointer type used in
+`switcher`'s constructor. (If C++11's `nullptr` is used, it still needs a
+conversion if multiple pointer types can be accepted in a constructor call
+but `std::nullptr_t` cannot.)
+
+[endsect]
+
+[section Acknowledgments]
+
+* [@http://www.boost.org/people/ed_brey.htm Ed Brey] suggested some interface
+changes.
+
+* [@http://www.moocat.org R. Samuel Klatchko] ([@mailto:rsk@moocat.org
+rsk@moocat.org], [@mailto:rsk@brightmail.com rsk@brightmail.com]) invented
+the idiom of how to use a class member for initializing a base class.
+
+* [@http://www.boost.org/people/dietmar_kuehl.htm Dietmar Kuehl] popularized the
+ base-from-member idiom in his [@http://www.informatik.uni-konstanz.de/~kuehl/c++/iostream/
+ IOStream example classes].
+
+* Jonathan Turkanis supplied an implementation of generating the constructor
+templates that can be controlled and automated with macros. The
+implementation uses the [@../../../preprocessor/index.html Preprocessor library].
+
+* [@http://www.boost.org/people/daryle_walker.html">Daryle Walker] started the
+library. Contributed the test file [@../../base_from_member_test.cpp
+base_from_member_test.cpp].
+
+[endsect]
+