utility/doc/base_from_member.qbk

[/
  Copyright 2001, 2003, 2004, 2012 Daryle Walker.
  Copyright (c) 2021, Alan Freitas

  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
]

[section:base_from_member Base from Member]
[block'''<?dbhtml stop-chunking?>''']

[section Introduction]

The class templates __base_from_member__ support the base-from-member idiom.
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 initialized 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 __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 Basic Usage]

With the starting example, the `fdoutbuf` sub-object needs to be
encapsulated in a base class that is inherited 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 Multiple Sub-Objects]

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]

[/===============]
[xinclude tmp/base_from_member_reference.xml]
[/===============]

[section Acknowledgments]

Author: Walker, Daryle

Copyright 2001, 2003, 2004, 2012 Daryle Walker

* [@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 [@boost:/libs/preprocessor/index.html Preprocessor library].

* [@http://www.boost.org/people/daryle_walker.html">Daryle Walker] started the
  library. Contributed the test file [@../../../test/base_from_member_test.cpp
  base_from_member_test.cpp].

[endsect]

[endsect]