libs/functional/forward/doc/forward.qbk

[library Boost.Functional/Forward
  [quickbook 1.3]
  [version 1.0]
  [authors [Schwinger, Tobias]]
  [copyright 2007 2008 Tobias Schwinger]
  [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])
  ]
  [purpose Function object adapters for generic argument forwarding.]
  [category higher-order]
  [category generic]
  [last-revision $Date: 2008/11/01 19:58:50 $]
]

[def __unspecified__            /unspecified/]
[def __boost_ref__              [@http://www.boost.org/doc/html/ref.html Boost.Ref]]
[def __boost_result_of__        [@http://www.boost.org/libs/utility/utility.htm#result_of Boost.ResultOf]]
[def __boost__result_of__       [@http://www.boost.org/libs/utility/utility.htm#result_of `boost::result_of`]]
[def __the_forwarding_problem__ [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm The Forwarding Problem]]
[def __boost_fusion__           [@http://www.boost.org/libs/fusion/doc/html/index.html Boost.Fusion]]

[section Brief Description]

`boost::forward_adapter` provides a reusable adapter template for function
objects. It forwards RValues as references to const, while leaving LValues
as-is.

    struct g // function object that only accept LValues
    {
        template< typename T0, typename T1, typename T2 >
        void operator()(T0 & t0, T1 & t1, T2 & t2) const;

        typedef void result_type;
    };

    // Adapted version also accepts RValues and forwards
    // them as references to const, LValues as-is
    typedef boost::forward_adapter<g> f;

Another adapter, `boost::lighweight_forward_adapter` allows forwarding with 
some help from the user accepting and unwrapping reference wrappers (see 
__boost_ref__) for reference arguments, const qualifying all other arguments. 

The target functions must be compatible with __boost_result_of__, and so are
the adapters.

[endsect]

[section Background]

Let's suppose we have some function `f` that we can call like this:

    f(123,a_variable);

Now we want to write another, generic function `g` that can be called the
same way and returns some object that calls `f` with the same arguments.

    f(123,a_variable) == g(f,123,a_variable).call_f()

[heading Why would we want to do it, anyway?]

Maybe we want to run `f` several times. Or maybe we want to run it within
another thread. Maybe we just want to encapsulate the call expression for now, 
and then use it with other code that allows to compose more complex expressions
in order to decompose it with C++ templates and have the compiler generate some 
machinery that eventually calls `f` at runtime (in other words; apply a
technique that is commonly referred to as Expression Templates). 

[heading Now, how do we do it?]

The bad news is: It's impossible. 

That is so because there is a slight difference between a variable and an
expression that evaluates to its value: Given

    int y;
    int const z = 0;

and

    template< typename T > void func1(T & x);

we can call

    func1(y); // x is a reference to a non-const object
    func1(z); // x is a reference to a const object

where

    func1(1); // fails to compile.

This way we can safely have `func1` store its reference argument and the 
compiler keeps us from storing a reference to an object with temporary lifetime.

It is important to realize that non-constness and whether an object binds to a
non-const reference parameter are two different properties. The latter is the
distinction between LValues and RValues. The names stem from the left hand side
and the right hand side of assignment expressions, thus LValues are typically
the ones you can assign to, and RValues the temporary results from the right
hand side expression. 

    y = 1+2;        // a is LValue, 1+2 is the expression producing the RValue,
    // 1+2 = a;     // usually makes no sense. 

    func1(y);       // works, because y is an LValue
    // func1(1+2);  // fails to compile, because we only got an RValue.

If we add const qualification on the parameter, our function also accepts
RValues: 

    template< typename T > void func2(T const & x);

    // [...] function scope:
    func2(1); // x is a reference to a const temporary, object,
    func2(y); // x is a reference to a const object, while y is not const, and
    func2(z); // x is a reference to a const object, just like z.

In all cases, the argument `x` in `func2` is a const-qualified LValue.
We can use function overloading to identify non-const LValues:

    template< typename T > void func3(T const & x); // #1
    template< typename T > void func3(T & x);       // #2

    // [...] function scope:
    func3(1); // x is a reference to a const, temporary object in #1,
    func3(y); // x is a reference to a non-const object in #2, and
    func3(z); // x is a reference to a const object in #1.

Note that all arguments `x` in the overloaded function `func3` are LValues.
In fact, there is no way to transport RValues into a function as-is in C++98.
Also note that we can't distinguish between what used to be a const qualified
LValue and an RValue. 

That's as close as we can get to a generic forwarding function `g` as
described above by the means of C++ 98. See __the_forwarding_problem__ for a
very detailed discussion including solutions that require language changes.

Now, for actually implementing it, we need 2^N overloads for N parameters
(each with and without const qualifier) for each number of arguments 
(that is 2^(Nmax+1) - 2^Nmin). Right, that means the compile-time complexity 
is O(2^N), however the factor is low so it works quite well for a reasonable
number (< 10) of arguments.

[endsect]

[section:reference Reference]

[section forward_adapter]

[heading Description]

Function object adapter template whose instances are callable with LValue and
RValue arguments. RValue arguments are forwarded as reference-to-const typed
LValues.

An arity can be given as second, numeric non-type template argument to restrict
forwarding to a specific arity.
If a third, numeric non-type template argument is present, the second and third 
template argument are treated as minimum and maximum arity, respectively.
Specifying an arity can be helpful to improve the readability of diagnostic
messages and compile time performance.

__boost_result_of__ can be used to determine the result types of specific call
expressions.

[heading Header]
    #include <boost/functional/forward_adapter.hpp>

[heading Synopsis]

    namespace boost
    {
        template< class Function,
            int Arity_Or_MinArity = __unspecified__, int MaxArity = __unspecified__ >
        class forward_adapter;
    }

[variablelist Notation
    [[`F`]            [a possibly const qualified function object type or reference type thereof]]
    [[`f`]            [an object convertible to `F`]]
    [[`FA`]           [the type `forward_adapter<F>`]]
    [[`fa`]           [an instance object of `FA`, initialized with `f`]]
    [[`a0`...`aN`]    [arguments to `fa`]]
]

The result type of a target function invocation must be

  __boost__result_of__<F*(TA0 [const]&...TAN [const]&])>::type

where `TA0`...`TAN` denote the argument types of `a0`...`aN`.

[heading Expression Semantics]

[table
    [[Expression]      [Semantics]]
    [[`FA(f)`]         [creates an adapter, initializes the target function with `f`.]]
    [[`FA()`]          [creates an adapter, attempts to use `F`'s default constructor.]]
    [[`fa(a0`...`aN)`] [calls `f` with with arguments `a0`...`aN`.]]
]

[heading Limits]

The macro BOOST_FUNCTIONAL_FORWARD_ADAPTER_MAX_ARITY can be defined to set the
maximum call arity. It defaults to 6.

[heading Complexity]

Preprocessing time: O(2^N), where N is the arity limit.
Compile time: O(2^N), where N depends on the arity range.
Run time: O(0) if the compiler inlines, O(1) otherwise.

[endsect]


[section lightweight_forward_adapter]

[heading Description]

Function object adapter template whose instances are callable with LValue and
RValue arguments. All arguments are forwarded as reference-to-const typed
LValues, except for reference wrappers which are unwrapped and may yield
non-const LValues.

An arity can be given as second, numeric non-type template argument to restrict
forwarding to a specific arity.
If a third, numeric non-type template argument is present, the second and third 
template argument are treated as minimum and maximum arity, respectively.
Specifying an arity can be helpful to improve the readability of diagnostic
messages and compile time performance.

__boost_result_of__ can be used to determine the result types of specific call
expressions.

[heading Header]
    #include <boost/functional/lightweight_forward_adapter.hpp>

[heading Synopsis]

    namespace boost
    {
        template< class Function,
            int Arity_Or_MinArity = __unspecified__, int MaxArity = __unspecified__ >
        struct lightweight_forward_adapter;
    }

[variablelist Notation
    [[`F`]         [a possibly const qualified function object type or reference type thereof]]
    [[`f`]         [an object convertible to `F`]]
    [[`FA`]        [the type `lightweight_forward_adapter<F>`]]
    [[`fa`]        [an instance of `FA`, initialized with `f`]]
    [[`a0`...`aN`] [arguments to `fa`]]
]

The result type of a target function invocation must be

  __boost__result_of__<F*(TA0 [const]&...TAN [const]&])>::type

where `TA0`...`TAN` denote the argument types of `a0`...`aN`.

[heading Expression Semantics]

[table
    [[Expression]      [Semantics]]
    [[`FA(f)`]         [creates an adapter, initializes the target function with `f`.]]
    [[`FA()`]          [creates an adapter, attempts to use `F`'s default constructor.]]
    [[`fa(a0`...`aN)`] [calls `f` with with const arguments `a0`...`aN`. If `aI` is a 
      reference wrapper it is unwrapped.]]
]

[heading Limits]

The macro BOOST_FUNCTIONAL_LIGHTWEIGHT_FORWARD_ADAPTER_MAX_ARITY can be defined
to set the maximum call arity. It defaults to 10.

[heading Complexity]

Preprocessing time: O(N), where N is the arity limit.
Compile time: O(N), where N is the effective arity of a call.
Run time: O(0) if the compiler inlines, O(1) otherwise.

[endsect]

[endsect]


[section Acknowledgements]

As these utilities are factored out of the __boost_fusion__ functional module, 
I want to thank Dan Marsden and Joel de Guzman for letting me participate in the
development of that great library in the first place.

Further, I want to credit the authors of the references below, for their
in-depth investigation of the problem and the solution implemented here.

Last but not least I want to thank Vesa Karnoven and Paul Mensonides for the
Boost Preprocessor library. Without it, I would have ended up with an external
code generator for this one.

[endsect]


[section References]

# [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm The Forwarding Problem],
  Peter Dimov, Howard E. Hinnant, David Abrahams, 2002

# [@http://www.boost.org/libs/utility/utility.htm#result_of Boost.ResultOf],
  Douglas Gregor, 2004

# [@http://www.boost.org/doc/html/ref.html Boost.Ref], 
  Jaakko Jarvi, Peter Dimov, Douglas Gregor, David Abrahams, 1999-2002

[endsect]