[/==============================================================================
Copyright (C) 2001-2010 Joel de Guzman
Copyright (C) 2001-2005 Dan Marsden
Copyright (C) 2001-2010 Thomas Heller
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 Function]
The `function` class template provides a mechanism for implementing lazily
evaluated functions. Syntactically, a lazy function looks like an ordinary C/C++
function. The function call looks familiar and feels the same as ordinary C++
functions. However, unlike ordinary functions, the actual function execution is
deferred.
#include <boost/phoenix/function.hpp>
Unlike ordinary function pointers or functor objects that need to be explicitly
bound through the bind function (see [link phoenix.modules.bind Bind]),
the argument types of these functions are automatically lazily bound.
In order to create a lazy function, we need to implement a model of the
__PFO__ concept. For a function that takes `N` arguments, a model of __PFO__ must
provide:
* An `operator()` that takes `N` arguments, and implements
the function logic. This is also true for ordinary function pointers.
* A nested metafunction `result<Signature>` or nested typedef `result_type`, following the __boost_result_of__ Protocol
[/
* A nested metafunction `result<A1, ... AN>` that takes the types of the `N` arguments to
the function and returns the result type of the function. (There is a special case for function
objects that accept no arguments. Such nullary functors are only required to define a typedef
`result_type` that reflects the return type of its `operator()`).
]
For example, the following type implements the FunctionEval concept, in order to provide a
lazy factorial function:
struct factorial_impl
{
template <typename Sig>
struct result;
template <typename This, typename Arg>
struct result<This(Arg const &)>
{
typedef Arg type;
};
template <typename Arg>
Arg operator()(Arg const & n) const
{
return (n <= 0) ? 1 : n * (*this)(n-1);
}
};
(See [@../../example/factorial.cpp factorial.cpp])
[/note The type of Arg is either a const-reference or non-const-reference
(depending on whether your argument to the actor evaluation is a const-ref or
non-const-ref).]
Having implemented the `factorial_impl` type, we can declare and instantiate a lazy
`factorial` function this way:
function<factorial_impl> factorial;
Invoking a lazy function such as `factorial` does not immediately execute the function
object `factorial_impl`. Instead, an [link phoenix.actor actor] object is
created and returned to the caller. Example:
factorial(arg1)
does nothing more than return an actor. A second function call will invoke
the actual factorial function. Example:
std::cout << factorial(arg1)(4);
will print out "24".
Take note that in certain cases (e.g. for function objects with state), an
instance of the model of __PFO__ may be passed on to the constructor. Example:
function<factorial_impl> factorial(ftor);
where ftor is an instance of factorial_impl (this is not necessary in this case
as `factorial_impl` does not require any state).
[important Take care though when using function objects with state because they are
often copied repeatedly, and state may change in one of the copies, rather than the
original.]
[section Adapting Functions]
If you want to adapt already existing functions or function objects it will become
a repetetive task. Therefor the following boilerplate macros are provided to help
you adapt already exsiting functions, thus reducing the need to
[link phoenix.modules.bind] functions.
[section BOOST_PHOENIX_ADAPT_FUNCTION_NULLARY]
[heading Description]
`BOOST_PHOENIX_ADAPT_FUNCTION_NULLARY` is a macro that can be used to generate
all the necessary boilerplate to make an arbitrary nullary function a lazy
function.
[note These macros generate no global objects. The resulting lazy functions are real functions
that create the lazy function expression object]
[heading Synopsis]
BOOST_PHOENIX_ADAPT_FUNCTION_NULLARY(
RETURN_TYPE
, LAZY_FUNCTION
, FUNCTION
)
[heading Semantics]
The above macro generates all necessary code to have a nullary lazy function
`LAZY_FUNCTION` which calls the nullary `FUNCTION` that has the return type
`RETURN_TYPE`
[heading Header]
#include <boost/phoenix/function/adapt_function.hpp>
[heading Example]
namespace demo
{
int foo()
{
return 42;
}
}
BOOST_PHOENIX_ADAPT_FUNCTION_NULLARY(int, foo, demo::foo)
int main()
{
using boost::phoenix::placeholders::_1;
assert((_1 + foo())(1) == 43);
}
[endsect]
[section BOOST_PHOENIX_ADAPT_FUNCTION]
[heading Description]
`BOOST_PHOENIX_ADAPT_FUNCTION` is a macro that can be used to generate
all the necessary boilerplate to make an arbitrary function a lazy function.
[heading Synopsis]
BOOST_PHOENIX_ADAPT_FUNCTION(
RETURN_TYPE
, LAZY_FUNCTION
, FUNCTION
, FUNCTION_ARITY
)
[heading Semantics]
The above macro generates all necessary code to have a lazy function
`LAZY_FUNCTION` which calls `FUNCTION` that has the return type
`RETURN_TYPE` with `FUNCTION_ARITY` number of arguments.
[heading Header]
#include <boost/phoenix/function/adapt_function.hpp>
[heading Example]
namespace demo
{
int plus(int a, int b)
{
return a + b;
}
template <typename T>
T
plus(T a, T b, T c)
{
return a + b + c;
}
}
BOOST_PHOENIX_ADAPT_FUNCTION(int, plus, demo::plus, 2)
BOOST_PHOENIX_ADAPT_FUNCTION(
typename remove_reference<A0>::type
, plus
, demo::plus
, 3
)
int main()
{
using boost::phoenix::arg_names::arg1;
using boost::phoenix::arg_names::arg2;
int a = 123;
int b = 256;
assert(plus(arg1, arg2)(a, b) == a+b);
assert(plus(arg1, arg2, 3)(a, b) == a+b+3);
}
[endsect]
[/
[section BOOST_PHOENIX_ADAPT_FUNCTION_VARARG]
[heading Description]
`BOOST_PHOENIX_ADAPT_FUNCTION_VARARG` is a macro that can be used to generate
all the necessary boilerplate to make an arbitrary function a lazy
function.
[heading Synopsis]
BOOST_PHOENIX_ADAPT_FUNCTION_VARARG(
RETURN_TYPE
, LAZY_FUNCTION
, FUNCTION
)
[heading Semantics]
[heading Header]
#include <boost/phoenix/function/adapt_function.hpp>
[heading Example]
[endsect]
]
[section BOOST_PHOENIX_ADAPT_CALLABLE_NULLARY]
[heading Description]
`BOOST_PHOENIX_ADAPT_CALLABLE_NULLARY` is a macro that can be used to generate
all the necessary boilerplate to make an arbitrary nullary function object a
lazy function.
[heading Synopsis]
BOOST_PHOENIX_ADAPT_CALLABLE_NULLARY(
LAZY_FUNCTION
, CALLABLE
)
[heading Semantics]
The above macro generates all necessary code to create `LAZY_FUNCTION` which
creates a lazy function object that represents a nullary call to `CALLABLE`.
The return type is specified by `CALLABLE` conforming to the __boost_result_of__
protocol.
[heading Header]
#include <boost/phoenix/function/adapt_callable.hpp>
[heading Example]
namespace demo
{
struct foo
{
typedef int result_type;
result_type operator()() const
{
return 42;
}
}
}
BOOST_PHOENIX_ADAPT_CALLABLE_NULLARY(foo, demo::foo)
int main()
{
using boost::phoenix::placeholders::_1;
assert((_1 + foo())(1) == 43);
}
[endsect]
[section BOOST_PHOENIX_ADAPT_CALLABLE]
[heading Description]
`BOOST_PHOENIX_ADAPT_CALLABLE` is a macro that can be used to generate
all the necessary boilerplate to make an arbitrary function object a lazy
function.
[heading Synopsis]
BOOST_PHOENIX_ADAPT_CALLABLE(
LAZY_FUNCTION
, FUNCTION_NAME
, FUNCTION_ARITY
)
[heading Semantics]
The above macro generates all necessary code to create `LAZY_FUNCTION` which
creates a lazy function object that represents a call to `CALLABLE` with `FUNCTION_ARITY`
arguments.
The return type is specified by `CALLABLE` conforming to the __boost_result_of__
protocol.
[heading Header]
#include <boost/phoenix/function/adapt_callable.hpp>
[heading Example]
namespace demo
{
struct plus
{
template <typename Sig>
struct result;
template <typename This, typename A0, typename A1>
struct result<This(A0, A1)>
: remove_reference<A0>
{};
template <typename This, typename A0, typename A1, typename A2>
struct result<This(A0, A1, A2)>
: remove_reference<A0>
{};
template <typename A0, typename A1>
A0 operator()(A0 const & a0, A1 const & a1) const
{
return a0 + a1;
}
template <typename A0, typename A1, typename A2>
A0 operator()(A0 const & a0, A1 const & a1, A2 const & a2) const
{
return a0 + a1 + a2;
}
};
}
BOOST_PHOENIX_ADAPT_CALLABLE(plus, demo::plus, 2)
BOOST_PHOENIX_ADAPT_CALLABLE(plus, demo::plus, 3)
int main()
{
using boost::phoenix::arg_names::arg1;
using boost::phoenix::arg_names::arg2;
int a = 123;
int b = 256;
assert(plus(arg1, arg2)(a, b) == a+b);
assert(plus(arg1, arg2, 3)(a, b) == a+b+3);
}
[endsect]
[/
[section BOOST_PHOENIX_ADAPT_CALLABLE_VARARG]
[heading Description]
`BOOST_PHOENIX_ADAPT_CALLABLE_VARARG` is a macro that can be used to generate
all the necessary boilerplate to make an arbitrary function object a lazy
function.
[heading Synopsis]
BOOST_PHOENIX_ADAPT_CALLABLE_VARARG(
LAZY_FUNCTION_NAME
, FUNCTION_NAME
)
[heading Semantics]
[heading Header]
#include <boost/phoenix/function/adapt_callable.hpp>
[heading Example]
[endsect]
/]
[endsect]
[endsect]