| -----Original Message----- | From: boost-bounces@lists.boost.org | [mailto:boost-bounces@lists.boost.org] On Behalf Of David Abrahams | Sent: 15 July 2005 19:52 | To: boost@lists.boost.org | Subject: [boost] [parameter] Tutorial checked in | | | I think the tutorial documentation is close to complete. Last minute | editorial comments would be appreciated. See | | libs/parameter/doc/html/index.html | There are some funnies in several places involving “ For example, using FireFox This “fancy dance” involving the unnamed namespace and references is all done to avoid violating the One Definition Rule (ODR) ... Paul Paul A Bristow Prizet Farmhouse, Kendal, Cumbria UK LA8 8AB +44 1539 561830 +44 7714 330204 mailto: pbristow@hetp.u-net.com

David Abrahams wrote:

I think the tutorial documentation is close to complete. Last minute editorial comments would be appreciated. See libs/parameter/doc/html/index.html

Dave, I have a question about documentation of *my* library that uses Boost.Parameter. Documentation of the original new_window example is straghtforward because the public interface is exactly what the user sees and uses: window* new_window( char const* name, int border_width = default_border_width, bool movable = true, bool initially_visible = true ); However, the interface to the equivalent function that uses Boost.Parameter is extremely complex, even if the resulting usage is beautifully simple: window* w = new_window("alert box", movable=false); // OK! So, my question. How would you go about documenting this Boost.Parameter-ized interface? I guess I'm suffering from a lack of imagination, but I'm genuinely stumped by this. Perhaps you could add such a documentation to the dfs example? Regards, Angus

Hi, A very minor change to the example code. I also see the funny chars: "... args[…] ...". I use Firefox 1.0.4 on Win2K. Adi ÿþ+++++++++++++++++++++++++++++++++++++++++++++++++ The Boost Parameter Library |(logo)|__ +++++++++++++++++++++++++++++++++++++++++++++++++ .. |(logo)| image:: ../../../../boost.png :alt: Boost __ ../../../../index.htm .. Firefox, at least, seems to need some help lowering subscripts. Without the following, subscripts seem not to drop at all. .. raw:: html <style type="text/css"> sub { vertical-align: -20% } </style> ------------------------------------- :Authors: David Abrahams, Daniel Wallin :Contact: dave@boost-consulting.com, dalwan01@student.umu.se :organization: `Boost Consulting`_ :date: $Date: 2005/07/18 20:34:31 $ :copyright: Copyright David Abrahams, Daniel Wallin 2005. 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) .. _`Boost Consulting`: http://www.boost-consulting.com :Abstract: Use this library to write functions that accept arguments by name: .. parsed-literal:: new_window("alert", **width=10**, **titlebar=false**); This capability is especially useful when a function has more than one argument with a useful default value, since named arguments can be passed in any order. .. _concepts: ../../../more/generic_programming.html#concept .. contents:: **Table of Contents** .. role:: concept :class: interpreted .. section-numbering:: ------------------------------------- ============== Introduction ============== In C++, arguments are normally given meaning by their positions with respect to a parameter list. That protocol is fine when there is at most one parameter with a default value, but when there are even a few useful defaults, the positional interface becomes burdensome: * Since an argument's meaning is given by its position, we have to choose an (often arbitrary) order for parameters with default values, making some combinations of defaults unusable: .. parsed-literal:: window* new_window( char const* name, **int border_width = default_border_width,** bool movable = true, bool initially_visible = true ); const bool movability = false; window* w = new_window("alert box", movability); In the example above we wanted to make an unmoveable window with a default ``border_width``, but instead we got a moveable window with a ``border_width`` of zero. To get the desired effect, we'd need to write: .. parsed-literal:: window* w = new_window( "alert box", **default_border_width**, movability); * It can become difficult for readers to understand the meaning of arguments at the call site:: window* w = new_window("alert", 1, true, false); Is this window moveable and initially invisible, or unmoveable and initially visible? The reader needs to remember the order of arguments to be sure. * The author of the call may not remember the order of the arguments either, leading to hard-to-find bugs. This library addresses the problems outlined above by associating each parameter with a keyword object. Now users can identify arguments by keyword, rather than by position: .. parsed-literal:: window* w = new_window("alert box", **movable=**\ false); // OK! .. I'm inclined to leave this part out. In particular, the 2nd point is kinda lame because even with the library, we need to introduce overloads -- dwa: C++ has two other limitations, with respect to default arguments, that are unrelated to its positional interface: * Default values cannot depend on the values of other function parameters: .. parsed-literal:: // Can we make resize windows to a square shape by default? void resize( window* w, int **width**, int height **= width** // nope, error! ); * Default values in function templates are useless for any argument whose type should be deduced when the argument is supplied explicitly:: template <class T> void f(T x = 0); f(3.14) // ok: x supplied explicitly; T is double f(); // error: can't deduce T from default argument 0! As a side effect of using the Boost Parameter library, you may find that you circumvent both of these limitations quite naturally. ========== Tutorial ========== In this section we'll show how the Parameter library can be used to build an expressive interface to the `Boost Graph library`__\ 's |dfs|_ algorithm. [#old_interface]_ After laying some groundwork and describing the algorithm's abstract interface, we'll show you how to build a basic implementation with keyword support. Then we'll add support for default arguments and we'll gradually refine the implementation with syntax improvements. Finally we'll show how to streamline the implementation of named parameter interfaces, improve their participation in overload resolution, and optimize their runtime efficiency. __ ../../../graph/index.html .. _dfs: ../../../graph/doc/depth_first_search.html .. |dfs| replace:: ``depth_first_search`` Headers And Namespaces ====================== Most components of the Parameter library are declared in a header named for the component. For example, :: #include <boost/parameter/keyword.hpp> will ensure ``boost::parameter::keyword`` is known to the compiler. There is also a combined header, ``boost/parameter.hpp``, that includes most of the library's components. For the the rest of this tutorial, unless we say otherwise, you can use the rule above to figure out which header to ``#include`` to access any given component of the library. Also, the examples below will also be written as if the namespace alias :: namespace parameter = boost::parameter; has been declared: we'll write ``parameter::xxx`` instead of ``boost::parameter::xxx``. The Abstract Interface to |dfs| =============================== The Graph library's |dfs| algorithm is a generic function accepting from one to four arguments by reference, as shown in the table below: .. RS -- Seeing the function described via table is harder to grasp. I suggest showing the function signature first, but omit the defaults for clarity. That will provide parameter names, in context, which will make the connection to the table simpler. .. _`parameter table`: .. _`default expressions`: .. table:: ``depth_first_search`` Parameters +----------------+----------+----------------------------------+ | Parameter Name | Dataflow | Default Value (if any) | +================+==========+==================================+ |``graph`` | in |none - this argument is required. | +----------------+----------+----------------------------------+ |``visitor`` | in |``boost::dfs_visitor<>()`` | +----------------+----------+----------------------------------+ |``root_vertex`` | in |``*vertices(graph).first`` | +----------------+----------+----------------------------------+ |``index_map`` | in |``get(boost::vertex_index,graph)``| +----------------+----------+----------------------------------+ |``color_map`` | out |an ``iterator_property_map`` | | | |created from a ``std::vector`` of | | | |``default_color_type`` of size | | | |``num_vertices(graph)`` and using | | | |``index_map`` for the index map. | +----------------+----------+----------------------------------+ Don't be intimidated by the complex default values. For the purposes of this exercise, you don't need to understand what they mean. Also, we'll show you how the default for ``color_map`` is computed later in the tutorial; trust us when we say that the complexity of its default will become valuable. Defining the Keywords ===================== The point of this exercise is to make it possible to call ``depth_first_search`` with keyword arguments, leaving out any arguments for which the default is appropriate: .. parsed-literal:: graphs::depth_first_search(g, **color_map = my_color_map**); To make that syntax legal, there needs to be an object called ``color_map`` with an assignment operator that can accept a ``my_color_map`` argument. In this step we'll create one such **keyword object** for each parameter. Each keyword object will be identified by a unique **keyword tag type**. We're going to define our interface in namespace ``graphs``. Since users need access to the keyword objects, but not the tag types, we'll define the keyword objects so they're acceessible through ``graphs``, and we'll hide the tag types away in a tested namespace, ``graphs::tag``. The library provides a convenient macro for that purpose (MSVC6.x users see this note__):: #include <boost/parameter/keyword.hpp> namespace graphs { BOOST_PARAMETER_KEYWORD(tag, graph) // Note: no semicolon BOOST_PARAMETER_KEYWORD(tag, visitor) BOOST_PARAMETER_KEYWORD(tag, root_vertex) BOOST_PARAMETER_KEYWORD(tag, index_map) BOOST_PARAMETER_KEYWORD(tag, color_map) } __ `Compiler Can't See References In Unnamed Namespace`_ The declaration of the ``visitor`` keyword you see here is equivalent to:: namespace graphs { namespace tag { struct visitor; } namespace { boost::parameter::keyword<tag::visitor>& visitor = boost::parameter::keyword<tag::visitor>::get(); } } This  fancy dance involving the unnamed namespace and references is all done to avoid violating the One Definition Rule (ODR) [#odr]_ when the named parameter interface is used by function templates that are instantiated in multiple translation units. Defining the Implementation Function ==================================== Next we can write the skeleton of the function that implements the core of ``depth_first_search``:: namespace graphs { namespace core { template <class ArgumentPack> void depth_first_search(ArgumentPack const& args) { // algorithm implementation goes here } }} .. |ArgumentPack| replace:: :concept:`ArgumentPack` ``core::depth_first_search`` has an |ArgumentPack| parameter: a bundle of references to the arguments that the caller passes to the algorithm, tagged with their keywords. To extract each parameter, just pass its keyword object to the |ArgumentPack|\ 's subscript operator. Just to get a feel for how things work, let's add some temporary code to print the arguments: .. parsed-literal:: namespace graphs { namespace core { template <class ArgumentPack> void depth_first_search(ArgumentPack const& args) { std::cout << "graph:\\t" << **args[graph]** << std::endl; std::cout << "visitor:\\t" << **args[visitor]** << std::endl; std::cout << "root_vertex:\\t" << **args[root_vertex]** << std::endl; std::cout << "index_map:\\t" << **args[index_map]** << std::endl; std::cout << "color_map:\\t" << **args[color_map]** << std::endl; } }} // graphs::core It's unlikely that many of the arguments the caller will eventually pass to ``depth_first_search`` can be printed, but for now the code above will give us something to experiment with. To see the keywords in action, we can write a little test driver: .. parsed-literal:: int main() { using namespace graphs; core::depth_first_search(**(** graph = 'G', visitor = 2, root_vertex = 3.5, index_map = "hello, world", color_map = false\ **)**); } An overloaded comma operator (``operator,``) combines the results of assigning to each keyword object into a single |ArgumentPack| object that gets passed on to ``core::depth_first_search``. The extra set of parentheses you see in the example above are required: without them, each assignment would be interpreted as a separate function argument and the comma operator wouldn't take effect. We'll show you how to get rid of the extra parentheses later in this tutorial. Of course, we can pass the arguments in any order:: int main() { using namespace graphs; core::depth_first_search(( root_vertex = 3.5, graph = 'G', color_map = false, index_map = "hello, world", visitor = 2)); } either of the two programs above will print:: graph: G visitor: 2 root_vertex: 3.5 index_map: hello, world color_map: false Adding Defaults =============== Currently, all the arguments to ``depth_first_search`` are required. If any parameter can't be found, there will be a compilation error where we try to extract it from the |ArgumentPack| using the subscript operator. To make it legal to omit an argument we need to give it a default value. Syntax ------ We can make any of the parameters optional by following its keyword with the ``|`` operator and the parameter's default value within the square brackets. In the following example, we've given ``root_vertex`` a default of ``42`` and ``color_map`` a default of ``"hello, world"``. .. parsed-literal:: namespace graphs { namespace core { template <class ArgumentPack> void depth_first_search(ArgumentPack const& args) { std::cout << "graph:\\t" << args[graph] << std::endl; std::cout << "visitor:\\t" << args[visitor] << std::endl; std::cout << "root_vertex:\\t" << args[root_vertex\ **|42**\ ] << std::endl; std::cout << "index_map:\\t" << args[index_map] << std::endl; std::cout << "color_map:\\t" << args[color_map\ **|"hello, world"**\ ] << std::endl; } }} // graphs::core Now we can invoke the function without supplying ``color_map`` or ``root_vertex``:: core::depth_first_search(( graph = 'G', index_map = "index", visitor = 6)); The call above would print:: graph: G visitor: 6 root_vertex: 42 index_map: index color_map: hello, world .. Important:: The index expression ``args[& ]`` always yields a *reference* that is bound either to the actual argument passed by the caller or, if no argument is passed explicitly, to the specified default value. Getting More Realistic ---------------------- Now it's time to put some more realistic defaults in place. We'll have to give up our print statements at least if we want to see the defaults work since, the default values of these parameters generally aren't printable. Instead, we'll connect local variables to the arguments and use those in our algorithm: .. parsed-literal:: namespace graphs { namespace core { template <class ArgumentPack> void depth_first_search(ArgumentPack const& args) { *Graph* g = args[graph]; *Visitor* v = args[visitor|\ *default-expression*\ :sub:`1`\ ]; *Vertex* s = args[root_vertex|\ *default-expression*\ :sub:`2`\ ]; *Index* i = args[index_map|\ *default-expression*\ :sub:`3`\ ]; *Color* c = args[visitor|\ *default-expression*\ :sub:`4`\ ]; *& use g, v, s, i, and c to implement the algorithm& * } }} // graphs::core We'll insert the `default expressions`_ in a moment, but first we need to come up with the types *Graph*, *Visitor*, *Vertex*, *Index*, and *Color*. The ``binding`` |Metafunction|_ ------------------------------- To compute the type of a parameter we can use a |Metafunction|_ called ``binding``: .. parsed-literal:: binding<ArgumentPack, Keyword, Default = void> { typedef *see text* type; }; where ``Default`` is the type of the default argument, if any. For example, to declare and initialize ``g`` above, we could write: .. parsed-literal:: typedef typename parameter::binding< ArgumentPack,\ **tag::graph**

::type Graph;

Graph g = args[graph]; As shown in the `parameter table`_, ``graph`` has no default, so the ``binding`` invocation for *Graph* takes only two arguments. The default ``visitor`` is ``boost::dfs_visitor<>()``, so the ``binding`` invocation for *Visitor* takes three arguments: .. parsed-literal:: typedef typename parameter::binding< ArgumentPack,\ **tag::visitor,boost::dfs_visitor<>**

::type Visitor;

Visitor v = args[visitor|\ **boost::dfs_visitor<>()**\ ]; Note that the default ``visitor`` is supplied as a *temporary* instance of ``dfs_visitor``. Because ``args[& ]`` always yields a reference, making ``v`` a reference would cause it to bind to that temporary, and immediately dangle. Therefore, it's crucial that we passed ``dfs_visitor<>``, and not ``dfs_visitor<> const&``, as the last argument to ``binding``. .. Important:: Never pass ``binding`` a reference type as the default unless you know that the default value passed to the |ArgumentPack|\ 's indexing operator will outlive the reference you'll bind to it. Sometimes there's no need to use ``binding`` at all. The ``root_vertex`` argument is required to be of the graph's ``vertex_descriptor`` type, [#vertex_descriptor]_ so we can just use that knowledge to bypass ``binding`` altogether. .. parsed-literal:: typename **boost::graph_traits<Graph>::vertex_descriptor** s = args[root_vertex|\ ***vertices(g).first**\ ]; .. _dangling: .. |Metafunction| replace:: :concept:`Metafunction` .. _Metafunction: ../../../mpl/doc/refmanual/metafunction.html Beyond Ordinary Default Arguments --------------------------------- Here's how you might write the declaration for the ``index_map`` parameter: .. parsed-literal:: typedef typename parameter::binding< ArgumentPack , tag::index_map , **typename boost::property_map<Graph, vertex_index_t>::const_type**

::type Index;

Index i = args[index_map|\ **get(boost::vertex_index,g)**\ ]; Notice two capabilities we've gained over what plain C++ default arguments provide: 1. The default value of the ``index`` parameter depends on the value of the ``graph`` parameter. That's illegal in plain C++: .. parsed-literal:: void f(int **graph**, int index = **graph** + 1); // error 2. The ``index`` parameter has a useful default, yet it is templated and its type can be deduced when an ``index`` argument is explicitly specified by the caller. In plain C++, you can *specify* a default value for a parameter with deduced type, but it's not very useful: .. parsed-literal:: template <class Index> int f(Index index **= 42**); // OK int y = f(); // **error; can't deduce Index** Syntactic Refinement ==================== In this section we'll describe how you can allow callers to invoke ``depth_first_search`` with just one pair of parentheses, and to omit keywords where appropriate. Describing the Positional Argument Order ---------------------------------------- .. _ParameterSpec: .. |ParameterSpec| replace:: :concept:`ParameterSpec` First, we'll need to build a type that describes the allowed parameters and their ordering when passed positionally. This type is known as a |ParameterSpec| (MSVC6.x users see this note__):: namespace graphs { typedef parameter::parameters< tag::graph , tag::visitor , tag::root_vertex , tag::index_map , tag::color_map > dfs_params; } __ `Can't Declare ParameterSpec Via typedef`_ The ``parameters`` template supplies a function-call operator that groups all its arguments into an |ArgumentPack|. Any arguments passed to it without a keyword label will be associated with a parameter according to its position in the |ParameterSpec|. So for example, given an object ``p`` of type ``dfs_params``, :: p('G', index_map=1) yields an |ArgumentPack| whose ``graph`` parameter has a value of ``'G'``, and whose ``index_map`` parameter has a value of ``1``. Forwarding Functions -------------------- Next we need a family of overloaded ``depth_first_search`` function templates that can be called with anywhere from one to five arguments. These *forwarding functions* will invoke an instance of ``dfs_params`` as a function object, passing their parameters to its ``operator()`` and forwarding the result on to ``core::depth_first_search``:: namespace graphs { template <class A0> void depth_first_search(A0 const& a0) { core::depth_first_search(dfs_params()(a0)); } template <class A0, class A1> void depth_first_search(A0 const& a0, A1 const& a1) { core::depth_first_search(dfs_params()(a0,a1)); } î" template <class A0, class A1, & class A4> void depth_first_search(A0 const& a0, A1 const& a1, & A4 const& a4) { core::depth_first_search(dfs_params()(a0,a1,a2,a3,a4)); } } That's it! We can now call ``graphs::depth_first_search`` with from one to five arguments passed positionally or via keyword.  Out Parameters ---------------- Well, that's not *quite* it. The overload set above works fine when ``color_map`` is passed by keyword, but it breaks down when it is passed positionally. As you may recall from the ``depth_first_search`` `parameter table`_, ``color_map`` is an  out parameter. That means the five-argument ``depth_first_search`` overload should really take its final argument by non-``const`` reference. On the other hand, when passing arguments by keyword, the keyword object's assignment operator yields a temporary |ArgumentPack| object, and a conforming C++ compiler will refuse to bind a non-``const`` reference to a temporary. To support an interface in which the last argument is passed by keyword, there must be a ``depth_first_search`` overload taking its argument by ``const`` reference. The simplest solution in this case is to add another overload: .. parsed-literal:: template <class A0, class A1, & class A4> void depth_first_search(A0 **const&** a0, A1 **const&** a1, & \ A4\ **&** a4) { core::depth_first_search(dfs_params()(a0,a1,a2,a3,a4)); } That works nicely, but only because there is only one  out parameter and it is in the last position. If ``color_map`` had been the first parameter, we would have needed *ten* overloads. In the worst case where the function has five  out parameters 2\ :sup:`5` or 32 overloads would be required. This  \ `forwarding problem`_\  is well-known to generic library authors, and the C++ standard committee is working on a proposal__ to address it. .. _`forwarding problem`: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm __ http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1690.html If it is impractical for you to write the overloads that would be required for positional  out arguments to be passed directly, you still have the option to ask users to pass them through |ref|_, which will ensure that the algorithm implementation sees a non-``const`` reference: .. parsed-literal:: depth_first_search(g, v, s, i, **boost::ref(c)**); .. |ref| replace:: ``boost::ref`` .. _ref: http://www.boost.org/doc/html/reference_wrapper.html Generating Forwarding Functions with Macros ------------------------------------------- To remove some of the tedium of writing overloaded forwarding functions, the library supplies a macro, suitably located in ``boost/parameter/macros.hpp``, that will generate free function overloads for you:: BOOST_PARAMETER_FUN(void, depth_first_search, 1, 5, dfs_params); will generate a family of five ``depth_first_search`` overloads, in the current scope, that pass their arguments through ``dfs_params``. Instead of ``core::depth_first_search``, these overloads will forward the |ArgumentPack| on to a function called ``depth_first_search_with_named_params``, also in the current scope. It's up to you to implement that function. You could simply transplant the body of ``core::depth_first_search`` into ``depth_first_search_with_named_params`` if you were going to use this approach. Note that ``BOOST_PARAMETER_FUN`` only takes arguments by ``const`` reference, so you will have to add any additional overloads required to handle positional  out parameters yourself. We are looking into providing a more sophisticated set of macros to address this problem and others, for an upcoming release of Boost. Controlling Overload Resolution =============================== The parameters of our templated forwarding functions are completely general; in fact, they're a perfect match for any argument type whatsoever. The problems with exposing such general function templates have been the subject of much discussion; especially in the presence of `unqualified calls`__. Probably the safest thing to do is to isolate the forwarding functions in a namespace containing no types [#using]_, but often we'd *like* our functions to play nicely with argument-dependent lookup and other function overloads. In that case, it's neccessary to remove the functions from the overload set when the passed argument types aren't appropriate. __ http://anubis.dkuug.dk/jtc1/sc22/wg21/docs/lwg-defects.html#225 Updating the |ParameterSpec| ---------------------------- This sort of overload control can be accomplished in C++ by taking advantage of the SFINAE (Substitution Failure Is Not An Error) rule. [#sfinae]_ You can take advantage of the Parameter library's built-in SFINAE support by using the following class templates in your |ParameterSpec|: .. parsed-literal:: template< class KeywordTag, class Predicate = *unspecified* > struct required; template< class KeywordTag, class Predicate = *unspecified* > struct optional; Instead of using keyword tags directly, we can wrap them in ``required`` and ``optional`` to indicate which function parameters are required, and optionally pass ``Predicate``\ s to describe the type requirements for each function parameter. The ``Predicate`` argument must be a unary `MPL lambda expression`_ that, when applied to the actual type of the argument, indicates whether that argument type meets the function's requirements for that parameter position. .. _`MPL lambda expression`: ../../../mpl/doc/refmanual/lambda-expression.html For example, let's say we want to restrict ``depth_first_search()`` so that the ``graph`` parameter is required and the ``root_vertex`` parameter is convertible to ``int``. We might write: .. parsed-literal:: #include <boost/type_traits/is_convertible.hpp> #include <boost/mpl/placeholders.hpp> namespace graphs { using namespace boost::mpl::placeholders; struct dfs_params : parameter::parameters< **parameter::required<tag::graph>** , parameter::optional<tag::visitor> , **parameter::optional< tag::root_vertex, boost::is_convertible<_,int> >** , parameter::optional<tag::index_map> , parameter::optional<tag::color_map> > {}; } Applying SFINAE to the Overload Set ----------------------------------- Now we add a special defaulted argument to each of our ``depth_first_search`` overloads: .. parsed-literal:: namespace graphs { template <class A0> void depth_first_search( A0 const& a0 , typename dfs_params::match<A0>::type p = dfs_params()) { core::depth_first_search(**p**\ (a0)); } template <class A0, class A1> void depth_first_search( A0 const& a0, A1 const& a1 , typename dfs_params::match<A0,A1>::type p = dfs_params()) { core::depth_first_search(**p**\ (a0,a1)); } î" template <class A0, class A1, & class A4> void depth_first_search( A0 const& a0, A1 const& a1, & A4 const& A4 , typename dfs_params::match<A0,A1,A2,A3,A4>::type p = dfs_params()) { core::depth_first_search(**p**\ (a0,a1,a2,a3,a4)); } } These additional parameters are not intended to be used directly by callers; they merely trigger SFINAE by becoming illegal types when the ``name`` argument is not convertible to ``const char*``. The ``BOOST_PARAMETER_FUN`` macro described earlier adds these extra function parameters for you (Borland users see this note__). .. _BOOST_PARAMETER_MATCH: __ `Default Arguments Unsupported on Nested Templates`_ Reducing BoilerPlate With Macros -------------------------------- The library provides a macro you can use to eliminate some of the repetetiveness of the declaring the optional parameters. ``BOOST_PARAMETER_MATCH`` takes three arguments: the |ParameterSpec|, a `Boost.Preprocessor sequence`__ of the function argument types, and a name for the defaulted function parameter (``p``, above). So we could shorten the overload set definition as follows: __ http://boost-consulting.com/mplbook/preprocessor.html#sequences .. parsed-literal:: namespace graphs { template <class A0> void depth_first_search( A0 const& a0 , **BOOST_PARAMETER_MATCH(dfs_params, (A0), p)**) { core::depth_first_search(p(a0)); } template <class A0, class A1> void depth_first_search( A0 const& a0, A1 const& a1 , **BOOST_PARAMETER_MATCH(dfs_params, (A0)(A1), p)**) { core::depth_first_search(p(a0,a1)); } î" template <class A0, class A1, & class A4> void depth_first_search( A0 const& a0, A1 const& a1, & A4 const& A4 , **BOOST_PARAMETER_MATCH(dfs_params, (A0)(A1)& (A4), p)**) { core::depth_first_search(p(a0,a1,a2,a3,a4)); } } Efficiency Issues ================= The ``color_map`` parameter gives us a few efficiency issues to consider. Here's a first cut at extraction and binding: .. parsed-literal:: typedef vector_property_map<boost::default_color_type, Index> default_color_map; typename parameter::binding< ArgumentPack , tag::color_map , default_color_map

::type color = args[color_map|\ **default_color_map(num_vertices(g),i)**\ ];

Eliminating Copies ------------------ The library has no way to know whether an explicitly-supplied argument is expensive to copy (or even if it is copyable at all), so ``binding<& ,k,& >::type`` is always a reference type when the *k* parameter is supplied by the caller. Since ``args[& ]`` yields a reference to the actual argument, ``color`` will be bound to the actual ``color_map`` argument and no copying will be done. As described above__, because the default is a temporary, it's important that ``color`` be a non-reference when the default is used. In that case, the default value will be *copied* into ``color``. If we store the default in a named variable, though, ``color`` can be a reference, thereby eliminating the copy: .. parsed-literal:: default_color_map default_color(num_vertices(g),i); typename parameter::binding< ArgumentPack , tag::color_map , **default_color_map&**

::type color = args[color_map|default_color];

__ dangling_ .. Hint:: To avoid making needless copies, pass a *reference to the default type* as the third argument to ``binding``. Lazy Default Computation ------------------------ Of course it's nice to avoid copying ``default_color``, but the more important cost is that of *constructing* it in the first place. A ``vector_property_map`` is cheap to copy, since it holds its elements via a |shared_ptr|_. On the other hand, construction of ``default_color`` costs at least two dynamic memory allocations and ``num_vertices(g)`` copies; it would be better to avoid doing this work when the default value won't be needed. .. |shared_ptr| replace:: ``shared_ptr`` .. _shared_ptr: ../../../smart_ptr/shared_ptr.htm To that end, the library allows us to supply a callable object that if no argument was supplied by the caller will be invoked to construct the default value. Instead of following the keyword with the ``|`` operator, we'll use ``||`` and follow it with a nullary (zero-argument) function object that constructs a default_color_map. Here, we build the function object using Boost.Lambda_: [#bind]_ .. _Boost.Lambda: ../../../lambda/index.html .. parsed-literal:: // After #include <boost/lambda/construct.hpp> typename parameter::binding< ArgumentPack , tag::color_map , default_color_map

::type color = args[ color_map **|| boost::lambda::construct<default_color_map>(num_vertices(g),i)** ];

.. sidebar:: Memnonics To remember the difference between ``|`` and ``||``, recall that ``||`` normally uses short-circuit evaluation: its second argument is only evaluated if its first argument is ``false``. Similarly, in ``color_map[param||f]``, ``f`` is only invoked if no ``color_map`` argument was supplied. Default Forwarding ------------------ Types that are expensive to construct yet cheap to copy aren't all that typical, and even copying the color map is more expensive than we might like. It might be nice to avoid both needless construction *and* needless copying of the default color map. The simplest way to achieve that is to avoid naming it altogether, at least not in ``core::depth_first_search``. Instead, we'll introduce another function template to implement the actual algorithm: .. parsed-literal:: namespace graphs { namespace core { template <class G, class V, class S, class I, class C> void **dfs_impl**\ (G& g, V& v, S& s, I& i, C& c) { *& actual algorithm implementation& * } }} Then, in ``core::depth_first_search``, we'll simply forward the result of indexing ``args`` to ``core::dfs_impl``:: core::dfs_impl( g,v,s,i , args[ color_map || boost::lambda::construct<default_color_map>(num_vertices(g),i) ]); In real code, after going to the trouble to write ``dfs_impl``, we'd probably just forward all the arguments. Dispatching Based on the Presence of a Default ---------------------------------------------- In fact, the Graph library itself constructs a slightly different ``color_map``, to avoid even the overhead of initializing a |shared_ptr|_:: std::vector<boost::default_color_type> color_vec(num_vertices(g)); boost::iterator_property_map< typename std::vector< boost::default_color_type >::iterator , Index

c(color_vec.begin(), i);

To avoid instantiating that code when it isn't needed, we'll have to find a way to select different function implementations, at compile time, based on whether a ``color_map`` argument was supplied. By using `tag dispatching`_ on the presence of a ``color_map`` argument, we can do just that: .. _`tag dispatching`: ../../../../more/generic_programming.html#tag_dispatching .. parsed-literal:: #include <boost/type_traits/is_same.hpp> #include <boost/mpl/bool.hpp> namespace graphs { namespace core { template <class ArgumentPack> void dfs_dispatch(ArgumentPack& args, **mpl::true_**) { *& use the color map computed in the previous example& * } template <class ArgumentPack> void dfs_dispatch(ArgumentPack& args, **mpl::false_**) { *& use args[color]& * } template <class ArgumentPack> void depth_first_search(ArgumentPack& args) { typedef typename binding<args,tag::color>::type color\_; core::dfs_dispatch(args, **boost::is_same<color\_,void>()**\ ); } }} We've used the fact that the default for ``binding``\ 's third argument is ``void``: because specializations of ``is_same`` are ``bool``-valued MPL |Integral Constant|_\ s derived either from ``mpl::true_`` or ``mpl::false_``, the appropriate ``dfs_dispatch`` implementation will be selected. .. |Integral Constant| replace:: :concept:`Integral Constant` .. _`Integral Constant`: ../../../mpl/doc/refmanual/integral-constant.html ============================ Portability Considerations ============================ Use the `regression test results`_ for the latest Boost release of the Parameter library to see how it fares on your favorite compiler. Additionally, you may need to be aware of the following issues and workarounds for particular compilers. .. _`regression test results`: http://www.boost.org/regression/release/user/parameter.html No SFINAE Support ================= Some older compilers don't support SFINAE. If your compiler meets that criterion, then Boost headers will ``#define`` the preprocessor symbol ``BOOST_NO_SFINAE``, and uses of ``parameters<& >::match`` and |BOOST_PARAMETER_MATCH| will be harmless, but will have no effect. No Support for |result_of|_ =========================== .. |result_of| replace:: ``result_of`` .. _result_of: ../../../utility/utility.htm#result_of `Lazy default computation`_ relies on the |result_of| class template to compute the types of default arguments given the type of the function object that constructs them. On compilers that don't support |result_of|, ``BOOST_NO_RESULT_OF`` will be ``#define``\ d, and the compiler will expect the function object to contain a nested type name, ``result_type``, that indicates its return type when invoked without arguments. To use an ordinary function as a default generator on those compilers, you'll need to wrap it in a class that provides ``result_type`` as a ``typedef`` and invokes the function via its ``operator()``. Can't Declare |ParameterSpec| via ``typedef`` ============================================= In principle you can declare a |ParameterSpec| as a ``typedef`` for a specialization of ``parameters<& >``, but Microsoft Visual C++ 6.x has been seen to choke on that usage. The workaround is to use inheritance and declare your |ParameterSpec| as a class: .. parsed-literal:: **struct dfs_parameters :** parameter::parameters< tag::graph, tag::visitor, tag::root_vertex , tag::index_map, tag::color_map > **{};** Default Arguments Unsupported on Nested Templates ================================================= As of this writing, Borland compilers don't support the use of default template arguments on member class templates. As a result, you have to supply ``BOOST_PARAMETER_MAX_ARITY`` arguments to every use of ``parameters<& >::match``. Since the actual defaults used are unspecified, the workaround is to use |BOOST_PARAMETER_MATCH|_ to declare default arguments for SFINAE. .. |BOOST_PARAMETER_MATCH| replace:: ``BOOST_PARAMETER_MATCH`` Compiler Can't See References In Unnamed Namespace ================================================== If you use Microsoft Visual C++ 6.x, you may find that the compiler has trouble finding your keyword objects. This problem has been observed, but only on this one compiler, and it disappeared as the test code evolved, so we suggest you use it only as a last resort rather than as a preventative measure. The solution is to add *using-declarations* to force the names to be available in the enclosing namespace without qualification:: namespace graphs { using graphs::graph; using graphs::visitor; using graphs::root_vertex; using graphs::index_map; using graphs::color_map; } -------------------------- .. [#old_interface] As of Boost 1.33.0 the Graph library was still using an `older named parameter mechanism`__, but there are plans to change it to use Boost.Parameter (this library) in an upcoming release, while keeping the old interface available for backward-compatibility. __ ../../../graph/doc/bgl_named_params.html .. [#odr] The **One Definition Rule** says that any given entity in a C++ program must have the same definition in all translation units (object files) that make up a program. .. [#vertex_descriptor] If you're not familiar with the Boost Graph Library, don't worry about the meaning of any Graph-library-specific details you encounter. In this case you could replace all mentions of vertex descriptor types with ``int`` in the text, and your understanding of the Parameter library wouldn't suffer. .. [#bind] The Lambda library is known not to work on `some less-conformant compilers`__. When using one of those you could define :: template <class T> struct construct2 { typedef T result_type; template <class A1, class A2> T operator() { return T(a1,a2); } }; and use Boost.Bind_ to generate the function object:: boost::bind(construct2<default_color_map>,num_vertices(g),i) __ http://www.boost.org/regression/release/user/lambda.html .. _Boost.Bind: ../../../libs/bind/index.html .. [#using] You can always give the illusion that the function lives in an outer namespace by applying a *using-declaration*:: namespace foo_overloads { // foo declarations here void foo() { ... } ... } using foo_overloads::foo; .. [#sfinae] If type substitution during the instantiation of a function template results in an invalid type, no compilation error is emitted; instead the overload is removed from the overload set. By producing an invalid type in the function signature depending on the result of some condition, whether or not an overload is considered during overload resolution can be controlled. The technique is formalized in the |enable_if|_ utility. See http://www.semantics.org/once_weakly/w02_SFINAE.pdf for more information on SFINAE. .. |enable_if| replace:: ``enable_if`` .. _enable_if: ../../../utility/enable_if.html