new MPL docs
[SVN r26212]
This commit is contained in:
Aleksey Gurtovoy committed
1 parent
d5e241cd2a
commit
54a5f8f55f
306 files changed
+27492
-7633
No files matched your search
@@ -0,0 +1,126 @@
|
||||
<?xml version="1.0" encoding="utf-8" ?>
|
||||
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
|
||||
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
||||
<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
|
||||
<title>THE BOOST MPL LIBRARY: Incomplete Support for Lambda Expressions</title>
|
||||
<link rel="stylesheet" href="../style.css" type="text/css" />
|
||||
</head>
|
||||
<body class="docframe">
|
||||
<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./broken-integral-constant.html" class="navigation-link">Prev</a> <a href="./eti.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./broken-integral-constant.html" class="navigation-link">Back</a> <a href="./eti.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./portability.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
|
||||
<td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./technical-details.html" class="navigation-link">Technical Details</a> / <a href="./portability.html" class="navigation-link">Portability</a> / <a href="./incomplete-support-for.html" class="navigation-link">Incomplete Support for Lambda Expressions</a></td>
|
||||
</tr></table><div class="header-separator"></div>
|
||||
<div class="section" id="incomplete-support-for">
|
||||
<h1><a class="toc-backref" href="./portability.html#id75" name="incomplete-support-for">Incomplete Support for Lambda Expressions</a></h1>
|
||||
<p>Seasoned MPL users will agree with us that if there is
|
||||
anything in the MPL that is seemingly magical both in power and
|
||||
its nature, then it's MPL lambda expressions. In reality, the
|
||||
mechanism that bring this to life is very straightforward and
|
||||
probably can be explained to anyone generally familiar with C++
|
||||
templates in less than 10 minutes.</p>
|
||||
<!-- People are going to hate you for saying so and not actually -->
|
||||
<!-- explaining it. -->
|
||||
<p>Unfortunately, this mechanism also happens to rely on support for
|
||||
partial template specialization and template template parameters.
|
||||
Among the so-called deficient compilers — basically, most of the
|
||||
compilers released before the year 2000 — the chances are poor
|
||||
that you'll find <em>complete</em> support for <em>both</em> of these features.
|
||||
Please see our <a class="reference" href="./portability.html#compatibility-table">compatibility table</a> for the list of the products
|
||||
which fall into this category.</p>
|
||||
<p>Although it's not possible to implement <em>fully transparent</em> lambda
|
||||
expressions without these two features, a slightly more limited
|
||||
implementation that requires some manual assistance from the
|
||||
metafunction author is possible. This section describes the manual
|
||||
work required and the limitations of the result.</p>
|
||||
<div class="section" id="incomplete-the-problem">
|
||||
<h2><a name="incomplete-the-problem">The Problem</a></h2>
|
||||
<p>If your compiler falls into the "deficient" category, the following
|
||||
valid MPL metaprogram will fail to compile for you:</p>
|
||||
<pre class="literal-block">
|
||||
#include <boost/mpl/apply.hpp>
|
||||
|
||||
using namespace boost::mpl;
|
||||
|
||||
template< typename T > struct add_const
|
||||
{
|
||||
typedef T const type;
|
||||
};
|
||||
|
||||
typedef apply1< add_const<_1>,int >::type t; // t == int const
|
||||
</pre>
|
||||
<p>Worse yet, chances are it wil fail with a diagnostic backtrace
|
||||
leading you into the inside of the library and possibly creating an
|
||||
impression that there's something's wrong there. The fact is, both
|
||||
the program and the library are defect free (for the
|
||||
purpose of this particular demonstraction), and it's your compiler
|
||||
that is to blame.</p>
|
||||
</div>
|
||||
<div class="section" id="incomplete-the-solution">
|
||||
<h2><a name="incomplete-the-solution">The Solution</a></h2>
|
||||
<p>As previously mentioned, the solution requires some work from
|
||||
metafunction authors, but for the users of those metafunctions, the
|
||||
result is relatively transparent. Here's what we have to do to our
|
||||
earlier example:</p>
|
||||
<pre class="literal-block">
|
||||
#include <boost/mpl/apply.hpp>
|
||||
<strong>#include <boost/mpl/aux_/lambda_support.hpp></strong>
|
||||
|
||||
using namespace boost::mpl;
|
||||
|
||||
template< typename T > struct add_const
|
||||
{
|
||||
typedef T const type;
|
||||
<strong>BOOST_MPL_AUX_LAMBDA_SUPPORT(1, add_const, (T))</strong>
|
||||
};
|
||||
|
||||
typedef apply1< add_const<_1>,int >::type t; // t == int const
|
||||
</pre>
|
||||
<p>With these two modifications, now the compiler that has been barking at us now
|
||||
happily accepts it. "Hey, that's not that bad at all!" you might say. Just put a
|
||||
little macro inside and be happy again.</p>
|
||||
</div>
|
||||
<div class="section" id="limitations">
|
||||
<h2><a name="limitations">Limitations</a></h2>
|
||||
<p>Unfortunately, that's not quite the end of the story. There are
|
||||
still cases where the above approach will fail and we will have to
|
||||
resort to writing out-of-line metafunction class. Here are the
|
||||
details:</p>
|
||||
<blockquote>
|
||||
<p>To make the lambda expression work without partial template
|
||||
specialization and
|
||||
template template parameters, the MPL has to implement some other way of
|
||||
pulling apart the template instantiations' expression tree, and the only way
|
||||
to do it is through an intrusive metafunction introspection
|
||||
mechanism. That's what hidden behind the <tt class="literal"><span class="pre">BOOST_MPL_AUX_LAMBDA_SUPPORT</span></tt> macro
|
||||
we've seen above.</p>
|
||||
<p>But then, after we've got the information we need (the metafunction's arity
|
||||
and its exact template arguments) stored inside the metafunction itself,
|
||||
the only way for the library to access it is to look inside the metafunction.
|
||||
The latter, in its turn, means instantiating the metafunction, prematurely,
|
||||
before the actuall call, <em>with one or more placeholder arguments</em>. This last
|
||||
part is a potential problem.</p>
|
||||
</blockquote>
|
||||
<p>In other words, the mechanism works as long as your metafunction is
|
||||
"placeholder-safe" (can be safely instantiated on placeholder
|
||||
arguments), which comes down to the follwing two criteria:</p>
|
||||
<ol class="arabic simple">
|
||||
<li>The metafunction doesn't access its arguments' nested members, or</li>
|
||||
<li>The only accessed members are types named <tt class="literal"><span class="pre">::tag</span></tt> or <tt class="literal"><span class="pre">::type</span></tt> (the
|
||||
placeholders do contain these).</li>
|
||||
</ol>
|
||||
<p>If these two hold, you can safely put <tt class="literal"><span class="pre">BOOST_MPL_AUX_LAMBDA_SUPPORT</span></tt> inside
|
||||
your metafunction and forget about the issue. If not, your are out of luck and
|
||||
probably have to write a metafunction class instead.</p>
|
||||
<p>The good news are that most of the MPL's own metafunctions and <a class="reference" href="../../../type_traits/index.html" target="_top">Boost.Type Traits</a>
|
||||
templates are "placeholder-safe" and have the workaround applied to them, so
|
||||
even on broken compilers things "just work" in about 90% of use cases.</p>
|
||||
<p>Please refer to the MPL <a class="reference" href="./reference-manual.html">reference manual</a> for the details on the
|
||||
<tt class="literal"><span class="pre">BOOST_MPL_AUX_LAMBDA_SUPPORT</span></tt> macro.</p>
|
||||
</div>
|
||||
</div>
|
||||
|
||||
<div class="footer-separator"></div>
|
||||
<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./broken-integral-constant.html" class="navigation-link">Prev</a> <a href="./eti.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./broken-integral-constant.html" class="navigation-link">Back</a> <a href="./eti.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./portability.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
|
||||
</tr></table></body>
|
||||
</html>
|
||||
Reference in new issue
Block a user