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| 4 | <!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> | |
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| 9 | <meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" /> | |
| 10 | <title>THE BOOST MPL LIBRARY: Placeholder Expression Definition</title> | |
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| 16 | </tr></table><div class="header-separator"></div> | |
| 17 | <div class="section" id="placeholder-expression"> | |
| 18 | <h1><a class="toc-backref" href="./lambda-details.html#id56" name="placeholder-expression">Placeholder Expression Definition</a></h1> | |
| 19 | <p>Now that you know just what <em>placeholder</em> means, we can define | |
| 20 | <em>placeholder expression</em>:</p> | |
| 21 | <div class="admonition-definition admonition"> | |
| 22 | <p class="admonition-title first">Definition</p> | |
| 23 | <p>A placeholder expression is either:</p> | |
| 24 | <blockquote> | |
| 25 | <blockquote> | |
| 26 | <ul class="simple"> | |
| 27 | <li>a placeholder</li> | |
| 28 | </ul> | |
| 29 | </blockquote> | |
| 30 | <p><em>or</em></p> | |
| 31 | <blockquote> | |
| 32 | <ul class="simple"> | |
| 33 | <li>a template specialization with at least one argument that | |
| 34 | is a placeholder expression.</li> | |
| 35 | </ul> | |
| 36 | </blockquote> | |
| 37 | </blockquote> | |
| 38 | </div> | |
| 39 | <p>In other words, a placeholder expression always involves a | |
| 40 | placeholder.</p> | |
| 41 | <!-- DWA: I'm still not sure we shouldn't be at least mentioning the | |
| 42 | pitfall, but for now it's commented out. | |
| 43 | ||
| 44 | Lambda and Nullary Metafunctions | |
| 45 | - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - | |
| 46 | ||
| 47 | The definition of *placeholder expression* above has an interesting | |
| 48 | implication: an ordinary nullary metafunction is never a placeholder | |
| 49 | expression. In other words, even though ``add_pointer<int>`` is a | |
| 50 | nullary metafunction, it won't be invoked in the expression below; | |
| 51 | the assertion will always fail: | |
| 52 | ||
| 53 | .. parsed-literal:: | |
| 54 | ||
| 55 | BOOST_STATIC_ASSERT(( | |
| 56 | mpl::apply< | |
| 57 | boost::is_same<**boost::add_pointer<int>**,_1> | |
| 58 | , int\* | |
| 59 | >::type::value | |
| 60 | )); | |
| 61 | ||
| 62 | In order to allow a nullary metafunction to be used as a lambda | |
| 63 | expression, MPL provides this definition of ``arg``: | |
| 64 | ||
| 65 | .. parsed-literal:: | |
| 66 | ||
| 67 | // primary template definition (not a specialization) | |
| 68 | template <class F> | |
| 69 | struct arg | |
| 70 | { | |
| 71 | template <class A1 = void\_, class A2 = void\_, ... class *Am* = void\_> | |
| 72 | struct apply : F | |
| 73 | { | |
| 74 | }; | |
| 75 | }; | |
| 76 | ||
| 77 | When applied to a lambda expression's actual arguments, ``arg<F>`` | |
| 78 | ignores them and simply returns ``F::type``. In other words, if | |
| 79 | ``F`` is a nullary metafunction, ``arg<F>`` is a metafunction class | |
| 80 | that invokes ``F`` and returns the result. So we can transform | |
| 81 | add_pointer<int> into a placeholder and get the desired result | |
| 82 | with: | |
| 83 | ||
| 84 | .. parsed-literal:: | |
| 85 | ||
| 86 | BOOST_STATIC_ASSERT(( | |
| 87 | mpl::apply< | |
| 88 | boost::is_same< | |
| 89 | **mpl::arg<boost::add_pointer<int> >** | |
| 90 | , _1 | |
| 91 | > | |
| 92 | , int\* | |
| 93 | >::type::value | |
| 94 | )); --> | |
| 95 | </div> | |
| 96 | ||
| 97 | <div class="footer-separator"></div> | |
| 98 | <table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./placeholders.html" class="navigation-link">Prev</a> <a href="./lambda-and-non.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./placeholders.html" class="navigation-link">Back</a> <a href="./lambda-and-non.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-details.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> | |
| 99 | </tr></table></body> | |
| 100 | </html> |