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| <div class="section" id="placeholder-expression"> |
| <h1><a class="toc-backref" href="./lambda-details.html#id56" name="placeholder-expression">Placeholder Expression Definition</a></h1> |
| <p>Now that you know just what <em>placeholder</em> means, we can define |
| <em>placeholder expression</em>:</p> |
| <div class="admonition-definition admonition"> |
| <p class="admonition-title first">Definition</p> |
| <p>A placeholder expression is either:</p> |
| <blockquote> |
| <blockquote> |
| <ul class="simple"> |
| <li>a placeholder</li> |
| </ul> |
| </blockquote> |
| <p><em>or</em></p> |
| <blockquote> |
| <ul class="simple"> |
| <li>a template specialization with at least one argument that |
| is a placeholder expression.</li> |
| </ul> |
| </blockquote> |
| </blockquote> |
| </div> |
| <p>In other words, a placeholder expression always involves a |
| placeholder.</p> |
| <!-- DWA: I'm still not sure we shouldn't be at least mentioning the |
| pitfall, but for now it's commented out. |
| |
| Lambda and Nullary Metafunctions |
| - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - |
| |
| The definition of *placeholder expression* above has an interesting |
| implication: an ordinary nullary metafunction is never a placeholder |
| expression. In other words, even though ``add_pointer<int>`` is a |
| nullary metafunction, it won't be invoked in the expression below; |
| the assertion will always fail: |
| |
| .. parsed-literal:: |
| |
| BOOST_STATIC_ASSERT(( |
| mpl::apply< |
| boost::is_same<**boost::add_pointer<int>**,_1> |
| , int\* |
| >::type::value |
| )); |
| |
| In order to allow a nullary metafunction to be used as a lambda |
| expression, MPL provides this definition of ``arg``: |
| |
| .. parsed-literal:: |
| |
| // primary template definition (not a specialization) |
| template <class F> |
| struct arg |
| { |
| template <class A1 = void\_, class A2 = void\_, ... class *Am* = void\_> |
| struct apply : F |
| { |
| }; |
| }; |
| |
| When applied to a lambda expression's actual arguments, ``arg<F>`` |
| ignores them and simply returns ``F::type``. In other words, if |
| ``F`` is a nullary metafunction, ``arg<F>`` is a metafunction class |
| that invokes ``F`` and returns the result. So we can transform |
| add_pointer<int> into a placeholder and get the desired result |
| with: |
| |
| .. parsed-literal:: |
| |
| BOOST_STATIC_ASSERT(( |
| mpl::apply< |
| boost::is_same< |
| **mpl::arg<boost::add_pointer<int> >** |
| , _1 |
| > |
| , int\* |
| >::type::value |
| )); --> |
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