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 125 <div class="title">boost::hana::default_ Struct Reference<div class="ingroups"><a class="el" href="group__group-core.html">Core</a></div></div>  </div>
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 128 <a name="details" id="details"></a><h2 class="groupheader">Description</h2>
 129 <div class="textblock"><p>Mark a tag-dispatched method implementation as a default implementation. </p>
 130 <p>When defining a new concept with tag-dispatched methods, it is sometimes possible to provide a default implementation for some method(s). Making <code><a class="el" href="structboost_1_1hana_1_1default__.html" title="Mark a tag-dispatched method implementation as a default implementation. ">default_</a></code> a base class of such a default implementation makes it possible to detect whether the method was dispatched to the default implementation afterwards.</p>
 131 <h2>Example </h2>
 132 <div class="fragment"><div class="line"><span class="comment">// Copyright Louis Dionne 2013-2017</span></div><div class="line"><span class="comment">// Distributed under the Boost Software License, Version 1.0.</span></div><div class="line"><span class="comment">// (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="assert_8hpp.html">boost/hana/assert.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="core_2default_8hpp.html">boost/hana/core/default.hpp</a>&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;<a class="code" href="core_2tag__of_8hpp.html">boost/hana/core/tag_of.hpp</a>&gt;</span></div><div class="line"></div><div class="line"><span class="preprocessor">#include &lt;algorithm&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;iterator&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;sstream&gt;</span></div><div class="line"><span class="preprocessor">#include &lt;vector&gt;</span></div><div class="line"><span class="keyword">namespace </span>hana = <a class="code" href="namespaceboost_1_1hana.html">boost::hana</a>;</div><div class="line"></div><div class="line"></div><div class="line"><span class="comment">// In the header defining the concept of a Printable</span></div><div class="line"><span class="keyword">template</span> &lt;<span class="keyword">typename</span> T&gt;</div><div class="line"><span class="keyword">struct </span>print_impl : hana::default_ {</div><div class="line">    <span class="keyword">template</span> &lt;<span class="keyword">typename</span> Stream, <span class="keyword">typename</span> X&gt;</div><div class="line">    <span class="keyword">static</span> <span class="keywordtype">void</span> <a class="code" href="group__group-functional.html#ga30027c383676084be151ef3c6cf2829f">apply</a>(Stream&amp; out, X <span class="keyword">const</span>&amp; x)</div><div class="line">    { out &lt;&lt; x; }</div><div class="line">};</div><div class="line"></div><div class="line"><span class="keyword">auto</span> <a class="code" href="group__group-experimental.html#ga660c0769106006a86948b5b355fad050">print</a> = [](<span class="keyword">auto</span>&amp; stream, <span class="keyword">auto</span> <span class="keyword">const</span>&amp; x) {</div><div class="line">    <span class="keywordflow">return</span> print_impl&lt;hana::tag_of_t&lt;decltype(x)&gt;&gt;<a class="code" href="group__group-functional.html#ga30027c383676084be151ef3c6cf2829f">::apply</a>(stream, x);</div><div class="line">};</div><div class="line"></div><div class="line"><span class="comment">// In some other header</span></div><div class="line"><span class="keyword">template</span> &lt;<span class="keyword">typename</span> T&gt;</div><div class="line"><span class="keyword">struct </span>print_impl&lt;<a class="code" href="namespacestd.html">std</a>::vector&lt;T&gt;&gt; {</div><div class="line">    <span class="keyword">template</span> &lt;<span class="keyword">typename</span> Stream&gt;</div><div class="line">    <span class="keyword">static</span> <span class="keywordtype">void</span> <a class="code" href="group__group-functional.html#ga30027c383676084be151ef3c6cf2829f">apply</a>(Stream&amp; out, std::vector&lt;T&gt; <span class="keyword">const</span>&amp; xs) {</div><div class="line">        out &lt;&lt; <span class="charliteral">&#39;[&#39;</span>;</div><div class="line">        std::copy(begin(xs), end(xs), std::ostream_iterator&lt;T&gt;{out, <span class="stringliteral">&quot;, &quot;</span>});</div><div class="line">        out &lt;&lt; <span class="charliteral">&#39;]&#39;</span>;</div><div class="line">    }</div><div class="line">};</div><div class="line"></div><div class="line">static_assert(hana::is_default&lt;print_impl&lt;int&gt;&gt;{}, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line">static_assert(!hana::is_default&lt;print_impl&lt;std::vector&lt;int&gt;&gt;&gt;{}, <span class="stringliteral">&quot;&quot;</span>);</div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() {</div><div class="line">    {</div><div class="line">        std::stringstream s;</div><div class="line">        <a class="code" href="group__group-experimental.html#ga660c0769106006a86948b5b355fad050">print</a>(s, std::vector&lt;int&gt;{1, 2, 3});</div><div class="line">        <a class="code" href="group__group-assertions.html#ga29b2b21ffa5513e5b706c50ffee980af">BOOST_HANA_RUNTIME_CHECK</a>(s.str() == <span class="stringliteral">&quot;[1, 2, 3, ]&quot;</span>);</div><div class="line">    }</div><div class="line"></div><div class="line">    {</div><div class="line">        std::stringstream s;</div><div class="line">        <a class="code" href="group__group-experimental.html#ga660c0769106006a86948b5b355fad050">print</a>(s, <span class="stringliteral">&quot;abcd&quot;</span>);</div><div class="line">        <a class="code" href="group__group-assertions.html#ga29b2b21ffa5513e5b706c50ffee980af">BOOST_HANA_RUNTIME_CHECK</a>(s.str() == <span class="stringliteral">&quot;abcd&quot;</span>);</div><div class="line">    }</div><div class="line">}</div></div><!-- fragment --> </div>
 133 <p>Inherited by boost::hana::accessors_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::adjust_if_impl&lt; Fun, when&lt; condition &gt; &gt;, boost::hana::adjust_impl&lt; Fun, when&lt; condition &gt; &gt;, boost::hana::all_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::all_of_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::and_impl&lt; L, when&lt; condition &gt; &gt;, boost::hana::any_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::any_of_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::ap_impl&lt; A, when&lt; condition &gt; &gt;, boost::hana::append_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::at_impl&lt; It, when&lt; condition &gt; &gt;, boost::hana::at_key_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::back_impl&lt; It, when&lt; condition &gt; &gt;, boost::hana::cartesian_product_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::chain_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::concat_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::contains_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::count_if_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::count_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::cycle_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::difference_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::div_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::drop_back_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::drop_front_exactly_impl&lt; It, when&lt; condition &gt; &gt;, boost::hana::drop_front_impl&lt; It, when&lt; condition &gt; &gt;, boost::hana::drop_while_impl&lt; It, when&lt; condition &gt; &gt;, boost::hana::duplicate_impl&lt; W, when&lt; condition &gt; &gt;, boost::hana::empty_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::equal_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::erase_key_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::eval_if_impl&lt; L, when&lt; condition &gt; &gt;, boost::hana::eval_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::extend_impl&lt; W, when&lt; condition &gt; &gt;, boost::hana::extract_impl&lt; W, when&lt; condition &gt; &gt;, boost::hana::fill_impl&lt; Fun, when&lt; condition &gt; &gt;, boost::hana::filter_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::find_if_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::find_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::first_impl&lt; P, when&lt; condition &gt; &gt;, boost::hana::flatten_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::fold_left_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::fold_right_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::for_each_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::front_impl&lt; It, when&lt; condition &gt; &gt;, boost::hana::greater_equal_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::greater_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::group_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::hash_impl&lt; Tag, when&lt; condition &gt; &gt;, boost::hana::if_impl&lt; L, when&lt; condition &gt; &gt;, boost::hana::insert_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::intersection_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::intersperse_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::is_disjoint_impl&lt; S1, S2, when&lt; condition &gt; &gt;, boost::hana::is_empty_impl&lt; It, when&lt; condition &gt; &gt;, boost::hana::is_subset_impl&lt; S1, S2, when&lt; condition &gt; &gt;, boost::hana::keys_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::length_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::less_equal_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::less_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::lexicographical_compare_impl&lt; It, when&lt; condition &gt; &gt;, boost::hana::lift_impl&lt; A, when&lt; condition &gt; &gt;, boost::hana::make_impl&lt; Datatype, when&lt; condition &gt; &gt;, boost::hana::max_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::maximum_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::maximum_pred_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::members_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::min_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::minimum_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::minimum_pred_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::minus_impl&lt; T, T, when&lt; condition &gt; &gt;, boost::hana::minus_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::mod_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::monadic_fold_left_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::monadic_fold_right_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::mult_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::negate_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::none_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::none_of_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::not_equal_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::not_impl&lt; L, when&lt; condition &gt; &gt;, boost::hana::one_impl&lt; R, when&lt; condition &gt; &gt;, boost::hana::or_impl&lt; L, when&lt; condition &gt; &gt;, boost::hana::partition_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::permutations_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::plus_impl&lt; T, U, when&lt; condition &gt; &gt;, boost::hana::power_impl&lt; R, when&lt; condition &gt; &gt;, boost::hana::prefix_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::prepend_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::product_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::remove_at_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::remove_if_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::remove_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::remove_range_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::repeat_impl&lt; I, when&lt; condition &gt; &gt;, boost::hana::replace_if_impl&lt; Fun, when&lt; condition &gt; &gt;, boost::hana::replace_impl&lt; Fun, when&lt; condition &gt; &gt;, boost::hana::replicate_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::reverse_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::scan_left_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::scan_right_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::second_impl&lt; P, when&lt; condition &gt; &gt;, boost::hana::slice_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::sort_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::span_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::suffix_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::sum_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::symmetric_difference_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::take_back_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::take_front_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::take_while_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::tap_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::then_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::transform_impl&lt; Fun, when&lt; condition &gt; &gt;, boost::hana::unfold_left_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::unfold_right_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::union_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::unique_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::unpack_impl&lt; T, when&lt; condition &gt; &gt;, boost::hana::value_impl&lt; C, when&lt; condition &gt; &gt;, boost::hana::while_impl&lt; L, hana::when&lt; condition &gt; &gt;, boost::hana::zero_impl&lt; M, when&lt; condition &gt; &gt;, boost::hana::zip_impl&lt; S, when&lt; condition &gt; &gt;, boost::hana::zip_shortest_impl&lt; S, when&lt; condition &gt; &gt;, and boost::hana::zip_shortest_with_impl&lt; S, when&lt; condition &gt; &gt;.</p>
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