deploy: d2013ee5952bbcf88906a832748783e372f3a939

1 files changed, 164 insertions, 5 deletions

diff --git a/development/c++.html b/development/c++.html
index 811aa7d..4ed1fad 100644
--- a/development/c++.html
+++ b/development/c++.html
@@ -83,7 +83,7 @@
 
         <nav id="sidebar" class="sidebar" aria-label="Table of contents">
             <div class="sidebar-scrollbox">
-                <ol class="chapter"><li class="chapter-item expanded affix "><a href="../intro.html">Introduction</a></li><li class="chapter-item expanded "><a href="../tools/index.html"><strong aria-hidden="true">1.</strong> Tools</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../tools/zsh.html"><strong aria-hidden="true">1.1.</strong> zsh</a></li><li class="chapter-item expanded "><a href="../tools/bash.html"><strong aria-hidden="true">1.2.</strong> bash</a></li><li class="chapter-item expanded "><a href="../tools/fish.html"><strong aria-hidden="true">1.3.</strong> fish</a></li><li class="chapter-item expanded "><a href="../tools/tmux.html"><strong aria-hidden="true">1.4.</strong> tmux</a></li><li class="chapter-item expanded "><a href="../tools/git.html"><strong aria-hidden="true">1.5.</strong> git</a></li><li class="chapter-item expanded "><a href="../tools/awk.html"><strong aria-hidden="true">1.6.</strong> awk</a></li><li class="chapter-item expanded "><a href="../tools/emacs.html"><strong aria-hidden="true">1.7.</strong> emacs</a></li><li class="chapter-item expanded "><a href="../tools/gpg.html"><strong aria-hidden="true">1.8.</strong> gpg</a></li><li class="chapter-item expanded "><a href="../tools/gdb.html"><strong aria-hidden="true">1.9.</strong> gdb</a></li><li class="chapter-item expanded "><a href="../tools/gdbserver.html"><strong aria-hidden="true">1.10.</strong> gdbserver</a></li><li class="chapter-item expanded "><a href="../tools/radare2.html"><strong aria-hidden="true">1.11.</strong> radare2</a></li><li class="chapter-item expanded "><a href="../tools/qemu.html"><strong aria-hidden="true">1.12.</strong> qemu</a></li><li class="chapter-item expanded "><a href="../tools/pacman.html"><strong aria-hidden="true">1.13.</strong> pacman</a></li><li class="chapter-item expanded "><a href="../tools/dot.html"><strong aria-hidden="true">1.14.</strong> dot</a></li></ol></li><li class="chapter-item expanded "><a href="../monitor/index.html"><strong aria-hidden="true">2.</strong> Resource analysis & monitor</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../monitor/lsof.html"><strong aria-hidden="true">2.1.</strong> lsof</a></li><li class="chapter-item expanded "><a href="../monitor/ss.html"><strong aria-hidden="true">2.2.</strong> ss</a></li><li class="chapter-item expanded "><a href="../monitor/pidstat.html"><strong aria-hidden="true">2.3.</strong> pidstat</a></li><li class="chapter-item expanded "><a href="../monitor/pgrep.html"><strong aria-hidden="true">2.4.</strong> pgrep</a></li><li class="chapter-item expanded "><a href="../monitor/pmap.html"><strong aria-hidden="true">2.5.</strong> pmap</a></li><li class="chapter-item expanded "><a href="../monitor/pstack.html"><strong aria-hidden="true">2.6.</strong> pstack</a></li></ol></li><li class="chapter-item expanded "><a href="../trace_profile/index.html"><strong aria-hidden="true">3.</strong> Trace and Profile</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../trace_profile/strace.html"><strong aria-hidden="true">3.1.</strong> strace</a></li><li class="chapter-item expanded "><a href="../trace_profile/ltrace.html"><strong aria-hidden="true">3.2.</strong> ltrace</a></li><li class="chapter-item expanded "><a href="../trace_profile/perf.html"><strong aria-hidden="true">3.3.</strong> perf</a></li><li class="chapter-item expanded "><a href="../trace_profile/oprofile.html"><strong aria-hidden="true">3.4.</strong> OProfile</a></li><li class="chapter-item expanded "><a href="../trace_profile/time.html"><strong aria-hidden="true">3.5.</strong> time</a></li></ol></li><li class="chapter-item expanded "><a href="../binary/index.html"><strong aria-hidden="true">4.</strong> Binary</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../binary/od.html"><strong aria-hidden="true">4.1.</strong> od</a></li><li class="chapter-item expanded "><a href="../binary/xxd.html"><strong aria-hidden="true">4.2.</strong> xxd</a></li><li class="chapter-item expanded "><a href="../binary/readelf.html"><strong aria-hidden="true">4.3.</strong> readelf</a></li><li class="chapter-item expanded "><a href="../binary/objdump.html"><strong aria-hidden="true">4.4.</strong> objdump</a></li><li class="chapter-item expanded "><a href="../binary/nm.html"><strong aria-hidden="true">4.5.</strong> nm</a></li></ol></li><li class="chapter-item expanded "><a href="../development/index.html"><strong aria-hidden="true">5.</strong> Development</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/c++filt.html"><strong aria-hidden="true">5.1.</strong> c++filt</a></li><li class="chapter-item expanded "><a href="../development/c++.html" class="active"><strong aria-hidden="true">5.2.</strong> c++</a></li><li class="chapter-item expanded "><a href="../development/glibc.html"><strong aria-hidden="true">5.3.</strong> glibc</a></li><li class="chapter-item expanded "><a href="../development/gcc.html"><strong aria-hidden="true">5.4.</strong> gcc</a></li><li class="chapter-item expanded "><a href="../development/make.html"><strong aria-hidden="true">5.5.</strong> make</a></li><li class="chapter-item expanded "><a href="../development/ld.so.html"><strong aria-hidden="true">5.6.</strong> ld.so</a></li><li class="chapter-item expanded "><a href="../development/symbolver.html"><strong aria-hidden="true">5.7.</strong> symbol versioning</a></li><li class="chapter-item expanded "><a href="../development/python.html"><strong aria-hidden="true">5.8.</strong> python</a></li></ol></li><li class="chapter-item expanded "><a href="../linux/index.html"><strong aria-hidden="true">6.</strong> Linux</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../linux/systemd.html"><strong aria-hidden="true">6.1.</strong> systemd</a></li><li class="chapter-item expanded "><a href="../linux/coredump.html"><strong aria-hidden="true">6.2.</strong> coredump</a></li><li class="chapter-item expanded "><a href="../linux/ptrace_scope.html"><strong aria-hidden="true">6.3.</strong> ptrace_scope</a></li><li class="chapter-item expanded "><a href="../linux/cryptsetup.html"><strong aria-hidden="true">6.4.</strong> cryptsetup</a></li><li class="chapter-item expanded "><a href="../linux/swap.html"><strong aria-hidden="true">6.5.</strong> swap</a></li><li class="chapter-item expanded "><a href="../linux/input.html"><strong aria-hidden="true">6.6.</strong> input</a></li></ol></li><li class="chapter-item expanded "><a href="../network/index.html"><strong aria-hidden="true">7.</strong> Network</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../network/tcpdump.html"><strong aria-hidden="true">7.1.</strong> tcpdump</a></li><li class="chapter-item expanded "><a href="../network/firewall-cmd.html"><strong aria-hidden="true">7.2.</strong> firewall-cmd</a></li><li class="chapter-item expanded "><a href="../network/nftables.html"><strong aria-hidden="true">7.3.</strong> nftables</a></li></ol></li><li class="chapter-item expanded "><a href="../web/index.html"><strong aria-hidden="true">8.</strong> Web</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../web/html.html"><strong aria-hidden="true">8.1.</strong> html</a></li><li class="chapter-item expanded "><a href="../web/chartjs.html"><strong aria-hidden="true">8.2.</strong> chartjs</a></li></ol></li><li class="chapter-item expanded "><a href="../arch/index.html"><strong aria-hidden="true">9.</strong> Arch</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../arch/x86_64.html"><strong aria-hidden="true">9.1.</strong> x86_64</a></li><li class="chapter-item expanded "><a href="../arch/arm64.html"><strong aria-hidden="true">9.2.</strong> arm64</a></li><li class="chapter-item expanded "><a href="../arch/armv7.html"><strong aria-hidden="true">9.3.</strong> armv7</a></li><li class="chapter-item expanded "><a href="../arch/riscv.html"><strong aria-hidden="true">9.4.</strong> riscv</a></li></ol></li></ol>
+                <ol class="chapter"><li class="chapter-item expanded affix "><a href="../intro.html">Introduction</a></li><li class="chapter-item expanded "><a href="../tools/index.html"><strong aria-hidden="true">1.</strong> Tools</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../tools/zsh.html"><strong aria-hidden="true">1.1.</strong> zsh</a></li><li class="chapter-item expanded "><a href="../tools/bash.html"><strong aria-hidden="true">1.2.</strong> bash</a></li><li class="chapter-item expanded "><a href="../tools/fish.html"><strong aria-hidden="true">1.3.</strong> fish</a></li><li class="chapter-item expanded "><a href="../tools/tmux.html"><strong aria-hidden="true">1.4.</strong> tmux</a></li><li class="chapter-item expanded "><a href="../tools/git.html"><strong aria-hidden="true">1.5.</strong> git</a></li><li class="chapter-item expanded "><a href="../tools/awk.html"><strong aria-hidden="true">1.6.</strong> awk</a></li><li class="chapter-item expanded "><a href="../tools/emacs.html"><strong aria-hidden="true">1.7.</strong> emacs</a></li><li class="chapter-item expanded "><a href="../tools/gpg.html"><strong aria-hidden="true">1.8.</strong> gpg</a></li><li class="chapter-item expanded "><a href="../tools/gdb.html"><strong aria-hidden="true">1.9.</strong> gdb</a></li><li class="chapter-item expanded "><a href="../tools/gdbserver.html"><strong aria-hidden="true">1.10.</strong> gdbserver</a></li><li class="chapter-item expanded "><a href="../tools/radare2.html"><strong aria-hidden="true">1.11.</strong> radare2</a></li><li class="chapter-item expanded "><a href="../tools/qemu.html"><strong aria-hidden="true">1.12.</strong> qemu</a></li><li class="chapter-item expanded "><a href="../tools/pacman.html"><strong aria-hidden="true">1.13.</strong> pacman</a></li><li class="chapter-item expanded "><a href="../tools/dot.html"><strong aria-hidden="true">1.14.</strong> dot</a></li></ol></li><li class="chapter-item expanded "><a href="../monitor/index.html"><strong aria-hidden="true">2.</strong> Resource analysis & monitor</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../monitor/lsof.html"><strong aria-hidden="true">2.1.</strong> lsof</a></li><li class="chapter-item expanded "><a href="../monitor/ss.html"><strong aria-hidden="true">2.2.</strong> ss</a></li><li class="chapter-item expanded "><a href="../monitor/pidstat.html"><strong aria-hidden="true">2.3.</strong> pidstat</a></li><li class="chapter-item expanded "><a href="../monitor/pgrep.html"><strong aria-hidden="true">2.4.</strong> pgrep</a></li><li class="chapter-item expanded "><a href="../monitor/pmap.html"><strong aria-hidden="true">2.5.</strong> pmap</a></li><li class="chapter-item expanded "><a href="../monitor/pstack.html"><strong aria-hidden="true">2.6.</strong> pstack</a></li></ol></li><li class="chapter-item expanded "><a href="../trace_profile/index.html"><strong aria-hidden="true">3.</strong> Trace and Profile</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../trace_profile/strace.html"><strong aria-hidden="true">3.1.</strong> strace</a></li><li class="chapter-item expanded "><a href="../trace_profile/ltrace.html"><strong aria-hidden="true">3.2.</strong> ltrace</a></li><li class="chapter-item expanded "><a href="../trace_profile/perf.html"><strong aria-hidden="true">3.3.</strong> perf</a></li><li class="chapter-item expanded "><a href="../trace_profile/oprofile.html"><strong aria-hidden="true">3.4.</strong> OProfile</a></li><li class="chapter-item expanded "><a href="../trace_profile/time.html"><strong aria-hidden="true">3.5.</strong> time</a></li></ol></li><li class="chapter-item expanded "><a href="../binary/index.html"><strong aria-hidden="true">4.</strong> Binary</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../binary/od.html"><strong aria-hidden="true">4.1.</strong> od</a></li><li class="chapter-item expanded "><a href="../binary/xxd.html"><strong aria-hidden="true">4.2.</strong> xxd</a></li><li class="chapter-item expanded "><a href="../binary/readelf.html"><strong aria-hidden="true">4.3.</strong> readelf</a></li><li class="chapter-item expanded "><a href="../binary/objdump.html"><strong aria-hidden="true">4.4.</strong> objdump</a></li><li class="chapter-item expanded "><a href="../binary/nm.html"><strong aria-hidden="true">4.5.</strong> nm</a></li></ol></li><li class="chapter-item expanded "><a href="../development/index.html"><strong aria-hidden="true">5.</strong> Development</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/c++filt.html"><strong aria-hidden="true">5.1.</strong> c++filt</a></li><li class="chapter-item expanded "><a href="../development/c++.html" class="active"><strong aria-hidden="true">5.2.</strong> c++</a></li><li class="chapter-item expanded "><a href="../development/glibc.html"><strong aria-hidden="true">5.3.</strong> glibc</a></li><li class="chapter-item expanded "><a href="../development/gcc.html"><strong aria-hidden="true">5.4.</strong> gcc</a></li><li class="chapter-item expanded "><a href="../development/make.html"><strong aria-hidden="true">5.5.</strong> make</a></li><li class="chapter-item expanded "><a href="../development/ld.so.html"><strong aria-hidden="true">5.6.</strong> ld.so</a></li><li class="chapter-item expanded "><a href="../development/symbolver.html"><strong aria-hidden="true">5.7.</strong> symbol versioning</a></li><li class="chapter-item expanded "><a href="../development/python.html"><strong aria-hidden="true">5.8.</strong> python</a></li><li class="chapter-item expanded "><a href="../development/gcov.html"><strong aria-hidden="true">5.9.</strong> gcov</a></li></ol></li><li class="chapter-item expanded "><a href="../linux/index.html"><strong aria-hidden="true">6.</strong> Linux</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../linux/systemd.html"><strong aria-hidden="true">6.1.</strong> systemd</a></li><li class="chapter-item expanded "><a href="../linux/coredump.html"><strong aria-hidden="true">6.2.</strong> coredump</a></li><li class="chapter-item expanded "><a href="../linux/ptrace_scope.html"><strong aria-hidden="true">6.3.</strong> ptrace_scope</a></li><li class="chapter-item expanded "><a href="../linux/cryptsetup.html"><strong aria-hidden="true">6.4.</strong> cryptsetup</a></li><li class="chapter-item expanded "><a href="../linux/swap.html"><strong aria-hidden="true">6.5.</strong> swap</a></li><li class="chapter-item expanded "><a href="../linux/input.html"><strong aria-hidden="true">6.6.</strong> input</a></li></ol></li><li class="chapter-item expanded "><a href="../network/index.html"><strong aria-hidden="true">7.</strong> Network</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../network/tcpdump.html"><strong aria-hidden="true">7.1.</strong> tcpdump</a></li><li class="chapter-item expanded "><a href="../network/firewall-cmd.html"><strong aria-hidden="true">7.2.</strong> firewall-cmd</a></li><li class="chapter-item expanded "><a href="../network/nftables.html"><strong aria-hidden="true">7.3.</strong> nftables</a></li></ol></li><li class="chapter-item expanded "><a href="../web/index.html"><strong aria-hidden="true">8.</strong> Web</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../web/html.html"><strong aria-hidden="true">8.1.</strong> html</a></li><li class="chapter-item expanded "><a href="../web/chartjs.html"><strong aria-hidden="true">8.2.</strong> chartjs</a></li></ol></li><li class="chapter-item expanded "><a href="../arch/index.html"><strong aria-hidden="true">9.</strong> Arch</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../arch/x86_64.html"><strong aria-hidden="true">9.1.</strong> x86_64</a></li><li class="chapter-item expanded "><a href="../arch/arm64.html"><strong aria-hidden="true">9.2.</strong> arm64</a></li><li class="chapter-item expanded "><a href="../arch/armv7.html"><strong aria-hidden="true">9.3.</strong> armv7</a></li><li class="chapter-item expanded "><a href="../arch/riscv.html"><strong aria-hidden="true">9.4.</strong> riscv</a></li></ol></li></ol>
             </div>
             <div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
         </nav>
@@ -148,6 +148,7 @@
                 <div id="content" class="content">
                     <main>
                         <h1 id="c"><a class="header" href="#c">c++</a></h1>
+<p>Source files of most examples is available <a href="https://github.com/johannst/notes/tree/master/src/development/c%2B%2B">here</a>.</p>
 <h2 id="type-deduction"><a class="header" href="#type-deduction">Type deduction</a></h2>
 <p>Force compile error to see what <code>auto</code> is deduced to.</p>
 <pre><code class="language-cpp">auto foo = bar();
@@ -193,6 +194,74 @@ int main() {
     show(1, 1.0, &quot;foo&quot;, 'a');
 }
 </code></pre>
+<h2 id="forwarding-reference-fwd-ref"><a class="header" href="#forwarding-reference-fwd-ref">Forwarding reference (<a href="https://en.cppreference.com/w/cpp/language/reference#Forwarding_references">fwd ref</a>)</a></h2>
+<p>A <code>forwarding reference</code> is a special references that preserves the <code>value category</code> of a function parameter and therefore allows for <code>perfect</code>
+forwarding.</p>
+<p>A forwarding reference is a parameter of a function template, which is declared
+as <code>rvalue</code> reference to a <code>non-cv</code> qualified <code>type</code> template parameter.</p>
+<pre><code class="language-cpp">template&lt;typename T&gt;
+void fn(T&amp;&amp; param); // param is a forwarding reference
+</code></pre>
+<p>Perfect forwarding can be achieved with <a href="https://en.cppreference.com/w/cpp/utility/forward"><code>std::forward</code></a>. This for
+example allows a wrapper function to pass a parameter with the <strong>exact</strong> same
+value category to a down-stream function which is being invoked in the wrapper.</p>
+<pre><code class="language-cpp">#include &lt;cstdio&gt;
+#include &lt;utility&gt;
+
+struct M {};
+
+// -- CONSUMER -----------------------------------------------------------------
+
+void use(M&amp;) {
+    puts(__PRETTY_FUNCTION__);
+}
+
+void use(M&amp;&amp;) {
+    puts(__PRETTY_FUNCTION__);
+}
+
+// -- TESTER -------------------------------------------------------------------
+
+template&lt;typename T&gt;
+void wrapper(T&amp;&amp; param) {  // forwarding reference
+    puts(__PRETTY_FUNCTION__);
+    // PARAM is an lvalue, therefore this always calls use(M&amp;).
+    use(param);
+}
+
+template&lt;typename T&gt;
+void fwd_wrapper(T&amp;&amp; param) {  // forwarding reference
+    puts(__PRETTY_FUNCTION__);
+    // PARAM is an lvalue, but std::forward returns PARAM with the same value
+    // category as the forwarding reference takes.
+    use(std::forward&lt;T&gt;(param));
+}
+
+// -- MAIN ---------------------------------------------------------------------
+
+int main() {
+    {
+        std::puts(&quot;==&gt; wrapper rvalue reference&quot;);
+        wrapper(M{});
+        // calls use(M&amp;).
+
+        std::puts(&quot;==&gt; wrapper lvalue reference&quot;);
+        struct M m;
+        wrapper(m);
+        // calls use(M&amp;).
+    }
+    {
+        std::puts(&quot;==&gt; fwd_wrapper rvalue reference&quot;);
+        fwd_wrapper(M{});
+        // calls use(M&amp;&amp;).
+
+        std::puts(&quot;==&gt; fwd_wrapper lvalue reference&quot;);
+        struct M m;
+        fwd_wrapper(m);
+        // calls use(M&amp;).
+    }
+}
+</code></pre>
 <h2 id="example-any_of-template-meta-function"><a class="header" href="#example-any_of-template-meta-function">Example: <code>any_of</code> template meta function</a></h2>
 <pre><code class="language-cpp">#include &lt;type_traits&gt;
 
@@ -275,7 +344,9 @@ void Invoke(const Ctx&amp; C, P... params) {
 // Custom context.
 
 struct Ctx {
-    void out(const char* s, unsigned v) const { printf(&quot;%s%x\n&quot;, s, v); }
+    void out(const char* s, unsigned v) const {
+        printf(&quot;%s%x\n&quot;, s, v);
+    }
 };
 
 // Operations to invoke.
@@ -286,7 +357,9 @@ struct OpA {
     using Return = int;
     static constexpr const char* const Name = &quot;OpA&quot;;
 
-    constexpr Return operator()(int a, int b) const { return a + b; }
+    constexpr Return operator()(int a, int b) const {
+        return a + b;
+    }
 };
 
 template&lt;typename Ctx&gt;
@@ -295,7 +368,9 @@ struct OpB {
     using Return = void;
     static constexpr const char* const Name = &quot;OpB&quot;;
 
-    Return operator()(const Ctx&amp; C, unsigned a) const { C.out(&quot;a = &quot;, a); }
+    Return operator()(const Ctx&amp; C, unsigned a) const {
+        C.out(&quot;a = &quot;, a);
+    }
 };
 
 int main() {
@@ -332,7 +407,9 @@ struct registry {
         return r.second;
     }
 
-    R invoke(const std::string&amp; nm, P... p) const { return invoke_impl&lt;R&gt;(nm, p...); }
+    R invoke(const std::string&amp; nm, P... p) const {
+        return invoke_impl&lt;R&gt;(nm, p...);
+    }
 
     void dump() const {
         for (const auto&amp; it : m_fns) {
@@ -594,6 +671,88 @@ int main() {
     // * No specialization matches, take the primary template.
 }
 </code></pre>
+<h1 id="example-perfect-forwarding"><a class="header" href="#example-perfect-forwarding">Example: Perfect forwarding</a></h1>
+<pre><code class="language-cpp">#include &lt;cassert&gt;
+#include &lt;cstdio&gt;
+#include &lt;new&gt;
+#include &lt;type_traits&gt;
+#include &lt;utility&gt;
+
+struct S {};
+
+struct M {
+    M() {
+        std::puts(&quot;M()&quot;);
+    }
+    M(const M&amp;) {
+        std::puts(&quot;M(M&amp;)&quot;);
+    }
+    M(M&amp;&amp;) {
+        std::puts(&quot;M(M&amp;&amp;)&quot;);
+    }
+    M&amp; operator=(const M&amp;) = delete;
+    M&amp; operator=(M&amp;&amp;) = delete;
+
+    M(S&amp;, int) {
+        std::puts(&quot;M(S&amp;)&quot;);
+    }
+    M(S&amp;&amp;, int) {
+        std::puts(&quot;M(S&amp;&amp;)&quot;);
+    }
+    ~M() {
+        std::puts(&quot;~M()&quot;);
+    }
+};
+
+template&lt;typename T&gt;
+struct option {
+    static_assert(!std::is_reference_v&lt;T&gt;);
+
+    constexpr option() = default;
+
+    template&lt;typename... Params&gt;
+    constexpr option(Params&amp;&amp;... params) : m_has_val(true) {
+        // BAD: does not perfectly forward!
+        //      eg, if option(S&amp;&amp;) is invoked, this would invoke M(S&amp;).
+        // new (&amp;m_val) T(params...);
+
+        // GOOD: perfectly forwards params to constructor of T.
+        new (m_val) T(std::forward&lt;Params&gt;(params)...);
+    }
+
+    ~option() {
+        reset();
+    }
+
+    constexpr T&amp; value() {
+        assert(m_has_val);
+        return *reinterpret_cast&lt;T*&gt;(m_val);
+    }
+
+  private:
+    constexpr void reset() {
+        if (!m_has_val) {
+            return;
+        }
+        if constexpr (!std::is_trivially_destructible_v&lt;T&gt;) {
+            value().~T();
+        };
+    }
+
+    alignas(T) char m_val[sizeof(T)];
+    bool m_has_val{false};
+};
+
+int main() {
+    std::puts(&quot;==&gt; case 1&quot;);
+    // invokes M(S&amp;&amp;, int)
+    option&lt;M&gt; opt1(S{}, 123);
+
+    std::puts(&quot;==&gt; case 2&quot;);
+    // invokes M() + M(M&amp;&amp;)
+    option&lt;M&gt; x /* option(M&amp;&amp;) + M(M&amp;&amp;) */ = M{} /* M() */;
+}
+</code></pre>
 
                     </main>