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                <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 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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 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                        <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();

// force compile error
typename decltype(foo)::_;
</code></pre>
<h2 id="variadic-templates-parameter-pack"><a class="header" href="#variadic-templates-parameter-pack">Variadic templates (<a href="https://en.cppreference.com/w/cpp/language/parameter_pack">parameter pack</a>)</a></h2>
<pre><code class="language-cpp">#include &lt;iostream&gt;

// -- Example 1 - print template value arguments.

// Base case with one parameter.
template&lt;int P&gt;
void show_int() {
    printf(&quot;%d\n&quot;, P);
}

// General case with at least two parameters, to disambiguate from base case.
template&lt;int P0, int P1, int... Params&gt;
void show_int() {
    printf(&quot;%d, &quot;, P0);
    show_int&lt;P1, Params...&gt;();
}

// -- Example 2 - print values of different types.

// Base case with one parameter.
template&lt;typename T&gt;
void show(const T&amp; t) {
    std::cout &lt;&lt; t &lt;&lt; '\n';
}

// General case with at least two parameters, to disambiguate from base case.
template&lt;typename T0, typename T1, typename... Types&gt;
void show(const T0&amp; t0, const T1&amp; t1, const Types&amp;... types) {
    std::cout &lt;&lt; t0 &lt;&lt; &quot;, &quot;;
    show(t1, types...);
}

int main() {
    show_int&lt;1, 2, 3, 4, 5&gt;();
    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;

template&lt;typename T, typename... U&gt;
struct any_of : std::false_type {};

// Found our type T in the list of types U.
template&lt;typename T, typename... U&gt;
struct any_of&lt;T, T, U...&gt; : std::true_type {};

// Pop off the first element in the list of types U,
// since it didn't match our type T.
template&lt;typename T, typename U0, typename... U&gt;
struct any_of&lt;T, U0, U...&gt; : any_of&lt;T, U...&gt; {};

// Convenience template variable to invoke meta function.
template&lt;typename T, typename... U&gt;
constexpr bool any_of_v = any_of&lt;T, U...&gt;::value;

static_assert(any_of_v&lt;int, char, bool, int&gt;, &quot;&quot;);
static_assert(!any_of_v&lt;int, char, bool, float&gt;, &quot;&quot;);
</code></pre>
<h2 id="example-sfinae-enable_if"><a class="header" href="#example-sfinae-enable_if">Example: <a href="https://en.cppreference.com/w/cpp/language/sfinae">SFINAE</a> (<a href="https://en.cppreference.com/w/cpp/types/enable_if">enable_if</a>)</a></h2>
<p>Provide a single entry point <code>Invoke</code> to call some <code>Operations</code>.
Use <code>enable_if</code> to enable/disable the template functions depending on the two
available traits an operation can have:</p>
<ul>
<li>Operation returns a result</li>
<li>Operation requires a context</li>
</ul>
<pre><code class="language-cpp">#include &lt;iostream&gt;
#include &lt;type_traits&gt;

// Helper meta fns.

template&lt;typename T&gt;
using enable_if_bool = std::enable_if_t&lt;T::value, bool&gt;;

template&lt;typename T&gt;
using disable_if_bool = std::enable_if_t&lt;!T::value, bool&gt;;

template&lt;typename T&gt;
using has_dst = std::integral_constant&lt;bool, !std::is_same&lt;typename T::Return, void&gt;::value&gt;;

// Template meta programming invoke machinery.

namespace impl {
    // Invoke an OPERATION which *USES* a context.
    template&lt;typename Ctx, template&lt;typename&gt; class Op, typename... P,
             enable_if_bool&lt;typename Op&lt;Ctx&gt;::HasCtx&gt; = true&gt;
    typename Op&lt;Ctx&gt;::Return Invoke(const Ctx&amp; C, P... params) {
        return Op&lt;Ctx&gt;()(C, params...);
    }

    // Invoke an OPERATION which uses *NO* context.
    template&lt;typename Ctx, template&lt;typename&gt; class Op, typename... P,
             disable_if_bool&lt;typename Op&lt;Ctx&gt;::HasCtx&gt; = true&gt;
    typename Op&lt;Ctx&gt;::Return Invoke(const Ctx&amp;, P... params) {
        return Op&lt;Ctx&gt;()(params...);
    }
}  // namespace impl

// Invoke an OPERATION which *HAS* a DESTINATION with arbitrary number of arguments.
template&lt;typename Ctx, template&lt;typename&gt; class Op, typename... P,
         enable_if_bool&lt;has_dst&lt;Op&lt;Ctx&gt;&gt;&gt; = true&gt;
void Invoke(const Ctx&amp; C, P... params) {
    std::cout &lt;&lt; &quot;Invoke &quot; &lt;&lt; Op&lt;Ctx&gt;::Name &lt;&lt; '\n';
    typename Op&lt;Ctx&gt;::Return R = impl::Invoke&lt;Ctx, Op&gt;(C, params...);
    std::cout &lt;&lt; &quot;returned -&gt; &quot; &lt;&lt; R &lt;&lt; '\n';
}

// Invoke an OPERATION which has *NOT* a DESTINATION with arbitrary number of arguments.
template&lt;typename Ctx, template&lt;typename&gt; class Op, typename... P,
         disable_if_bool&lt;has_dst&lt;Op&lt;Ctx&gt;&gt;&gt; = true&gt;
void Invoke(const Ctx&amp; C, P... params) {
    std::cout &lt;&lt; &quot;Invoke &quot; &lt;&lt; Op&lt;Ctx&gt;::Name &lt;&lt; &quot; without destination.&quot; &lt;&lt; '\n';
    impl::Invoke&lt;Ctx, Op&gt;(C, params...);
}

// Custom context.

struct Ctx {
    void out(const char* s, unsigned v) const {
        printf(&quot;%s%x\n&quot;, s, v);
    }
};

// Operations to invoke.

template&lt;typename Ctx&gt;
struct OpA {
    using HasCtx = std::false_type;
    using Return = int;
    static constexpr const char* const Name = &quot;OpA&quot;;

    constexpr Return operator()(int a, int b) const {
        return a + b;
    }
};

template&lt;typename Ctx&gt;
struct OpB {
    using HasCtx = std::true_type;
    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);
    }
};

int main() {
    Ctx C;

    Invoke&lt;Ctx, OpA&gt;(C, 1, 2);
    Invoke&lt;Ctx, OpB&gt;(C, 0xf00du);

    return 0;
}
</code></pre>
<h2 id="example-minimal-templatized-test-registry"><a class="header" href="#example-minimal-templatized-test-registry">Example: Minimal templatized test registry</a></h2>
<p>A small test function registry bringing together a few different template
features.</p>
<pre><code class="language-cpp">#include &lt;cstdio&gt;
#include &lt;functional&gt;
#include &lt;map&gt;
#include &lt;string&gt;
#include &lt;type_traits&gt;

template&lt;typename R, typename... P&gt;
struct registry {
    using FUNC = R (*)(P...);
    using SELF = registry&lt;R, P...&gt;;
    using RET = R;

    static SELF&amp; get() {
        static SELF r;
        return r;
    }

    bool add(std::string nm, FUNC fn) {
        const auto r = m_fns.insert({std::move(nm), std::move(fn)});
        return r.second;
    }

    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) {
            std::puts(it.first.c_str());
        }
    }

  private:
    std::map&lt;std::string, FUNC&gt; m_fns;

    template&lt;typename RET&gt;
    std::enable_if_t&lt;std::is_same_v&lt;RET, void&gt;&gt; invoke_impl(const std::string&amp; nm, P... p) const {
        const auto it = m_fns.find(nm);
        if (it == m_fns.end()) {
            return;
        }
        std::invoke(it-&gt;second, p...);
    }

    template&lt;typename RET&gt;
    std::enable_if_t&lt;!std::is_same_v&lt;RET, void&gt;, RET&gt; invoke_impl(const std::string&amp; nm,
                                                                  P... p) const {
        const auto it = m_fns.find(nm);
        if (it == m_fns.end()) {
            static_assert(std::is_default_constructible_v&lt;RET&gt;,
                          &quot;RET must be default constructible&quot;);
            return {};
        }
        return std::invoke(it-&gt;second, p...);
    }
};

#define TEST_REGISTER(REGISTRY, NAME)                                                      \
    static bool regfn_##REGISTRY##NAME() {                                                 \
        const bool r = REGISTRY::get().add(#NAME, NAME);                                   \
        if (!r) {                                                                          \
            std::puts(&quot;Failed to register test &quot; #NAME &quot;, same name already registered!&quot;); \
            std::abort();                                                                  \
        }                                                                                  \
        return r;                                                                          \
    }                                                                                      \
    static const bool reg_##REGISTRY##NAME = regfn_##REGISTRY##NAME();

#define TEST(REGISTRY, NAME, ...)    \
    REGISTRY::RET NAME(__VA_ARGS__); \
    TEST_REGISTER(REGISTRY, NAME);   \
    REGISTRY::RET NAME(__VA_ARGS__)

// -- Usage 1 simple usage.

using REG1 = registry&lt;void&gt;;
TEST(REG1, test1) {
    std::puts(&quot;REG1::test1&quot;);
}
TEST(REG1, test2) {
    std::puts(&quot;REG1::test2&quot;);
}

// -- Usage 2 with convenience macro wrapper.

using REG2 = registry&lt;void, bool&gt;;
#define TEST2(NAME, ...) TEST(REG2, NAME, ##__VA_ARGS__)

TEST2(test1, bool val) {
    printf(&quot;REG2::test1 val %d\n&quot;, val);
}

int main() {
    const auto&amp; R1 = REG1::get();
    R1.dump();
    R1.invoke(&quot;test1&quot;);
    R1.invoke(&quot;test2&quot;);

    const auto&amp; R2 = REG2::get();
    R2.dump();
    R2.invoke(&quot;test1&quot;, true);

    return 0;
}
</code></pre>
<h2 id="example-concepts-pre-c20"><a class="header" href="#example-concepts-pre-c20">Example: Concepts pre c++20</a></h2>
<p>Prior to c++20's concepts, <code>SFINAE</code> and <code>std::void_t</code> can be leveraged to build
something similar allowing to define an interface (aka trait) for a template
parameter.</p>
<pre><code class="language-cpp">
template&lt;typename T, template&lt;typename&gt; class Checker, typename = void&gt;
struct is_valid : std::false_type {};

template&lt;typename T, template&lt;typename&gt; class Checker&gt;
struct is_valid&lt;T, Checker, std::void_t&lt;Checker&lt;T&gt;&gt;&gt; : std::true_type {};

template&lt;typename T, template&lt;typename&gt; class Checker&gt;
static constexpr bool is_valid_v = is_valid&lt;T, Checker&gt;::value;

// -----------------------------------------------------------------------------

template&lt;typename T, typename R, template&lt;typename&gt; class Checker, typename = void&gt;
struct is_valid_with_ret : std::false_type {};

template&lt;typename T, typename R, template&lt;typename&gt; class Checker&gt;
struct is_valid_with_ret&lt;T, R, Checker, std::void_t&lt;Checker&lt;T&gt;&gt;&gt; : std::is_same&lt;R, Checker&lt;T&gt;&gt; {};

template&lt;typename T, typename R, template&lt;typename&gt; class Checker&gt;
static constexpr bool is_valid_with_ret_v = is_valid_with_ret&lt;T, R, Checker&gt;::value;

// -----------------------------------------------------------------------------

template&lt;typename T&gt;
struct is_entry {
    template&lt;typename TT&gt;
    using init = decltype(std::declval&lt;TT&gt;().init());
    template&lt;typename TT&gt;
    using tag = decltype(std::declval&lt;TT&gt;().tag());
    template&lt;typename TT&gt;
    using val = decltype(std::declval&lt;TT&gt;().val());

    static constexpr bool value = is_valid_v&lt;T, init&gt; &amp;&amp; is_valid_with_ret_v&lt;T, int, tag&gt; &amp;&amp;
                                  is_valid_with_ret_v&lt;T, typename T::Type, val&gt;;
};

template&lt;typename T&gt;
static constexpr bool is_entry_v = is_entry&lt;T&gt;::value;

template&lt;typename E&gt;
struct Entry {
    using Type = E;
    void init();
    int tag() const;
    E val() const;
};

int main() {
    static_assert(is_entry_v&lt;Entry&lt;bool&gt;&gt;, &quot;&quot;);
}
</code></pre>
<p>The main mechanic can be explained with the following reduced example. If one
of the <code>decltype(std:declval&lt;T&gt;...</code> expressions is ill-formed, the template
specialization for <code>is_valid</code> will be removed from the candidate set due to
<a href="https://en.cppreference.com/w/cpp/language/sfinae">SFINAE</a>.</p>
<pre><code class="language-cpp">#include &lt;type_traits&gt;

// (1) Primary template.
template&lt;typename T, typename = void&gt;
struct is_valid : std::false_type {};

// (2) Partial template specialization.
template&lt;typename T&gt;
struct is_valid&lt;T, std::void_t&lt;decltype(std::declval&lt;T&gt;().some_fun1()),
                               decltype(std::declval&lt;T&gt;().some_fun2())&gt;&gt; : std::true_type {};
struct A {
    void some_fun1() {}
    void some_fun2() {}
};

struct B {};

static_assert(is_valid&lt;A&gt;::value, &quot;is true&quot;);
// * Compare template arg list with primary template, we only supplied one
//   arg, the second one will be defaulted as
//   is_valid&lt;A, void&gt;
// * Compare template arg list against available specializations, this will
//   try to match the pattern &lt;A, void&gt; against the patterns defined in the
//   partial specializations.
// * Try specialization (2)
//   * T -&gt; A
//   * Evaluate std::void_t -&gt; decltype's are well-formed
//     std::void_t&lt;...&gt; -&gt; void
//   * Specialization (2) matches &lt;A, void&gt;
// * Pick the most specialized version -&gt; (2)

static_assert(!is_valid&lt;B&gt;::value, &quot;is false&quot;);
// * Compare template arg list with primary template, we only supplied one
//   arg, the second one will be defaulted as
//   is_valid&lt;A, void&gt;
// * Compare template arg list against available specializations, this will
//   try to match the pattern &lt;B, void&gt; against the patterns defined in the
//   partial specializations.
// * Try specialization (2)
//   * T -&gt; B
//   * Evaluate std::void_t -&gt; decltype's are ill-formed
//   * Specialization (2) is removed from candidate set, no hard error (SFINAE)
// * No specialization matches, take the primary template.
</code></pre>
<blockquote>
<p><code>std::declval&lt;T&gt;()</code> creates an instance of type T in an unevaluated context.</p>
</blockquote>
<p>A more detailed description is available in the SO discussion <a href="https://stackoverflow.com/a/27688405">How does
<code>void_t</code> work</a>.</p>
<h2 id="template-selection-with-partially--fully-specializations"><a class="header" href="#template-selection-with-partially--fully-specializations">Template selection with partially / fully specializations.</a></h2>
<pre><code class="language-cpp">enum Kind {
    kPrimary,
    kTT,
    kIntBool,
    kIntInt,
};

// (1) Primary template.
template&lt;typename T, typename U = bool&gt;
struct pair {
    static constexpr Kind kind = kPrimary;
};

// (2) Partial template specialization.
template&lt;typename T&gt;
struct pair&lt;T, T&gt; {
    static constexpr Kind kind = kTT;
};

// (3) Template specialization.
template&lt;&gt;
struct pair&lt;int, bool&gt; {
    static constexpr Kind kind = kIntBool;
};

// (4) Template specialization.
template&lt;&gt;
struct pair&lt;int, int&gt; {
    static constexpr Kind kind = kIntInt;
};

int main() {
    static_assert(pair&lt;int&gt;::kind == kIntBool, &quot;&quot;);
    // * Compare template arg list with primary template, we only supplied one
    //   arg, the second one will be defaulted as
    //   pair&lt;int, bool&gt;
    // * Compare template arg list against available specializations, this will
    //   try to match the pattern &lt;int, bool&gt; against the patterns defined in the
    //   partial specializations.
    // * (2) &lt;int, bool&gt; pattern does not match
    // * (3) &lt;int, bool&gt; pattern does match
    // * (4) &lt;int, bool&gt; pattern does not match
    // * Pick the most specialized version -&gt; (3)

    static_assert(pair&lt;char, char&gt;::kind == kTT, &quot;&quot;);
    // * Compare template arg list against available specializations, this will
    //   try to match the pattern &lt;char, char&gt; against the patterns defined in the
    //   partial specializations.
    // * (2) &lt;char, char&gt; pattern does match
    // * (3) &lt;char, char&gt; pattern does not match
    // * (4) &lt;char, char&gt; pattern does not match
    // * Pick the most specialized version -&gt; (2)

    static_assert(pair&lt;int, int&gt;::kind == kIntInt, &quot;&quot;);
    // * Compare template arg list against available specializations, this will
    //   try to match the pattern &lt;int, int&gt; against the patterns defined in the
    //   partial specializations.
    // * (2) &lt;int, int&gt; pattern does match
    // * (3) &lt;int, int&gt; pattern does match
    // * (4) &lt;int, int&gt; pattern does not match
    // * Pick the most specialized version -&gt; (3)

    static_assert(pair&lt;char, short&gt;::kind == kPrimary, &quot;&quot;);
    // * Compare template arg list against available specializations, this will
    //   try to match the pattern &lt;char, short&gt; against the patterns defined in the
    //   partial specializations.
    // * (2) &lt;char, short&gt; pattern does not match
    // * (3) &lt;char, short&gt; pattern does not match
    // * (4) &lt;char, short&gt; pattern does not match
    // * 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>

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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>
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                        <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();

// force compile error
typename decltype(foo)::_;
</code></pre>
<h2 id="variadic-templates-parameter-pack"><a class="header" href="#variadic-templates-parameter-pack">Variadic templates (<a href="https://en.cppreference.com/w/cpp/language/parameter_pack">parameter pack</a>)</a></h2>
<pre><code class="language-cpp">#include &lt;iostream&gt;

// -- Example 1 - print template value arguments.

// Base case with one parameter.
template&lt;int P&gt;
void show_int() {
    printf(&quot;%d\n&quot;, P);
}

// General case with at least two parameters, to disambiguate from base case.
template&lt;int P0, int P1, int... Params&gt;
void show_int() {
    printf(&quot;%d, &quot;, P0);
    show_int&lt;P1, Params...&gt;();
}

// -- Example 2 - print values of different types.

// Base case with one parameter.
template&lt;typename T&gt;
void show(const T&amp; t) {
    std::cout &lt;&lt; t &lt;&lt; '\n';
}

// General case with at least two parameters, to disambiguate from base case.
template&lt;typename T0, typename T1, typename... Types&gt;
void show(const T0&amp; t0, const T1&amp; t1, const Types&amp;... types) {
    std::cout &lt;&lt; t0 &lt;&lt; &quot;, &quot;;
    show(t1, types...);
}

int main() {
    show_int&lt;1, 2, 3, 4, 5&gt;();
    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;

template&lt;typename T, typename... U&gt;
struct any_of : std::false_type {};

// Found our type T in the list of types U.
template&lt;typename T, typename... U&gt;
struct any_of&lt;T, T, U...&gt; : std::true_type {};

// Pop off the first element in the list of types U,
// since it didn't match our type T.
template&lt;typename T, typename U0, typename... U&gt;
struct any_of&lt;T, U0, U...&gt; : any_of&lt;T, U...&gt; {};

// Convenience template variable to invoke meta function.
template&lt;typename T, typename... U&gt;
constexpr bool any_of_v = any_of&lt;T, U...&gt;::value;

static_assert(any_of_v&lt;int, char, bool, int&gt;, &quot;&quot;);
static_assert(!any_of_v&lt;int, char, bool, float&gt;, &quot;&quot;);
</code></pre>
<h2 id="example-sfinae-enable_if"><a class="header" href="#example-sfinae-enable_if">Example: <a href="https://en.cppreference.com/w/cpp/language/sfinae">SFINAE</a> (<a href="https://en.cppreference.com/w/cpp/types/enable_if">enable_if</a>)</a></h2>
<p>Provide a single entry point <code>Invoke</code> to call some <code>Operations</code>.
Use <code>enable_if</code> to enable/disable the template functions depending on the two
available traits an operation can have:</p>
<ul>
<li>Operation returns a result</li>
<li>Operation requires a context</li>
</ul>
<pre><code class="language-cpp">#include &lt;iostream&gt;
#include &lt;type_traits&gt;

// Helper meta fns.

template&lt;typename T&gt;
using enable_if_bool = std::enable_if_t&lt;T::value, bool&gt;;

template&lt;typename T&gt;
using disable_if_bool = std::enable_if_t&lt;!T::value, bool&gt;;

template&lt;typename T&gt;
using has_dst = std::integral_constant&lt;bool, !std::is_same&lt;typename T::Return, void&gt;::value&gt;;

// Template meta programming invoke machinery.

namespace impl {
    // Invoke an OPERATION which *USES* a context.
    template&lt;typename Ctx, template&lt;typename&gt; class Op, typename... P,
             enable_if_bool&lt;typename Op&lt;Ctx&gt;::HasCtx&gt; = true&gt;
    typename Op&lt;Ctx&gt;::Return Invoke(const Ctx&amp; C, P... params) {
        return Op&lt;Ctx&gt;()(C, params...);
    }

    // Invoke an OPERATION which uses *NO* context.
    template&lt;typename Ctx, template&lt;typename&gt; class Op, typename... P,
             disable_if_bool&lt;typename Op&lt;Ctx&gt;::HasCtx&gt; = true&gt;
    typename Op&lt;Ctx&gt;::Return Invoke(const Ctx&amp;, P... params) {
        return Op&lt;Ctx&gt;()(params...);
    }
}  // namespace impl

// Invoke an OPERATION which *HAS* a DESTINATION with arbitrary number of arguments.
template&lt;typename Ctx, template&lt;typename&gt; class Op, typename... P,
         enable_if_bool&lt;has_dst&lt;Op&lt;Ctx&gt;&gt;&gt; = true&gt;
void Invoke(const Ctx&amp; C, P... params) {
    std::cout &lt;&lt; &quot;Invoke &quot; &lt;&lt; Op&lt;Ctx&gt;::Name &lt;&lt; '\n';
    typename Op&lt;Ctx&gt;::Return R = impl::Invoke&lt;Ctx, Op&gt;(C, params...);
    std::cout &lt;&lt; &quot;returned -&gt; &quot; &lt;&lt; R &lt;&lt; '\n';
}

// Invoke an OPERATION which has *NOT* a DESTINATION with arbitrary number of arguments.
template&lt;typename Ctx, template&lt;typename&gt; class Op, typename... P,
         disable_if_bool&lt;has_dst&lt;Op&lt;Ctx&gt;&gt;&gt; = true&gt;
void Invoke(const Ctx&amp; C, P... params) {
    std::cout &lt;&lt; &quot;Invoke &quot; &lt;&lt; Op&lt;Ctx&gt;::Name &lt;&lt; &quot; without destination.&quot; &lt;&lt; '\n';
    impl::Invoke&lt;Ctx, Op&gt;(C, params...);
}

// Custom context.

struct Ctx {
    void out(const char* s, unsigned v) const {
        printf(&quot;%s%x\n&quot;, s, v);
    }
};

// Operations to invoke.

template&lt;typename Ctx&gt;
struct OpA {
    using HasCtx = std::false_type;
    using Return = int;
    static constexpr const char* const Name = &quot;OpA&quot;;

    constexpr Return operator()(int a, int b) const {
        return a + b;
    }
};

template&lt;typename Ctx&gt;
struct OpB {
    using HasCtx = std::true_type;
    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);
    }
};

int main() {
    Ctx C;

    Invoke&lt;Ctx, OpA&gt;(C, 1, 2);
    Invoke&lt;Ctx, OpB&gt;(C, 0xf00du);

    return 0;
}
</code></pre>
<h2 id="example-minimal-templatized-test-registry"><a class="header" href="#example-minimal-templatized-test-registry">Example: Minimal templatized test registry</a></h2>
<p>A small test function registry bringing together a few different template
features.</p>
<pre><code class="language-cpp">#include &lt;cstdio&gt;
#include &lt;functional&gt;
#include &lt;map&gt;
#include &lt;string&gt;
#include &lt;type_traits&gt;

template&lt;typename R, typename... P&gt;
struct registry {
    using FUNC = R (*)(P...);
    using SELF = registry&lt;R, P...&gt;;
    using RET = R;

    static SELF&amp; get() {
        static SELF r;
        return r;
    }

    bool add(std::string nm, FUNC fn) {
        const auto r = m_fns.insert({std::move(nm), std::move(fn)});
        return r.second;
    }

    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) {
            std::puts(it.first.c_str());
        }
    }

  private:
    std::map&lt;std::string, FUNC&gt; m_fns;

    template&lt;typename RET&gt;
    std::enable_if_t&lt;std::is_same_v&lt;RET, void&gt;&gt; invoke_impl(const std::string&amp; nm, P... p) const {
        const auto it = m_fns.find(nm);
        if (it == m_fns.end()) {
            return;
        }
        std::invoke(it-&gt;second, p...);
    }

    template&lt;typename RET&gt;
    std::enable_if_t&lt;!std::is_same_v&lt;RET, void&gt;, RET&gt; invoke_impl(const std::string&amp; nm,
                                                                  P... p) const {
        const auto it = m_fns.find(nm);
        if (it == m_fns.end()) {
            static_assert(std::is_default_constructible_v&lt;RET&gt;,
                          &quot;RET must be default constructible&quot;);
            return {};
        }
        return std::invoke(it-&gt;second, p...);
    }
};

#define TEST_REGISTER(REGISTRY, NAME)                                                      \
    static bool regfn_##REGISTRY##NAME() {                                                 \
        const bool r = REGISTRY::get().add(#NAME, NAME);                                   \
        if (!r) {                                                                          \
            std::puts(&quot;Failed to register test &quot; #NAME &quot;, same name already registered!&quot;); \
            std::abort();                                                                  \
        }                                                                                  \
        return r;                                                                          \
    }                                                                                      \
    static const bool reg_##REGISTRY##NAME = regfn_##REGISTRY##NAME();

#define TEST(REGISTRY, NAME, ...)    \
    REGISTRY::RET NAME(__VA_ARGS__); \
    TEST_REGISTER(REGISTRY, NAME);   \
    REGISTRY::RET NAME(__VA_ARGS__)

// -- Usage 1 simple usage.

using REG1 = registry&lt;void&gt;;
TEST(REG1, test1) {
    std::puts(&quot;REG1::test1&quot;);
}
TEST(REG1, test2) {
    std::puts(&quot;REG1::test2&quot;);
}

// -- Usage 2 with convenience macro wrapper.

using REG2 = registry&lt;void, bool&gt;;
#define TEST2(NAME, ...) TEST(REG2, NAME, ##__VA_ARGS__)

TEST2(test1, bool val) {
    printf(&quot;REG2::test1 val %d\n&quot;, val);
}

int main() {
    const auto&amp; R1 = REG1::get();
    R1.dump();
    R1.invoke(&quot;test1&quot;);
    R1.invoke(&quot;test2&quot;);

    const auto&amp; R2 = REG2::get();
    R2.dump();
    R2.invoke(&quot;test1&quot;, true);

    return 0;
}
</code></pre>
<h2 id="example-concepts-pre-c20"><a class="header" href="#example-concepts-pre-c20">Example: Concepts pre c++20</a></h2>
<p>Prior to c++20's concepts, <code>SFINAE</code> and <code>std::void_t</code> can be leveraged to build
something similar allowing to define an interface (aka trait) for a template
parameter.</p>
<pre><code class="language-cpp">
template&lt;typename T, template&lt;typename&gt; class Checker, typename = void&gt;
struct is_valid : std::false_type {};

template&lt;typename T, template&lt;typename&gt; class Checker&gt;
struct is_valid&lt;T, Checker, std::void_t&lt;Checker&lt;T&gt;&gt;&gt; : std::true_type {};

template&lt;typename T, template&lt;typename&gt; class Checker&gt;
static constexpr bool is_valid_v = is_valid&lt;T, Checker&gt;::value;

// -----------------------------------------------------------------------------

template&lt;typename T, typename R, template&lt;typename&gt; class Checker, typename = void&gt;
struct is_valid_with_ret : std::false_type {};

template&lt;typename T, typename R, template&lt;typename&gt; class Checker&gt;
struct is_valid_with_ret&lt;T, R, Checker, std::void_t&lt;Checker&lt;T&gt;&gt;&gt; : std::is_same&lt;R, Checker&lt;T&gt;&gt; {};

template&lt;typename T, typename R, template&lt;typename&gt; class Checker&gt;
static constexpr bool is_valid_with_ret_v = is_valid_with_ret&lt;T, R, Checker&gt;::value;

// -----------------------------------------------------------------------------

template&lt;typename T&gt;
struct is_entry {
    template&lt;typename TT&gt;
    using init = decltype(std::declval&lt;TT&gt;().init());
    template&lt;typename TT&gt;
    using tag = decltype(std::declval&lt;TT&gt;().tag());
    template&lt;typename TT&gt;
    using val = decltype(std::declval&lt;TT&gt;().val());

    static constexpr bool value = is_valid_v&lt;T, init&gt; &amp;&amp; is_valid_with_ret_v&lt;T, int, tag&gt; &amp;&amp;
                                  is_valid_with_ret_v&lt;T, typename T::Type, val&gt;;
};

template&lt;typename T&gt;
static constexpr bool is_entry_v = is_entry&lt;T&gt;::value;

template&lt;typename E&gt;
struct Entry {
    using Type = E;
    void init();
    int tag() const;
    E val() const;
};

int main() {
    static_assert(is_entry_v&lt;Entry&lt;bool&gt;&gt;, &quot;&quot;);
}
</code></pre>
<p>The main mechanic can be explained with the following reduced example. If one
of the <code>decltype(std:declval&lt;T&gt;...</code> expressions is ill-formed, the template
specialization for <code>is_valid</code> will be removed from the candidate set due to
<a href="https://en.cppreference.com/w/cpp/language/sfinae">SFINAE</a>.</p>
<pre><code class="language-cpp">#include &lt;type_traits&gt;

// (1) Primary template.
template&lt;typename T, typename = void&gt;
struct is_valid : std::false_type {};

// (2) Partial template specialization.
template&lt;typename T&gt;
struct is_valid&lt;T, std::void_t&lt;decltype(std::declval&lt;T&gt;().some_fun1()),
                               decltype(std::declval&lt;T&gt;().some_fun2())&gt;&gt; : std::true_type {};
struct A {
    void some_fun1() {}
    void some_fun2() {}
};

struct B {};

static_assert(is_valid&lt;A&gt;::value, &quot;is true&quot;);
// * Compare template arg list with primary template, we only supplied one
//   arg, the second one will be defaulted as
//   is_valid&lt;A, void&gt;
// * Compare template arg list against available specializations, this will
//   try to match the pattern &lt;A, void&gt; against the patterns defined in the
//   partial specializations.
// * Try specialization (2)
//   * T -&gt; A
//   * Evaluate std::void_t -&gt; decltype's are well-formed
//     std::void_t&lt;...&gt; -&gt; void
//   * Specialization (2) matches &lt;A, void&gt;
// * Pick the most specialized version -&gt; (2)

static_assert(!is_valid&lt;B&gt;::value, &quot;is false&quot;);
// * Compare template arg list with primary template, we only supplied one
//   arg, the second one will be defaulted as
//   is_valid&lt;A, void&gt;
// * Compare template arg list against available specializations, this will
//   try to match the pattern &lt;B, void&gt; against the patterns defined in the
//   partial specializations.
// * Try specialization (2)
//   * T -&gt; B
//   * Evaluate std::void_t -&gt; decltype's are ill-formed
//   * Specialization (2) is removed from candidate set, no hard error (SFINAE)
// * No specialization matches, take the primary template.
</code></pre>
<blockquote>
<p><code>std::declval&lt;T&gt;()</code> creates an instance of type T in an unevaluated context.</p>
</blockquote>
<p>A more detailed description is available in the SO discussion <a href="https://stackoverflow.com/a/27688405">How does
<code>void_t</code> work</a>.</p>
<h2 id="template-selection-with-partially--fully-specializations"><a class="header" href="#template-selection-with-partially--fully-specializations">Template selection with partially / fully specializations.</a></h2>
<pre><code class="language-cpp">enum Kind {
    kPrimary,
    kTT,
    kIntBool,
    kIntInt,
};

// (1) Primary template.
template&lt;typename T, typename U = bool&gt;
struct pair {
    static constexpr Kind kind = kPrimary;
};

// (2) Partial template specialization.
template&lt;typename T&gt;
struct pair&lt;T, T&gt; {
    static constexpr Kind kind = kTT;
};

// (3) Template specialization.
template&lt;&gt;
struct pair&lt;int, bool&gt; {
    static constexpr Kind kind = kIntBool;
};

// (4) Template specialization.
template&lt;&gt;
struct pair&lt;int, int&gt; {
    static constexpr Kind kind = kIntInt;
};

int main() {
    static_assert(pair&lt;int&gt;::kind == kIntBool, &quot;&quot;);
    // * Compare template arg list with primary template, we only supplied one
    //   arg, the second one will be defaulted as
    //   pair&lt;int, bool&gt;
    // * Compare template arg list against available specializations, this will
    //   try to match the pattern &lt;int, bool&gt; against the patterns defined in the
    //   partial specializations.
    // * (2) &lt;int, bool&gt; pattern does not match
    // * (3) &lt;int, bool&gt; pattern does match
    // * (4) &lt;int, bool&gt; pattern does not match
    // * Pick the most specialized version -&gt; (3)

    static_assert(pair&lt;char, char&gt;::kind == kTT, &quot;&quot;);
    // * Compare template arg list against available specializations, this will
    //   try to match the pattern &lt;char, char&gt; against the patterns defined in the
    //   partial specializations.
    // * (2) &lt;char, char&gt; pattern does match
    // * (3) &lt;char, char&gt; pattern does not match
    // * (4) &lt;char, char&gt; pattern does not match
    // * Pick the most specialized version -&gt; (2)

    static_assert(pair&lt;int, int&gt;::kind == kIntInt, &quot;&quot;);
    // * Compare template arg list against available specializations, this will
    //   try to match the pattern &lt;int, int&gt; against the patterns defined in the
    //   partial specializations.
    // * (2) &lt;int, int&gt; pattern does match
    // * (3) &lt;int, int&gt; pattern does match
    // * (4) &lt;int, int&gt; pattern does not match
    // * Pick the most specialized version -&gt; (3)

    static_assert(pair&lt;char, short&gt;::kind == kPrimary, &quot;&quot;);
    // * Compare template arg list against available specializations, this will
    //   try to match the pattern &lt;char, short&gt; against the patterns defined in the
    //   partial specializations.
    // * (2) &lt;char, short&gt; pattern does not match
    // * (3) &lt;char, short&gt; pattern does not match
    // * (4) &lt;char, short&gt; pattern does not match
    // * 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>

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