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                        <h1 id="c"><a class="header" href="#c">c++</a></h1>
<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="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>

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