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<address align=right>
Document number: P0644R1 <br />
Date: 2017-10-08 <br />
Audience: Evolution Working Group <br />
Reply-To: Barry Revzin &lt;barry.revzin@gmail.com>
</address>
<hr/>
<h1 align=center>Forward without <code>forward</code></h1>

<h2>Contents</h2>

<ul>
<li><a href="#Motivation">Motivation</a></li>
<li><a href="#Proposal">Proposal</a></li>
<li><a href="#Revision">Revision History</a></li>
</ul>

<a name="Motivation"> </a><h2>Motivation</h2>

Consider the following example:

<pre><code class="language-cpp">template &lt;class X, class Y>
decltype(auto) foo(X&& x, Y&& y) {
    return std::forward&lt;X>(x)(std::forward&lt;Y>(y));
}</code></pre>

It's a simple function, but it takes some time to figure out what's actually going on. It's even more awkward when we consider the same function in lambda form (although this awkwardness has been partially alleviated with the adoption of <a href="https://wg21.link/p0428">[P0428]</a>):

<pre><code class="language-cpp">// C++17
auto bar = [](auto&& x, auto&& y) {
    return std::forward&lt;decltype(x)>(x)(std::forward&lt;decltype(y)>(y));
};

// C++2a
auto bar = []&lt;class X, class Y>(X&& x, Y&& y) {
    return std::forward&lt;X>(x)(std::forward&lt;Y>(y));
};
</code></pre>

This verbosity and lack of clarity lead many people to actually use a <i>macro</i> to make forwarding shorter:

<pre><code class="language-cpp">#define FWD(x) std::forward&lt;decltype(x)>(x)

template &lt;class X, class Y>
decltype(auto) foo(X&& x, Y&& y) {
    return FWD(x)(FWD(y));
}

auto bar = [](auto&& x, auto&& y) {
    return FWD(x)(FWD(y));
};</code></pre>

<p>A search on GitHub shows over 100 such definitions. That is substantially clearer, but are we really going to suggest that people use a macro?! Let's not. Forwarding references even got their name from the fact that they are intended to be <code class="language-cpp">std::forward()</code>-ed, so it's awkward when the expected and typical usage is just so very verbose. The body of the lambda with <code class="language-cpp">std::forward</code> is 73 characters, the body with <code>FWD</code> is 23. It is the contention of this proposal that those extra characters harm readability and don't help anybody. 

<p>Code that forwards is just so much more verbose than code that doesn't. But forwarding in itself doesn't justify the verbosity or resulting complexity. Unlike <code class="language-cpp">std::move</code> and <code class="language-cpp">std::ref</code> which are used to do non-typical things and deserve to be visible markers for code readability and understanding, <code class="language-cpp">std::forward</code> is much more typical in these contexts and does not need as much of a sign post. We can do better.

<a name="Proposal"></a><h2>Proposal</h2>

<p>This paper would like to see a shorter way to forward arguments and proposes non-overloadable unary <code class="language-cpp">operator>></code>, where <code class="language-cpp">>>expr</code> is defined as <code class="language-cpp">static_cast&lt;decltype(expr)&&>(expr)</code>. This addition would make it easier to write and, more importantly, read code that uses forwarding.

<p>Note that <code class="language-cpp">>>x</code>, where <code class="language-cpp">x</code> is not an lvalue reference, is also equivalent to <code class="language-cpp">std::move(x)</code>.

<p>Examples demonstrating the improvement:

<table style="width:100%">
<tr><th style="width:50%">C++17</th><th style="width:50%">This proposal</th></tr>
<tr><td><pre style="background:transparent;border:none;"><code class="language-cpp">template &lt;class X, class Y>
decltype(auto) foo(X&& x, Y&& y) {
    return std::forward&lt;X>(x)(std::forward&lt;Y>(y));
}

auto bar = [](auto&& x, auto&& y) {
    return std::forward&lt;decltype(x)>(x)(std::forward&lt;decltype(y)>(y));
};</code></pre></td>
<td><pre style="background:transparent;border:none;"><code class="language-cpp">template &lt;class X, class Y>
decltype(auto) foo(X&& x, Y&& y) {
    return (>>x)(>>y);
}

auto bar = [](auto&& x, auto&& y) {
    return (>>x)(>>y);
};</code></pre></td></tr>
<tr><td colspan="2"><center><b>Implementing <code>std::apply</code> (code from <a href="http://en.cppreference.com/w/cpp/utility/apply#Possible_implementation">cppreference</a>)</b></center></td></tr>
<tr><td><pre style="background:transparent;border:none;"><code class="language-cpp">namespace detail {
template &lt;class F, class Tuple, std::size_t... I>
constexpr decltype(auto) apply_impl(F&& f, Tuple&& t,
    std::index_sequence&lt;I...>)
{
    return std::invoke(std::forward&lt;F>(f),
        std::get&lt;I>(std::forward&lt;Tuple>(t))...);
}
}  // namespace detail
 
template &lt;class F, class Tuple>
constexpr decltype(auto) apply(F&& f, Tuple&& t)
{
    return detail::apply_impl(
        std::forward&lt;F>(f), std::forward&lt;Tuple>(t),
        std::make_index_sequence&lt;
            std::tuple_size_v&lt;std::decay_t&lt;Tuple>>>{});
}</code></pre></td>
<td><pre style="background:transparent;border:none;"><code class="language-cpp">namespace detail {
template &lt;class F, class Tuple, std::size_t... I>
constexpr decltype(auto) apply_impl(F&& f, Tuple&& t,
    std::index_sequence&lt;I...>)
{
    return std::invoke(>>f, std::get&lt;I>(>>t)...);
    
}
}  // namespace detail
 
template &lt;class F, class Tuple>
constexpr decltype(auto) apply(F&& f, Tuple&& t)
{
    return detail::apply_impl(
        >>f, >>t,
        std::make_index_sequence&lt;
            std::tuple_size_v&lt;std::decay_t&lt;Tuple>>>{});
}</code></pre></td>
</tr>
<tr><td colspan="2"><center><b>Delaying invocation (assuming pack expansion in init-capture - P0780)</b></center></td></tr>
<tr><td><pre style="background:transparent;border:none;"><code class="language-cpp">template &lt;class F, class... Args>
auto delay_invoke(F&& f, Args&&... args) {
    return [f=std::forward&lt;F>(f),
            args=std::forward&lt;Args>(args)...)]() -> decltype(auto) {
        return std::invoke(f, args...);
    };
}</code></pre></td>
<td valign="top"><pre style="background:transparent;border:none;"><code class="language-cpp">template &lt;class F, class... Args>
auto delay_invoke(F&& f, Args&&... args) {
    return [f=>>f, args=>>args...]() -> decltype(auto) { 
    
        return std::invoke(f, args...);
    };        
}</code></pre></td>
</tr>
<tr><td colspan="2"><center><b>Overload set as function object (assuming abbreviated lambda - P0573)</b></center></td></tr>
<tr><td><pre style="background:transparent;border:none;"><code class="language-cpp">[](auto&&... args) => f(std::forward&lt;decltype(args)>(args)...)
[&](args...) => obj.bar(std::forward&lt;decltype(args)>(args)...)
</code></pre></td>
<td><pre style="background:transparent;border:none;"><code class="language-cpp">[](auto&&... args) => f(>>args...)
[&](args...) => obj.bar(>>args...)</code></pre></td>
</table>

<p>This proposal does not suggest deprecating <code class="language-cpp">std::forward()</code>. There is nothing actually wrong with using it, and moreover there is another functionality of this function template that is quite important: forwarding expressions to a <i>different</i> type. This isn't common, but is more important to highlight as different from simple forwarding. This extension allows for making simple forwarding look simple, while making complex forwarding look complex.

<p>Unary <code class="language-cpp">>></code> is ill-formed today so this is purely a language extension.

<h3>Impact on Compile Times</h3>

<p> There are two ways that you can forward a variable: you can use <code class="language-cpp">std::forward</code> or you can use <code class="language-cpp">static_cast</code> directly (as this proposal's forwarding operator does):

<pre><code class="language-cpp">template &lt;class X, class Y>
decltype(auto) foo(X&& x, Y&& y) {
    return std::forward&lt;X>(x)(std::forward&lt;Y>(y));    // with std::forward
    return static_cast&lt;X&&>(x)(static_cast&lt;Y&&>(y));  // with static_cast, exactly equivalent
}</code></pre>

Louis Dionne <a href="https://github.com/boostorg/hana/commit/540f665e5132d75bbf6eda704638622727c0c01c">rewrote Boost.Hana</a> to use <code class="language-cpp">static_cast</code> instead of <code class="language-cpp">std::forward</code> after tests showed a 14% speed-up in compile times for template-heavy code. Granted, this is <i>very</i> specific use-case, but it does suggest a nice side benefit that a forwarding operator would provide.

<h3>Implementation Experience</h3>

An implementation of this proposal for gcc has been graciously provided by Bastien Penavayre on <a href="https://github.com/DaemonSnake/gcc-abbreviated-cpp-lambda">github</a>, along with a version of the <a href="http://www.gcc-abbreviated-lambdas-proposal.tk/">compiler explorer</a>.


<a name="Revision"></a><h2>Revision History</h2>

<b>Changes from r0</b>: Dropped the proposal of using <code class="language-cpp">>></code> as a <i>capture-default</i>, in favor of simply proposing allowing pack expansion in <i>init-capture</i> (see P0780). Added reference to P0428 for forwarding from generic lambdas, included compile time stats about <code class="language-cpp">std::forward</code> vs <code class="language-cpp">static_cast</code>, corrected some grammar and incorrect syntax in examples.

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