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<tbody><tr>
<td>Document number</td>
<td>P3207R0</td>
</tr>
<tr>
<td>Date</td>
<td>2024-03-24</td>
</tr>
<tr>
<td>Reply-to</td>
<td>
<p><span>Jarrad J. Waterloo &lt;descender76 at gmail dot com&gt;</span></p>
</td>
</tr>
<tr>
<td>Audience</td>
<td>Library Evolution Working Group (LEWG)</td>
</tr>
</tbody></table><h1 id="More-amp-like" data-id="More-amp-like"><a class="anchor hidden-xs" href="#More-amp-like" title="More-amp-like"><span class="octicon octicon-link"></span></a><span>More &amp; like</span></h1><style>
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</style><h2 id="Table-of-contents" data-id="Table-of-contents"><a class="anchor hidden-xs" href="#Table-of-contents" title="Table-of-contents"><span class="octicon octicon-link"></span></a><span>Table of contents</span></h2><ul>
<li><a href="#More-reference-like"><span>More &amp; like</span></a>
<ul>
<li><a href="#Abstract"><span>Abstract</span></a></li>
<li><a href="#Motivational-Example"><span>Motivational Example</span></a>
<ul>
<li><a href="#function_ref"><span>function_ref</span></a></li>
<li><a href="#optional"><span>optional</span></a></li>
<li><a href="#Where-else-and-where-not"><span>Where else and where not</span></a></li>
<li><a href="#Language-feature"><span>Language feature</span></a></li>
</ul>
</li>
<li><a href="#Summary"><span>Summary</span></a></li>
<li><a href="#References"><span>References</span></a></li>
</ul>
</li>
</ul><h2 id="Abstract" data-id="Abstract"><a class="anchor hidden-xs" href="#Abstract" title="Abstract"><span class="octicon octicon-link"></span></a><span>Abstract</span></h2><p data-original-title="" title=""><span>Our new and existing reference types could be made to be more like references. This will reduce the misuse of these reference types and make it easier to reason about dangling.</span></p><h2 id="Motivational-Example" data-id="Motivational-Example"><a class="anchor hidden-xs" href="#Motivational-Example" title="Motivational-Example"><span class="octicon octicon-link"></span></a><span>Motivational Example</span></h2><h3 id="function_ref" data-id="function_ref"><a class="anchor hidden-xs" href="#function_ref" title="function_ref"><span class="octicon octicon-link"></span></a><span>function_ref</span></h3><p><span>A stripped down </span><code>function_ref</code><span> has been created to better illustrate the requested changes. This new version of </span><code>function_ref</code><span> has been renamed </span><code>function_ptr</code><span>. Its address of operators has been deleted. Further, an alias to a </span><code>const function_ptr</code><span> has been created and named </span><code>function_ref</code><span>.</span></p><h4 id="given" data-id="given"><a class="anchor hidden-xs" href="#given" title="given"><span class="octicon octicon-link"></span></a><span>given</span></h4><pre data-original-title="" title=""><code class="cpp hljs"><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string">&lt;string&gt;</span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string">&lt;vector&gt;</span></span>
<span class="token comment">//#include &lt;functional&gt;</span>

<span class="token comment">// NOTE: function_ref renamed to function_ptr since it is reseatable.</span>
<span class="token keyword">template</span><span class="token operator">&lt;</span> <span class="token keyword">class</span> <span class="token operator">&gt;</span>
<span class="token keyword">class</span> <span class="token class-name">function_ptr</span><span class="token punctuation">;</span>

<span class="token keyword">template</span><span class="token operator">&lt;</span> <span class="token keyword">class</span> <span class="token class-name">R</span><span class="token punctuation">,</span> <span class="token keyword">class</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> Args <span class="token operator">&gt;</span>
<span class="token keyword">class</span> <span class="token class-name">function_ptr</span><span class="token operator">&lt;</span><span class="token function">R</span><span class="token punctuation">(</span>Args<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">)</span> <span class="token keyword">noexcept</span><span class="token operator">&gt;</span>
<span class="token punctuation">{</span>
<span class="token keyword">public</span><span class="token operator">:</span>
    <span class="token function">function_ptr</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token keyword">delete</span><span class="token punctuation">;</span>
    <span class="token keyword">constexpr</span> <span class="token function">function_ptr</span><span class="token punctuation">(</span><span class="token keyword">const</span> function_ptr<span class="token operator">&amp;</span> other<span class="token punctuation">)</span> <span class="token keyword">noexcept</span> <span class="token operator">=</span> <span class="token keyword">default</span><span class="token punctuation">;</span>
    <span class="token keyword">constexpr</span> <span class="token function">function_ptr</span><span class="token punctuation">(</span>function_ptr<span class="token operator">&amp;&amp;</span> other<span class="token punctuation">)</span> <span class="token keyword">noexcept</span> <span class="token operator">=</span> <span class="token keyword">default</span><span class="token punctuation">;</span>
    <span class="token keyword">constexpr</span> function_ptr<span class="token operator">&amp;</span> <span class="token keyword">operator</span><span class="token operator">=</span><span class="token punctuation">(</span> <span class="token keyword">const</span> function_ptr<span class="token operator">&amp;</span> <span class="token punctuation">)</span> <span class="token keyword">noexcept</span> <span class="token operator">=</span> <span class="token keyword">default</span><span class="token punctuation">;</span>
    <span class="token keyword">constexpr</span> function_ptr<span class="token operator">&amp;</span> <span class="token keyword">operator</span><span class="token operator">=</span><span class="token punctuation">(</span> function_ptr<span class="token operator">&amp;&amp;</span> other <span class="token punctuation">)</span> <span class="token keyword">noexcept</span> <span class="token operator">=</span> <span class="token keyword">default</span><span class="token punctuation">;</span>
    <span class="token operator">~</span><span class="token function">function_ptr</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">noexcept</span> <span class="token operator">=</span> <span class="token keyword">default</span><span class="token punctuation">;</span>
    <span class="token comment">// NOTE: The 'address of' operators were deleted to prevent</span>
    <span class="token comment">// making a reference to a reference type</span>
    <span class="token comment">// &amp; &amp; &amp; &amp; &amp; = &amp;</span>
    <span class="token comment">// a reference to a reference type should just be a copy</span>
    function_ptr<span class="token operator">*</span> <span class="token keyword">operator</span><span class="token operator">&amp;</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token punctuation">)</span> <span class="token operator">=</span> <span class="token keyword">delete</span><span class="token punctuation">;</span>
    function_ptr <span class="token keyword">const</span> <span class="token operator">*</span><span class="token keyword">operator</span><span class="token operator">&amp;</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token punctuation">)</span> <span class="token keyword">const</span> <span class="token operator">=</span> <span class="token keyword">delete</span><span class="token punctuation">;</span>
    function_ptr <span class="token keyword">volatile</span> <span class="token operator">*</span><span class="token keyword">operator</span><span class="token operator">&amp;</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token punctuation">)</span> <span class="token keyword">volatile</span> <span class="token operator">=</span> <span class="token keyword">delete</span><span class="token punctuation">;</span>
    function_ptr <span class="token keyword">const</span> <span class="token keyword">volatile</span> <span class="token operator">*</span><span class="token keyword">operator</span><span class="token operator">&amp;</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token punctuation">)</span> <span class="token keyword">const</span> <span class="token keyword">volatile</span> <span class="token operator">=</span> <span class="token keyword">delete</span><span class="token punctuation">;</span>

    <span class="token comment">// MISSING; guaranteed efficiency for 2 pointer design</span>
    <span class="token keyword">constexpr</span> <span class="token function">function_ptr</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token operator">*</span> s<span class="token punctuation">,</span> <span class="token function">R</span> <span class="token punctuation">(</span><span class="token operator">*</span>f<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token operator">*</span> state<span class="token punctuation">,</span> Args<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">)</span> <span class="token keyword">noexcept</span><span class="token punctuation">)</span> <span class="token keyword">noexcept</span>
     <span class="token operator">:</span> state<span class="token punctuation">{</span>s<span class="token punctuation">}</span><span class="token punctuation">,</span> thunk<span class="token punctuation">{</span>f<span class="token punctuation">}</span> <span class="token punctuation">{</span><span class="token punctuation">}</span>

    R <span class="token keyword">operator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">(</span> Args<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> args <span class="token punctuation">)</span> <span class="token keyword">const</span> <span class="token keyword">noexcept</span>
    <span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token function">thunk</span><span class="token punctuation">(</span>state<span class="token punctuation">,</span> std<span class="token double-colon punctuation">::</span><span class="token generic-function"><span class="token function">forward</span><span class="token generic class-name"><span class="token operator">&lt;</span>Args<span class="token operator">&gt;</span></span></span><span class="token punctuation">(</span>args<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token keyword">private</span><span class="token operator">:</span>
    <span class="token keyword">void</span><span class="token operator">*</span> state<span class="token punctuation">;</span>
    <span class="token function">R</span> <span class="token punctuation">(</span><span class="token operator">*</span>thunk<span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token operator">*</span> state<span class="token punctuation">,</span> Args<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">)</span> <span class="token keyword">noexcept</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>

<span class="token comment">// NOTE: reference types should by default be const to make it more &amp; like</span>
<span class="token keyword">template</span><span class="token operator">&lt;</span> <span class="token keyword">class</span> <span class="token class-name">T</span> <span class="token operator">&gt;</span>
<span class="token keyword">using</span> function_ref <span class="token operator">=</span> <span class="token keyword">const</span> function_ptr<span class="token operator">&lt;</span>T<span class="token operator">&gt;</span><span class="token punctuation">;</span>
</code></pre><p><span>So what do these minor changes buy us.</span></p><ol>
<li><span>By deleting the address of operators, we ensure proper usage of the reference type. Reference types are meant to be passed by copy not by reference. This removes additional avenues of dangling. This is consistent with references in general. If we retained these operators than they should return the address of the referent instead of the address of the reference type.</span></li>
<li><span>Lvalue references are immutable. "A reference is similar to a const pointer such as int* const p (as opposed to a pointer to const such as const int* p)" </span><sup class="footnote-ref"><a href="#fn1" id="fnref1">[1]</a></sup><span> References and constant pointers are easier to reason about dangling. Since references are not reseatable and constant pointers are not by default reseatable, they are just aliases to the referent that they point to. This means at compile time, once they point to a global, local or temporary, they always point to said global, local or temporary. Further since the constness is baked in, a programmer is less likely to forget to add it.</span></li>
</ol><pre><code class="cpp hljs">
<span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
    function_ptr<span class="token operator">&lt;</span><span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span> <span class="token keyword">noexcept</span><span class="token operator">&gt;</span> fp<span class="token punctuation">{</span><span class="token keyword">nullptr</span><span class="token punctuation">,</span>
        <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token operator">*</span> state<span class="token punctuation">,</span> <span class="token keyword">int</span> i<span class="token punctuation">)</span> <span class="token keyword">noexcept</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token function">test</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    fp <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token keyword">nullptr</span><span class="token punctuation">,</span> <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token operator">*</span> state<span class="token punctuation">,</span> <span class="token keyword">int</span> i<span class="token punctuation">)</span> <span class="token keyword">noexcept</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token function">test</span><span class="token punctuation">(</span>i <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    function_ref<span class="token operator">&lt;</span><span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span> <span class="token keyword">noexcept</span><span class="token operator">&gt;</span> fr<span class="token punctuation">{</span><span class="token keyword">nullptr</span><span class="token punctuation">,</span>
        <span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token operator">*</span> state<span class="token punctuation">,</span> <span class="token keyword">int</span> i<span class="token punctuation">)</span> <span class="token keyword">noexcept</span> <span class="token punctuation">{</span> <span class="token keyword">return</span> <span class="token function">test</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">}</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token comment">//auto a = &amp;fr;// deleted because function_ref is a reference type</span>
    <span class="token comment">// &amp; &amp; &amp; &amp; &amp; = &amp;</span>
    <span class="token comment">// shouldn't reassign function_ref to make it more like ref</span>
    <span class="token comment">//fr = {nullptr, [](void* state, int i) noexcept { return test(i + 1); }};</span>
    <span class="token comment">// NOTE: function_ptr can still be created from function_ref</span>
    <span class="token comment">// ensuring its use in collections</span>
    fp <span class="token operator">=</span> fr<span class="token punctuation">;</span>
    std<span class="token double-colon punctuation">::</span>vector<span class="token operator">&lt;</span>function_ptr<span class="token operator">&lt;</span><span class="token keyword">int</span> <span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span> <span class="token keyword">noexcept</span><span class="token operator">&gt;&gt;</span> fps<span class="token punctuation">;</span>
    <span class="token comment">//std::vector&lt;function_ref&lt;int (int) noexcept&gt;&gt; frs;// correct compiler error</span>

    <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre><h3 id="optional" data-id="optional"><a class="anchor hidden-xs" href="#optional" title="optional"><span class="octicon octicon-link"></span></a><span>optional</span></h3><p><span>It has been proposed to allow </span><code>std::optional</code><span> to support being a pure reference type: </span><code>std::optional&lt;T&amp;&gt;</code><span> </span><sup class="footnote-ref"><a href="#fn2" id="fnref2">[2]</a></sup><span>. While one must pause to decide whether this specialization should have its address of operators deleted since </span><code>std::optional</code><span> is an existing type, there shouldn't be any impediment to adding one alias to bake constness in as a convenience. This is especially important as there are plenty of methods in the standard library that current return a reference with undefined behavior which would be better served by returning a </span><code>const optional&lt;T&amp;&gt;</code><span> with defined behavior. Making sure to add </span><code>const</code><span> to our </span><code>optional&lt;T&amp;&gt;</code><span> would also be imporatant for our future proposals such as </span><code>Better lookups for map and unordered_map</code><span> </span><sup class="footnote-ref"><a href="#fn3" id="fnref3">[3]</a></sup><span>.</span></p><h4 id="given1" data-id="given"><a class="anchor hidden-xs" href="#given1" title="given1"><span class="octicon octicon-link"></span></a><span>given</span></h4><pre><code class="cpp hljs"><span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string">&lt;vector&gt;</span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string">&lt;optional&gt;</span><span class="token comment">// plus optional&lt;&amp;&gt;</span></span>

<span class="token comment">// const baked in by default</span>
<span class="token keyword">template</span><span class="token operator">&lt;</span><span class="token keyword">class</span> <span class="token class-name">T</span><span class="token operator">&gt;</span>
<span class="token keyword">using</span> optional_ref <span class="token operator">=</span> <span class="token keyword">const</span> optional<span class="token operator">&lt;</span>T<span class="token operator">&amp;</span><span class="token operator">&gt;</span><span class="token punctuation">;</span><span class="token comment">// OR opt_ref</span>
</code></pre><p><span>Here the proposed </span><code>std::optional&lt;T&amp;&gt;</code><span> </span><sup class="footnote-ref"><a href="#fn2" id="fnref2:1">[2:1]</a></sup><span> behaves correctly like a pointer with a constant pointer able to be implicitly converted to a non constant pointer.</span></p><table>
<tbody><tr>
<td>
<pre><code class="cpp hljs"><span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
    <span class="token keyword">int</span> value <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    <span class="token keyword">int</span> <span class="token operator">*</span> p00 <span class="token operator">=</span> <span class="token operator">&amp;</span>value<span class="token punctuation">;</span>
    <span class="token keyword">int</span> <span class="token keyword">const</span> <span class="token operator">*</span> p10 <span class="token operator">=</span> <span class="token operator">&amp;</span>value<span class="token punctuation">;</span>
    <span class="token keyword">int</span> <span class="token operator">*</span> <span class="token keyword">const</span> p01 <span class="token operator">=</span> <span class="token operator">&amp;</span>value<span class="token punctuation">;</span>
    <span class="token keyword">int</span> <span class="token keyword">const</span> <span class="token operator">*</span> <span class="token keyword">const</span> p11 <span class="token operator">=</span> <span class="token operator">&amp;</span>value<span class="token punctuation">;</span>
    p00 <span class="token operator">=</span> p00<span class="token punctuation">;</span>
    <span class="token comment">//p00 = p10;// error</span>
    p00 <span class="token operator">=</span> p01<span class="token punctuation">;</span>
    <span class="token comment">//p00 = p11;// error</span>
    p10 <span class="token operator">=</span> p00<span class="token punctuation">;</span>
    p10 <span class="token operator">=</span> p10<span class="token punctuation">;</span>
    p10 <span class="token operator">=</span> p01<span class="token punctuation">;</span>
    p10 <span class="token operator">=</span> p11<span class="token punctuation">;</span>
    <span class="token comment">//p01 = p00;// error</span>
    <span class="token comment">//p01 = p10;// error</span>
    <span class="token comment">//p01 = p01;// error</span>
    <span class="token comment">//p01 = p11;// error</span>
    <span class="token comment">//p11 = p00;// error</span>
    <span class="token comment">//p11 = p10;// error</span>
    <span class="token comment">//p11 = p01;// error</span>
    <span class="token comment">//p11 = p11;// error</span>
    <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre>
</td>
<td>
<pre><code class="cpp hljs"><span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
    <span class="token keyword">int</span> value <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    optional<span class="token operator">&lt;</span><span class="token keyword">int</span><span class="token operator">&amp;</span><span class="token operator">&gt;</span> op00<span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    optional<span class="token operator">&lt;</span><span class="token keyword">const</span> <span class="token keyword">int</span><span class="token operator">&amp;</span><span class="token operator">&gt;</span> op10<span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token keyword">const</span> optional<span class="token operator">&lt;</span><span class="token keyword">int</span><span class="token operator">&amp;</span><span class="token operator">&gt;</span> op01<span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token keyword">const</span> optional<span class="token operator">&lt;</span><span class="token keyword">const</span> <span class="token keyword">int</span><span class="token operator">&amp;</span><span class="token operator">&gt;</span> op11<span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    op00 <span class="token operator">=</span> op00<span class="token punctuation">;</span>
    <span class="token comment">//op00 = op10;// error</span>
    op00 <span class="token operator">=</span> op01<span class="token punctuation">;</span>
    <span class="token comment">//op00 = op11;// error</span>
    op10 <span class="token operator">=</span> op00<span class="token punctuation">;</span>
    op10 <span class="token operator">=</span> op10<span class="token punctuation">;</span>
    op10 <span class="token operator">=</span> op01<span class="token punctuation">;</span>
    op10 <span class="token operator">=</span> op11<span class="token punctuation">;</span>
    <span class="token comment">//op01 = op00;// error</span>
    <span class="token comment">//op01 = op10;// error</span>
    <span class="token comment">//op01 = op01;// error</span>
    <span class="token comment">//op01 = op11;// error</span>
    <span class="token comment">//op11 = op00;// error</span>
    <span class="token comment">//op11 = op10;// error</span>
    <span class="token comment">//op11 = op01;// error</span>
    <span class="token comment">//op11 = op11;// error</span>
    <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre>
</td>
</tr>
</tbody></table><p><span>The </span><code>optional_ref</code><span> alias similary behaves correctly but this time </span><code>const</code><span> is baked in by default.</span></p><table>
<tbody><tr>
<td>
<pre><code class="cpp hljs"><span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
    <span class="token keyword">int</span> value <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    optional<span class="token operator">&lt;</span><span class="token keyword">int</span><span class="token operator">&amp;</span><span class="token operator">&gt;</span> op00<span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    optional<span class="token operator">&lt;</span><span class="token keyword">const</span> <span class="token keyword">int</span><span class="token operator">&amp;</span><span class="token operator">&gt;</span> op10<span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token keyword">const</span> optional<span class="token operator">&lt;</span><span class="token keyword">int</span><span class="token operator">&amp;</span><span class="token operator">&gt;</span> op01<span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    <span class="token keyword">const</span> optional<span class="token operator">&lt;</span><span class="token keyword">const</span> <span class="token keyword">int</span><span class="token operator">&amp;</span><span class="token operator">&gt;</span> op11<span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    op00 <span class="token operator">=</span> op00<span class="token punctuation">;</span>
    <span class="token comment">//op00 = op10;// error</span>
    op00 <span class="token operator">=</span> op01<span class="token punctuation">;</span>
    <span class="token comment">//op00 = op11;// error</span>
    op10 <span class="token operator">=</span> op00<span class="token punctuation">;</span>
    op10 <span class="token operator">=</span> op10<span class="token punctuation">;</span>
    op10 <span class="token operator">=</span> op01<span class="token punctuation">;</span>
    op10 <span class="token operator">=</span> op11<span class="token punctuation">;</span>
    <span class="token comment">//op01 = op00;// error</span>
    <span class="token comment">//op01 = op10;// error</span>
    <span class="token comment">//op01 = op01;// error</span>
    <span class="token comment">//op01 = op11;// error</span>
    <span class="token comment">//op11 = op00;// error</span>
    <span class="token comment">//op11 = op10;// error</span>
    <span class="token comment">//op11 = op01;// error</span>
    <span class="token comment">//op11 = op11;// error</span>
    <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre>
</td>
<td>
<pre><code class="cpp hljs"><span class="token keyword">int</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">{</span>
    <span class="token keyword">int</span> value <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    optional<span class="token operator">&lt;</span><span class="token keyword">int</span><span class="token operator">&amp;</span><span class="token operator">&gt;</span> op00<span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    optional<span class="token operator">&lt;</span><span class="token keyword">const</span> <span class="token keyword">int</span><span class="token operator">&amp;</span><span class="token operator">&gt;</span> op10<span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    optional_ref<span class="token operator">&lt;</span><span class="token keyword">int</span><span class="token operator">&gt;</span> op01<span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    optional_ref<span class="token operator">&lt;</span><span class="token keyword">const</span> <span class="token keyword">int</span><span class="token operator">&gt;</span> op11<span class="token punctuation">{</span>value<span class="token punctuation">}</span><span class="token punctuation">;</span>
    op00 <span class="token operator">=</span> op00<span class="token punctuation">;</span>
    <span class="token comment">//op00 = op10;// error</span>
    op00 <span class="token operator">=</span> op01<span class="token punctuation">;</span>
    <span class="token comment">//op00 = op11;// error</span>
    op10 <span class="token operator">=</span> op00<span class="token punctuation">;</span>
    op10 <span class="token operator">=</span> op10<span class="token punctuation">;</span>
    op10 <span class="token operator">=</span> op01<span class="token punctuation">;</span>
    op10 <span class="token operator">=</span> op11<span class="token punctuation">;</span>
    <span class="token comment">//op01 = op00;// error</span>
    <span class="token comment">//op01 = op10;// error</span>
    <span class="token comment">//op01 = op01;// error</span>
    <span class="token comment">//op01 = op11;// error</span>
    <span class="token comment">//op11 = op00;// error</span>
    <span class="token comment">//op11 = op10;// error</span>
    <span class="token comment">//op11 = op01;// error</span>
    <span class="token comment">//op11 = op11;// error</span>
    <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre>
</td>
</tr>
</tbody></table><h3 id="Where-else-and-where-not" data-id="Where-else-and-where-not"><a class="anchor hidden-xs" href="#Where-else-and-where-not" title="Where-else-and-where-not"><span class="octicon octicon-link"></span></a><span>Where else and where not</span></h3><p><span>Similar changes could be performed on existing pure reference types in the standard library such as </span><code>std::span</code><span>, </span><code>std::string_view</code><span> and </span><code>std::mdspan</code><span>. These changes would not be recommended for hybrid reference types such as </span><code>std::tuple</code><span> and </span><code>std::variant</code><span> should reference support be added.</span></p><h3 id="Language-feature" data-id="Language-feature"><a class="anchor hidden-xs" href="#Language-feature" title="Language-feature"><span class="octicon octicon-link"></span></a><span>Language feature?</span></h3><p><span>This could better be implemented as a language feature. I would be glad to write such a proposal, if there was sufficient interest.</span></p><pre><code class="cpp hljs"><span class="token punctuation">[</span><span class="token punctuation">[</span>const_by_default<span class="token punctuation">]</span><span class="token punctuation">]</span>
<span class="token keyword">class</span> <span class="token class-name">function_ref</span><span class="token punctuation">;</span>

<span class="token comment">// usage</span>
function_ref fr1<span class="token punctuation">;</span><span class="token comment">// const by default</span>
<span class="token keyword">mutable</span> function_ref fr2<span class="token punctuation">;</span><span class="token comment">// const has been disengaged</span>
</code></pre><h2 id="Summary" data-id="Summary"><a class="anchor hidden-xs" href="#Summary" title="Summary"><span class="octicon octicon-link"></span></a><span>Summary</span></h2><p><span>Deleting address of operators from pure reference types minimizes the misuse of said types and reduces the avenues to dangling. Ensuring pure reference types are immutable by default makes it easier for programmers to reason about the lifetimes of the referents that these reference type instances points to.</span></p><h2 id="References" data-id="References"><a class="anchor hidden-xs" href="#References" title="References"><span class="octicon octicon-link"></span></a><span>References</span></h2><hr class="footnotes-sep"><section class="footnotes">
<ol class="footnotes-list">
<li id="fn1" class="footnote-item"><p><a href="https://isocpp.org/wiki/faq/references#reseating-refs" target="_blank" rel="noopener"><span>https://isocpp.org/wiki/faq/references#reseating-refs</span></a> <a href="#fnref1" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn2" class="footnote-item"><p><a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2988r3.pdf" target="_blank" rel="noopener"><span>https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2988r3.pdf</span></a> <a href="#fnref2" class="footnote-backref">↩︎</a> <a href="#fnref2:1" class="footnote-backref">↩︎</a></p>
</li>
<li id="fn3" class="footnote-item"><p><a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3091r0.html" target="_blank" rel="noopener"><span>https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p3091r0.html</span></a> <a href="#fnref3" class="footnote-backref">↩︎</a></p>
</li>
</ol>
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</body>

</html>
