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<title>Issue 297: const_mem_fun_t&lt;&gt;::argument_type should be const T*</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#CD1">CD1</a> status.</em></p>
<h3 id="297"><a href="lwg-defects.html#297">297</a>. const_mem_fun_t&lt;&gt;::argument_type should be const T*</h3>
<p><b>Section:</b> 22.10.10 <a href="https://wg21.link/logical.operations">[logical.operations]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Martin Sebor <b>Opened:</b> 2001-01-06 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>The class templates <code>const_mem_fun_t</code> in 20.5.8, p8 and
<code>const_mem_fun1_t</code>
in 20.5.8, p9 derive from <code>unary_function&lt;T*, S></code>, and
<code>binary_function&lt;T*,
A, S></code>, respectively. Consequently, their <code>argument_type</code>, and
<code>first_argument_type</code>
members, respectively, are both defined to be <code>T*</code> (non-const).
However, their function call member operator takes a <code>const T*</code>
argument. It is my opinion that <code>argument_type</code> should be <code>const
T*</code> instead, so that one can easily refer to it in generic code. The
example below derived from existing code fails to compile due to the
discrepancy:
</p>

<p><code>template &lt;class T></code>
<br/><code>void foo (typename T::argument_type arg)&nbsp;&nbsp; // #1</code>
<br/><code>{</code>
<br/><code>&nbsp;&nbsp;&nbsp; typename T::result_type (T::*pf) (typename
T::argument_type)
const =&nbsp;&nbsp; // #2</code>
<br/><code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &amp;T::operator();</code>
<br/><code>}</code>
</p>

<p><code>struct X { /* ... */ };</code></p>

<p><code>int main ()</code>
<br/><code>{</code>
<br/><code>&nbsp;&nbsp;&nbsp; const X x;</code>
<br/><code>&nbsp;&nbsp;&nbsp; foo&lt;std::const_mem_fun_t&lt;void, X>
>(&amp;x);&nbsp;&nbsp;
// #3</code>
<br/><code>}</code>
</p>

<p>#1 <code>foo()</code> takes a plain unqualified <code>X*</code> as an argument
<br/>#2 the type of the pointer is incompatible with the type of the member
function
<br/>#3 the address of a constant being passed to a function taking a non-const
<code>X*</code>
</p>


<p id="res-297"><b>Proposed resolution:</b></p>
<p>Replace the top portion of the definition of the class template
const_mem_fun_t in 20.5.8, p8
</p>
<p><code>template &lt;class S, class T> class const_mem_fun_t</code>
<br/><code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; : public
unary_function&lt;T*, S> {</code>
</p>
<p>with</p>
<p><code>template &lt;class S, class T> class const_mem_fun_t</code>
<br/><code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; : public
unary_function&lt;<b>const</b> T*, S> {</code>
</p>
<p>Also replace the top portion of the definition of the class template
const_mem_fun1_t in 20.5.8, p9</p>
<p><code>template &lt;class S, class T, class A> class const_mem_fun1_t</code>
<br/><code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; : public
binary_function&lt;T*, A, S> {</code>
</p>
<p>with</p>
<p><code>template &lt;class S, class T, class A> class const_mem_fun1_t</code>
<br/><code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; : public
binary_function&lt;<b>const</b> T*, A, S> {</code>
</p>


<p><b>Rationale:</b></p>
<p>This is simply a contradiction: the <code>argument_type</code> typedef,
and the argument type itself, are not the same.</p>





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