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Add docs for C++11 ref-qualifiers

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William S Fulton 2017-08-25 22:52:56 +01:00
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@ -42,6 +42,7 @@
<li><a href="#CPlusPlus11_noexcept">Exception specifications and noexcept</a>
<li><a href="#CPlusPlus11_alignment">Control and query object alignment</a>
<li><a href="#CPlusPlus11_attributes">Attributes</a>
<li><a href="#CPlusPlus11_ref_qualifiers">Methods with ref-qualifiers</a>
</ul>
<li><a href="#CPlusPlus11_standard_library_changes">Standard library changes</a>
<ul>
@ -971,6 +972,104 @@ int [[attr1]] i [[attr2, attr3]];
[[noreturn, nothrow]] void f [[noreturn]] ();
</pre></div>
<H3><a name="CPlusPlus11_ref_qualifiers">7.2.29 Methods with ref-qualifiers</a></H3>
<p>
C++11 non-static member functions can be declared with ref-qualifiers.
Member functions declared with a <tt>&amp;</tt> lvalue ref-qualifiers are wrapped like any other function without ref-qualifiers.
Member functions declared with a <tt>&amp;&amp;</tt> rvalue ref-qualifiers are ignored by default
as they are unlikely to be required from non-C++ languages where the concept of <i>rvalue-ness</i>
for the implied *this pointer does not apply.
The warning is hidden by default, but can be displayed as described in the section on <a href="Warnings.html#Warnings_nn4">Enabling extra warnings</a>.
</p>
<p>
Consider:
</p>
<div class="code"><pre>
struct RQ {
void m1(int x) &amp;;
void m2(int x) &amp;&amp;;
};
</pre></div>
<p>
The only wrapped method will be the lvalue ref-qualified method <tt>m1</tt>
and if SWIG is run with the <tt>-Wextra</tt> command-line option, the following warning will be issued indicating <tt>m2</tt> is not wrapped:
</p>
<div class="shell">
<pre>
example.i:7: Warning 405: Method with rvalue ref-qualifier m2(int) &amp;&amp; ignored.
</pre>
</div>
<p>
If you unignore the method as follows, wrappers for <tt>m2</tt> will be generated:
</p>
<div class="code"><pre>
%feature("ignore", "0") RQ::m2(int x) &amp;&amp;;
struct RQ {
void m1(int x) &amp;;
void m2(int x) &amp;&amp;;
};
</pre></div>
<p>
Inspection of the generated C++ code, will show that <tt>std::move</tt> is used on the instance
of the <tt>RQ *</tt> class:
</p>
<div class="code"><pre>
RQ *arg1 = (RQ *) 0 ;
int arg2 ;
arg1 = ...marshalled from target language...
arg2 = ...marshalled from target language...
std::move(*arg1).m2(arg2);
</pre></div>
<p>
This will compile but when run, the move effects may not be what you want.
As stated earlier, rvalue ref-qualifiers aren't really applicable outside the world of C++.
However, if you really know what you are doing, full control over the call to the method is
possible via the low-level "action" feature.
This feature completely replaces the call to the underlying function, that is, the last line in the snippet of code above.
</p>
<div class="code"><pre>
%feature("ignore", "0") RQ::m2(int x) &amp;&amp;;
%feature("action") RQ::m2(int x) &amp;&amp; %{
RQ().m2(arg2);
%}
struct RQ {
void m1(int x) &amp;;
void m2(int x) &amp;&amp;;
};
</pre></div>
<p>
resulting in:
</p>
<div class="code"><pre>
RQ *arg1 = (RQ *) 0 ;
int arg2 ;
arg1 = ...marshalled from target language...
arg2 = ...marshalled from target language...
RQ().m2(arg2);
</pre></div>
<p>
<b>Compatibility note:</b> SWIG-4.0.0 was the first version to support ref-qualifiers.
</p>
<H2><a name="CPlusPlus11_standard_library_changes">7.3 Standard library changes</a></H2>
@ -1177,5 +1276,6 @@ Phew, that is a lot of hard work to get a callback working.
You could just go with the more attractive option of just using <tt>double</tt> as the return type in the function declaration instead of <tt>result_of</tt>!
</p>
</body>
</html>