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Hello,

There are many old APIs that are using double pointer as argument to return
an object. Wide usage examples are WinAPI, SDL, etc.
Currently you have to use one more temporary variable to pass it and then
assign it to std::unique_ptr.

*// Some utility header file...*
*struct ComCloser*
*{*
* constexpr ComCloser() noexcept = default;*
* void operator()( IUnknown* comObj ) noexcept*
* {*
* comObj->Release();*
* }*
*};*

*template<typename T>*
*using ComResource = std::unique_ptr<T, ComCloser>;*

*// Some cpp file*
*IWbemLocator* pLoc = nullptr;*
*HRESULT hres = CoCreateInstance( CLSID_WbemLocator, 0,
CLSCTX_INPROC_SERVER, IID_IWbemLocator, (LPVOID*)&pLoc );*
*_com_util::CheckError( hres ); // will throw exception on error*

*ComResource<IWbemLocator> locator( pLoc );*

By adding
*pointer* operator&()*
*{*

* reset();*
* return &_ptr;*
*}*

We can omit most of the cpp file code to become like
*// Some cpp file*
*ComResource<IWbemLocator> locator;*
*HRESULT hres = ::CoCreateInstance( CLSID_WbemLocator, 0,
CLSCTX_INPROC_SERVER, IID_IWbemLocator, (LPVOID*)&locator );*
*_com_util::CheckError( hres );*

Of course when using operator&() if there is any previous pointer it will
be destroyed by reset(); as it can be seen from my example.
Please provide feedback and suggestions. :)

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<div dir=3D"ltr">Hello,<div><br></div><div>There are many old APIs that are=
 using double pointer as argument to return an object. Wide usage examples =
are WinAPI, SDL, etc.</div><div>Currently you have to use one more temporar=
y variable to pass it and then assign it to std::unique_ptr.</div><div><br>=
</div><div><div><i>// Some utility header file...</i></div><div><i>struct C=
omCloser</i></div><div><i>{</i></div><div><i><span style=3D"white-space:pre=
"> </span>constexpr ComCloser() noexcept =3D default;</i></div><div><i><spa=
n style=3D"white-space:pre"> </span>void operator()( IUnknown* comObj ) noe=
xcept</i></div><div><i><span style=3D"white-space:pre"> </span>{</i></div><=
div><i><span style=3D"white-space:pre">  </span>comObj-&gt;Release();</i></=
div><div><i><span style=3D"white-space:pre"> </span>}</i></div><div><i>};</=
i></div><div><i><br></i></div><div><i>template&lt;typename T&gt;</i></div><=
div><i>using ComResource =3D std::unique_ptr&lt;T, ComCloser&gt;;</i></div>=
<div><i><br></i></div><div><i>// Some cpp file</i></div><div><i>IWbemLocato=
r* pLoc =3D nullptr;</i></div><div><i>HRESULT hres =3D CoCreateInstance( CL=
SID_WbemLocator, 0, CLSCTX_INPROC_SERVER, IID_IWbemLocator, (LPVOID*)&amp;p=
Loc );</i></div><div><i>_com_util::CheckError( hres ); // will throw except=
ion on error</i></div><div><i><br></i></div><div><i>ComResource&lt;IWbemLoc=
ator&gt; locator( pLoc );</i></div></div><div><br></div><div>By adding</div=
><div><div><i>pointer* operator&amp;()</i></div><div><i>{</i></div><div><i>=
<span style=3D"white-space: pre;"> </span>reset();<br></i></div><div><i><sp=
an style=3D"white-space:pre"> </span>return &amp;_ptr;</i></div><div><i>}</=
i></div></div><div><br></div><div>We can omit most of the cpp file code to =
become like</div><div><div><i>// Some cpp file</i></div><div><i>ComResource=
&lt;IWbemLocator&gt; locator;</i></div><div><i>HRESULT hres =3D ::CoCreateI=
nstance( CLSID_WbemLocator, 0, CLSCTX_INPROC_SERVER, IID_IWbemLocator, (LPV=
OID*)&amp;locator );</i></div><div><i>_com_util::CheckError( hres );</i></d=
iv></div><div><br></div><div>Of course when using operator&amp;() if there =
is any previous pointer it will be destroyed by reset(); as it can be seen =
from my example.</div><div>Please provide feedback and suggestions. :)</div=
></div>

<p></p>

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40isocpp.org?utm_medium=3Demail&utm_source=3Dfooter">https://groups.google.=
com/a/isocpp.org/d/msgid/std-proposals/5198c390-26d7-4750-8065-eb6fe653b86c=
%40isocpp.org</a>.<br />

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