Topic: Copy elision and replacing copy by move in
Author: Mikhail Semenov <mikhailsemenov1957@gmail.com>
Date: Sun, 5 Jan 2014 05:31:44 -0800 (PST)
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In the following cases it is natural to allow copy elision to take place:
class A
{
double *v;
std::size_t n;
public:
.. . .
A operator+(A x) const &
{
for (std::size_t i = 0; i < n; i++)
{
x.v[i] += v[i];
}
return x; // copy elision will take place here
}
A operator+(const A& x) && // rvalue-ref-qualified
{
for (std::size_t i = 0; i < n; i++)
{
v[i] += x.v[i];
}
return std::move(*this); // At present, std::move is
required here in order to get the desired effect. But it seems that return
*this; should be enough.
}
.. . .
};
There are two sides for this proposal:
-- when a member function returns a whole object of its class then a copy
elision can take place;
-- when a member function returns a member variable, then copy can be
replaced with move.
Here is another example:
*class P*
*{*
* S r;*
* . . .*
*public:*
* P();*
* . . .*
* S f() &&*
* {*
* . . .*
* return a.r; // copy is replaced by move*
* }*
*};*
*S w = P().f();*
In this case, the value *a.r* can be moved into *w*, not copied.
This proposal is available here:
*https://dl.dropboxusercontent.com/u/35715999/copy_elision_for_r_value_qualified_members.htm*<https://dl.dropboxusercontent.com/u/35715999/copy_elision_for_r_value_qualified_members.htm>
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<div dir=3D"ltr"><div> </div><div>In the following cases it is natural=
to allow copy elision to take place: </div><div> </div><div>class A<b=
r> {<br> double *v;<br> s=
td::size_t n; <br><br><br> public:<br> . . .<br> &n=
bsp; A operator+(A x) const &<br> &=
nbsp; {<br> &nb=
sp; for (std::size_t i =3D 0; i < n;=
i++)<br>  =
; {<br> &=
nbsp; &nbs=
p; x.v[i] +=3D v[i];<br> &n=
bsp; }<br> &nbs=
p; &=
nbsp; return x; // copy elision will take place here<br> =
}<br><br> &nbs=
p; <font color=3D"#0000ff">A operator+(const A& x) && // =
rvalue-ref-qualified<br> {=
<br> &nb=
sp; for (std::size_t i =3D 0; i < n; i++)<br> &n=
bsp;  =
; {<br> &=
nbsp; &nbs=
p; v[i] +=3D x.v[i];<br> &n=
bsp; }<br> &nbs=
p; &=
nbsp; return std::move(*this); // At present, std::move is requ=
ired here in order to get the desired effect. But it seems that <font=
color=3D"#274e13">return *this;</font> should be enough.<br> &=
nbsp; }</font><br> . . .<br> }; </div><d=
iv> </div><div><font color=3D"#000000" face=3D"Times New Roman" size=
=3D"3">
</font></div><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 8pt;"><font si=
ze=3D"3"><font color=3D"#000000"><font face=3D"Times New Roman">There are t=
wo sides for this proposal: </font></font></font></p><p class=3D"MsoNormal"=
style=3D"margin: 0cm 0cm 8pt;"><font size=3D"3"><font color=3D"#000000"><f=
ont face=3D"Times New Roman">-- when a member function returns a whole obje=
ct of its
class then a copy elision can take place;<?xml:namespace prefix =3D "o" ns =
=3D "urn:schemas-microsoft-com:office:office" /><o:p></o:p></font></font></=
font></p><div><font color=3D"#000000" face=3D"Times New Roman" size=3D"3">
</font></div><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 8pt;"><font si=
ze=3D"3"><font color=3D"#000000"><font face=3D"Times New Roman">-- when a m=
ember function returns a member variable, then
copy can be replaced with move.<o:p></o:p></font></font></font></p><div><fo=
nt color=3D"#000000" face=3D"Times New Roman" size=3D"3">
</font></div><div><font color=3D"#000000" face=3D"Times New Roman" size=3D"=
3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 8pt;"><font size=3D"=
3"><font color=3D"#000000"><font face=3D"Times New Roman">Here is another&n=
bsp;example:<o:p></o:p></font></font></font></p><font color=3D"#000000" fac=
e=3D"Times New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>class P</span></b><o:p></o:p></font></p><font color=3D"=
#000000" face=3D"Times New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>{</span></b><o:p></o:p></font></p><font color=3D"#00000=
0" face=3D"Times New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>
S r;</span></b><o:p></o:p></font></p><font color=3D"#000000" face=3D"Times =
New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>
.. . .</span></b><o:p></o:p></font></p><font color=3D"#000000" face=3D"Times=
New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>public:</span></b><o:p></o:p></font></p><font color=3D"=
#000000" face=3D"Times New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>
P();</span></b><o:p></o:p></font></p><font color=3D"#000000" face=3D"Times =
New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>
.. . .</span></b><o:p></o:p></font></p><font color=3D"#000000" face=3D"Times=
New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>
S f() &&</span></b><o:p></o:p></font></p><font color=3D"#000000" fa=
ce=3D"Times New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>
{</span></b><o:p></o:p></font></p><font color=3D"#000000" face=3D"Times New=
Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>
.. . .</span></b><o:p></o:p></font></p><font color=3D"#000000" face=3D"Times=
New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><b><span style=3D'font-family: "Courier New"; font-size: 11pt;'><fon=
t color=3D"#000000">
</font><span style=3D"background: black; color: white;">return =
a.r</span><font color=3D"#000000">; //
copy is replaced by move</font></span></b><o:p></o:p></p><font color=3D"#00=
0000" face=3D"Times New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>
}</span></b><o:p></o:p></font></p><font color=3D"#000000" face=3D"Times New=
Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>};</span></b><o:p></o:p></font></p><font color=3D"#0000=
00" face=3D"Times New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'> </span></b><o:p></o:p></font></p><font color=3D"#=
000000" face=3D"Times New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font color=3D"#000000"><b><span style=3D'font-family: "Courier New"=
; font-size: 11pt;'>S w =3D
P().f();</span></b><o:p></o:p></font></p><font color=3D"#000000" face=3D"Ti=
mes New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font size=3D"3"><font color=3D"#000000"><font face=3D"Times New Rom=
an"> <o:p></o:p></font></font></font></p><font color=3D"#000000" face=
=3D"Times New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 0pt; line-height: no=
rmal;"><font size=3D"3"><font color=3D"#000000"><font face=3D"Times New Rom=
an">In this case, the value <b>a.r</b> can be moved into <b>w</b>, not
copied.</font></font></font></p><p class=3D"MsoNormal" style=3D"margin: 0cm=
0cm 0pt; line-height: normal;"><font size=3D"3"><font color=3D"#000000"><f=
ont face=3D"Times New Roman"></font></font></font> </p><p class=3D"Mso=
Normal" style=3D"margin: 0cm 0cm 0pt; line-height: normal;"><font size=3D"3=
"><font color=3D"#000000"><font face=3D"Times New Roman">This proposal is a=
vailable here:</font></font></font></p><p class=3D"MsoNormal" style=3D"marg=
in: 0cm 0cm 0pt; line-height: normal;"><a onmousedown=3D"this.href=3D'https=
://www.google.com/url?q\75https%3A%2F%2Fdl.dropboxusercontent.com%2Fu%2F357=
15999%2Fcopy_elision_for_r_value_qualified_members.htm\46sa\75D\46sntz\0751=
\46usg\75AFQjCNHazxh1EhnflBbFqlpJPeuiVbyBFw';return true;" onclick=3D"this.=
href=3D'https://www.google.com/url?q\75https%3A%2F%2Fdl.dropboxusercontent.=
com%2Fu%2F35715999%2Fcopy_elision_for_r_value_qualified_members.htm\46sa\75=
D\46sntz\0751\46usg\75AFQjCNHazxh1EhnflBbFqlpJPeuiVbyBFw';return true;" hre=
f=3D"https://dl.dropboxusercontent.com/u/35715999/copy_elision_for_r_value_=
qualified_members.htm" target=3D"_blank"><u><font color=3D"#0000ff">https:/=
/dl.dropboxusercontent.<wbr>com/u/35715999/copy_elision_<wbr>for_r_value_qu=
alified_members.<wbr>htm</font></u></a></p><p class=3D"MsoNormal" style=3D"=
margin: 0cm 0cm 0pt; line-height: normal;"> </p><font color=3D"#000000=
" face=3D"Times New Roman" size=3D"3">
</font><p class=3D"MsoNormal" style=3D"margin: 0cm 0cm 8pt;"><font size=3D"=
3"><font color=3D"#000000"><font face=3D"Times New Roman"> <o:p></o:p>=
</font></font></font></p><font color=3D"#000000" face=3D"Times New Roman" s=
ize=3D"3">
</font></div></div>
<p></p>
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