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*Current status*
Copy elision is guaranteed when a prvalue is returned from a function:

T foo() {
    return T();
}

Additionally, the compiler may perform copy elision if the object is named
in the function:
T foo(){
    T myFoo{};
    return myFoo; //Maybe performs copy elision
}

*Proposal*
Guarantee copy elision of object x returned from a function if:

   - x is not static
   - x is not declared outside the body of the function
   - the address of or a reference to x is not exported* from the function
   (*I will define this word below)
   - x is a complete object


struct A {
    A* ptr;
    A() { ptr = this; }
}
A foo(){
    A a {};
    //Address is exported; copy elision is no longer guaranteed
    return a;
}

T* GLOBAL_T_PTR;
T foo() {
    T myFoo();
    GLOBAL_T_PTR = &myFoo; //Address is exported; copy elision is no longer
guaranteed
    return myFoo;
}

struct A {
};
struct B {
    A& ref;
    B(A& _ref) : ref(_ref) {}
};
A foo(B* &ptr){
    A val;
    ptr = new B(val);
    return val; //Reference is exported; copy elision is no longer
guaranteed
}

struct A;
A* GLOBAL_A_PTR;
struct A {
    A() {}
    A(const A& val) { GLOBAL_A_PTR = this; }
};
A foo(){
    A a;
    return a; //address of copy-constructed object would have been
exported; copy elision is no longer guaranteed
}



*Definition of export*
A function exports an address or reference if that address or reference is
(potentially) stored externally as a result of a side-effect of the
function. If it is not known at compile time whether or not an export will
occur, copy elision is not guaranteed. If a reference or address is passed
to a function with external linkage, copy elision is not guaranteed. If a
address or reference is passed to a function whose definition is visible to
the compiler, then it is not exported if that function does not export it.



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<div dir=3D"ltr"><div><b>Current status</b></div>Copy elision is guaranteed=
 when a prvalue is returned from a function:<div><br></div><div><div class=
=3D"prettyprint" style=3D"border: 1px solid rgb(187, 187, 187); word-wrap: =
break-word;"><code class=3D"prettyprint"><div class=3D"subprettyprint" styl=
e=3D"caret-color: rgb(102, 0, 102);"><span style=3D"background-color: rgb(2=
55, 255, 255);"><span style=3D"color: #000;" class=3D"styled-by-prettify">T=
 foo</span><span style=3D"color: #660;" class=3D"styled-by-prettify">()</sp=
an><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span =
style=3D"color: #660;" class=3D"styled-by-prettify">{</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </span><span s=
tyle=3D"color: #008;" class=3D"styled-by-prettify">return</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"> T</span><span style=3D"colo=
r: #660;" class=3D"styled-by-prettify">();</span><span style=3D"color: #000=
;" class=3D"styled-by-prettify"><br></span><span style=3D"color: #660;" cla=
ss=3D"styled-by-prettify">}</span></span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br></span></div></code></div><br>Additionally, the=
 compiler may perform copy elision if the object is named in the function:<=
/div><div><div class=3D"prettyprint" style=3D"border: 1px solid rgb(187, 18=
7, 187); word-wrap: break-word;"><code class=3D"prettyprint"><div class=3D"=
subprettyprint" style=3D"caret-color: rgb(102, 0, 102);"><font color=3D"#66=
0066"><span style=3D"background-color: rgb(255, 255, 255);"><span style=3D"=
color: #000;" class=3D"styled-by-prettify">T foo</span><span style=3D"color=
: #660;" class=3D"styled-by-prettify">(){</span><span style=3D"color: #000;=
" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 T myFoo</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">{};</span><span style=3D"col=
or: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </span><span styl=
e=3D"color: #008;" class=3D"styled-by-prettify">return</span><span style=3D=
"color: #000;" class=3D"styled-by-prettify"> myFoo</span><span style=3D"col=
or: #660;" class=3D"styled-by-prettify">;</span><span style=3D"color: #000;=
" class=3D"styled-by-prettify"> </span><span style=3D"color: #800;" class=
=3D"styled-by-prettify">//Maybe performs copy elision</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"color:=
 #660;" class=3D"styled-by-prettify">}</span></span><span style=3D"color: #=
000;" class=3D"styled-by-prettify"><br></span></font></div></code></div><br=
><b>Proposal</b></div><div>Guarantee copy elision of object x<i>=C2=A0</i>r=
eturned from a function<i>=C2=A0</i>if:</div><div><ul><li>x is not static</=
li><li>x is not declared outside the body of the function</li><li>the addre=
ss of or a reference to x is not exported* from the function (*I will defin=
e this word below)</li><li>x is a complete object</li></ul></div><div><br><=
/div><div><div class=3D"prettyprint" style=3D"border: 1px solid rgb(187, 18=
7, 187); word-wrap: break-word;"><code class=3D"prettyprint"><div class=3D"=
subprettyprint" style=3D"background-color: rgb(255, 255, 255);"><span style=
=3D"color: #008;" class=3D"styled-by-prettify">struct</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify"> A </span><span style=3D"color: =
#660;" class=3D"styled-by-prettify">{</span><span style=3D"color: #000;" cl=
ass=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 A</span><span style=3D"color: =
#660;" class=3D"styled-by-prettify">*</span><span style=3D"color: #000;" cl=
ass=3D"styled-by-prettify"> ptr</span><span style=3D"color: #660;" class=3D=
"styled-by-prettify">;</span><span style=3D"color: #000;" class=3D"styled-b=
y-prettify"><br>=C2=A0 =C2=A0 A</span><span style=3D"color: #660;" class=3D=
"styled-by-prettify">()</span><span style=3D"color: #000;" class=3D"styled-=
by-prettify"> </span><font color=3D"#666600"><span style=3D"color: #660;" c=
lass=3D"styled-by-prettify">{</span><span style=3D"color: #000;" class=3D"s=
tyled-by-prettify"> ptr </span><span style=3D"color: #660;" class=3D"styled=
-by-prettify">=3D</span><span style=3D"color: #000;" class=3D"styled-by-pre=
ttify"> </span><span style=3D"color: #008;" class=3D"styled-by-prettify">th=
is</span><span style=3D"color: #660;" class=3D"styled-by-prettify">;</span>=
<span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span sty=
le=3D"color: #660;" class=3D"styled-by-prettify">}</span></font><span style=
=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">}</span><span style=3D"color: #000=
;" class=3D"styled-by-prettify"><br>A foo</span><span style=3D"color: #660;=
" class=3D"styled-by-prettify">(){</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 A a </span><span style=3D"color: =
#660;" class=3D"styled-by-prettify">{};</span><font><span style=3D"color: #=
000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </span><span style=3D"=
caret-color: rgb(102, 0, 102); color: rgb(136, 0, 0);"><span style=3D"color=
: #800;" class=3D"styled-by-prettify">//Address is exported; copy elision i=
s no longer guaranteed </span></span><span style=3D"color: #000;" class=3D"=
styled-by-prettify"><br>=C2=A0 =C2=A0 </span><span style=3D"color: #008;" c=
lass=3D"styled-by-prettify">return</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> a</span><span style=3D"color: #660;" class=3D"styl=
ed-by-prettify">;</span><span style=3D"color: #000;" class=3D"styled-by-pre=
ttify"> <br></span><span style=3D"color: #660;" class=3D"styled-by-prettify=
">}</span></font></div></code></div><br><div class=3D"prettyprint" style=3D=
"border: 1px solid rgb(187, 187, 187); word-wrap: break-word;"><code class=
=3D"prettyprint"><div class=3D"subprettyprint"><font style=3D"background-co=
lor: rgb(255, 255, 255);"><span style=3D"color: #000;" class=3D"styled-by-p=
rettify">T</span><span style=3D"color: #660;" class=3D"styled-by-prettify">=
*</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> GLOBAL_T=
_PTR</span><span style=3D"color: #660;" class=3D"styled-by-prettify">;</spa=
n><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>T foo</span=
><span style=3D"color: #660;" class=3D"styled-by-prettify">(</span><font co=
lor=3D"#000000"><span style=3D"caret-color: rgb(102, 0, 102);"><span style=
=3D"color: #660;" class=3D"styled-by-prettify">)</span><span style=3D"color=
: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #660;" =
class=3D"styled-by-prettify">{</span><span style=3D"color: #000;" class=3D"=
styled-by-prettify"><br>=C2=A0 =C2=A0 T myFoo</span><span style=3D"color: #=
660;" class=3D"styled-by-prettify">();</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 GLOBAL_T_PTR </span><span sty=
le=3D"color: #660;" class=3D"styled-by-prettify">=3D</span><span style=3D"c=
olor: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #66=
0;" class=3D"styled-by-prettify">&amp;</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify">myFoo</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">;</span><span style=3D"color: #000;" class=3D"style=
d-by-prettify"> </span><span style=3D"color: #800;" class=3D"styled-by-pret=
tify">//Address is exported; copy elision is no longer guaranteed </span><s=
pan style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 <=
/span><span style=3D"color: #008;" class=3D"styled-by-prettify">return</spa=
n><span style=3D"color: #000;" class=3D"styled-by-prettify"> myFoo</span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">;</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">}</span></span></font></font></div=
></code></div><div><br></div><div class=3D"prettyprint" style=3D"border: 1p=
x solid rgb(187, 187, 187); word-wrap: break-word;"><code class=3D"prettypr=
int"><div class=3D"subprettyprint" style=3D"caret-color: rgb(102, 0, 102); =
background-color: rgb(255, 255, 255);"><span style=3D"color: #008;" class=
=3D"styled-by-prettify">struct</span><span style=3D"color: #000;" class=3D"=
styled-by-prettify"> A </span><span style=3D"color: #660;" class=3D"styled-=
by-prettify">{</span><span style=3D"color: #000;" class=3D"styled-by-pretti=
fy"><br></span><span style=3D"color: #660;" class=3D"styled-by-prettify">};=
</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br></span=
><span style=3D"color: #008;" class=3D"styled-by-prettify">struct</span><sp=
an style=3D"color: #000;" class=3D"styled-by-prettify"> B </span><span styl=
e=3D"color: #660;" class=3D"styled-by-prettify">{</span><span style=3D"colo=
r: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 A</span><span styl=
e=3D"color: #660;" class=3D"styled-by-prettify">&amp;</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #0=
08;" class=3D"styled-by-prettify">ref</span><span style=3D"color: #660;" cl=
ass=3D"styled-by-prettify">;</span><span style=3D"color: #000;" class=3D"st=
yled-by-prettify"><br>=C2=A0 =C2=A0 B</span><span style=3D"color: #660;" cl=
ass=3D"styled-by-prettify">(</span><span style=3D"color: #000;" class=3D"st=
yled-by-prettify">A</span><span style=3D"color: #660;" class=3D"styled-by-p=
rettify">&amp;</span><span style=3D"color: #000;" class=3D"styled-by-pretti=
fy"> _ref</span><span style=3D"color: #660;" class=3D"styled-by-prettify">)=
</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">:</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"> </span><font color=3D"#6666=
00"><span style=3D"color: #008;" class=3D"styled-by-prettify">ref</span><sp=
an style=3D"color: #660;" class=3D"styled-by-prettify">(</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify">_ref</span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">)</span><span style=3D"color: #000=
;" class=3D"styled-by-prettify"> </span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">{}</span></font><span style=3D"color: #000;" class=
=3D"styled-by-prettify"><br></span><span style=3D"color: #660;" class=3D"st=
yled-by-prettify">};</span><span style=3D"color: #000;" class=3D"styled-by-=
prettify"><br>A foo</span><span style=3D"color: #660;" class=3D"styled-by-p=
rettify">(</span><span style=3D"color: #000;" class=3D"styled-by-prettify">=
B</span><span style=3D"color: #660;" class=3D"styled-by-prettify">*</span><=
span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span styl=
e=3D"color: #660;" class=3D"styled-by-prettify">&amp;</span><span style=3D"=
color: #000;" class=3D"styled-by-prettify">ptr</span><span style=3D"color: =
#660;" class=3D"styled-by-prettify">){</span><span style=3D"color: #000;" c=
lass=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 A val</span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">;</span><span style=3D"color: #000=
;" class=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 ptr </span><span style=3D=
"color: #660;" class=3D"styled-by-prettify">=3D</span><span style=3D"color:=
 #000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #008;" c=
lass=3D"styled-by-prettify">new</span><span style=3D"color: #000;" class=3D=
"styled-by-prettify"> B</span><span style=3D"color: #660;" class=3D"styled-=
by-prettify">(</span><span style=3D"color: #000;" class=3D"styled-by-pretti=
fy">val</span><span style=3D"color: #660;" class=3D"styled-by-prettify">);<=
/span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=A0 =
=C2=A0 </span><span style=3D"color: #008;" class=3D"styled-by-prettify">ret=
urn</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> val</s=
pan><span style=3D"color: #660;" class=3D"styled-by-prettify">;</span><span=
 style=3D"color: #000;" class=3D"styled-by-prettify"> </span><span style=3D=
"color: #800;" class=3D"styled-by-prettify">//Reference is exported; copy e=
lision is no longer guaranteed</span><span style=3D"color: #000;" class=3D"=
styled-by-prettify"><br></span><span style=3D"color: #660;" class=3D"styled=
-by-prettify">}</span></div></code></div><br><div class=3D"prettyprint" sty=
le=3D"border: 1px solid rgb(187, 187, 187); word-wrap: break-word;"><code c=
lass=3D"prettyprint"><div class=3D"subprettyprint" style=3D"caret-color: rg=
b(102, 0, 102);"><font color=3D"#660066" style=3D"background-color: rgb(255=
, 255, 255);"><span style=3D"color: #008;" class=3D"styled-by-prettify">str=
uct</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> A</spa=
n><span style=3D"color: #660;" class=3D"styled-by-prettify">;</span><span s=
tyle=3D"color: #000;" class=3D"styled-by-prettify"><br>A</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">*</span><span style=3D"color=
: #000;" class=3D"styled-by-prettify"> GLOBAL_A_PTR</span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">;</span><span style=3D"color: #000=
;" class=3D"styled-by-prettify"><br></span><span style=3D"color: #008;" cla=
ss=3D"styled-by-prettify">struct</span><span style=3D"color: #000;" class=
=3D"styled-by-prettify"> A </span><span style=3D"color: #660;" class=3D"sty=
led-by-prettify">{</span><span style=3D"color: #000;" class=3D"styled-by-pr=
ettify"><br>=C2=A0 =C2=A0 A</span><span style=3D"color: #660;" class=3D"sty=
led-by-prettify">()</span><span style=3D"color: #000;" class=3D"styled-by-p=
rettify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify">=
{}</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=C2=
=A0 =C2=A0 A</span><span style=3D"color: #660;" class=3D"styled-by-prettify=
">(</span><span style=3D"color: #008;" class=3D"styled-by-prettify">const</=
span><span style=3D"color: #000;" class=3D"styled-by-prettify"> A</span><sp=
an style=3D"color: #660;" class=3D"styled-by-prettify">&amp;</span><span st=
yle=3D"color: #000;" class=3D"styled-by-prettify"> val</span><span style=3D=
"color: #660;" class=3D"styled-by-prettify">)</span><span style=3D"color: #=
000;" class=3D"styled-by-prettify"> </span><span style=3D"color: #660;" cla=
ss=3D"styled-by-prettify">{</span><span style=3D"color: #000;" class=3D"sty=
led-by-prettify"> GLOBAL_A_PTR </span><span style=3D"color: #660;" class=3D=
"styled-by-prettify">=3D</span><span style=3D"color: #000;" class=3D"styled=
-by-prettify"> </span><span style=3D"color: #008;" class=3D"styled-by-prett=
ify">this</span><span style=3D"color: #660;" class=3D"styled-by-prettify">;=
</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> </span><s=
pan style=3D"color: #660;" class=3D"styled-by-prettify">}</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"co=
lor: #660;" class=3D"styled-by-prettify">};</span><span style=3D"color: #00=
0;" class=3D"styled-by-prettify"><br>A foo</span><span style=3D"color: #660=
;" class=3D"styled-by-prettify">(){</span><span style=3D"color: #000;" clas=
s=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 A a</span><span style=3D"color: =
#660;" class=3D"styled-by-prettify">;</span><span style=3D"color: #000;" cl=
ass=3D"styled-by-prettify"><br>=C2=A0 =C2=A0 </span><span style=3D"color: #=
008;" class=3D"styled-by-prettify">return</span><span style=3D"color: #000;=
" class=3D"styled-by-prettify"> a</span><span style=3D"color: #660;" class=
=3D"styled-by-prettify">;</span><span style=3D"color: #000;" class=3D"style=
d-by-prettify"> </span><span style=3D"color: #800;" class=3D"styled-by-pret=
tify">//address of copy-constructed object would have been exported; copy e=
lision is no longer guaranteed</span><span style=3D"color: #000;" class=3D"=
styled-by-prettify"><br></span><span style=3D"color: #660;" class=3D"styled=
-by-prettify">}</span></font></div></code></div><div><br></div><div><br></d=
iv><br><b>Definition of export</b></div><div>A function exports an address =
or reference if that address or reference is (potentially) stored externall=
y as a result of a side-effect of the function. If it is not known at compi=
le time whether or not an export will occur, copy elision is not guaranteed=
.. If a reference or address is passed to a function with external linkage, =
copy elision is not guaranteed. If a address or reference is passed to a fu=
nction whose definition is visible to the compiler, then it is not exported=
 if that function does not export it.=C2=A0</div><div><br></div><div><br></=
div><div><br></div></div>

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Addendum: if an object X (for whatever reason) stores, references, or
contains a pointer to another object Y, such that Y references X or stores
a pointer to X, X was "exported." If the move or copy constructor of X
exports the address of the move or copy-constructed object, X was
"exported". If X is "exported", then copy elision of X is not guaranteed
when X is returned.

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<div dir=3D"ltr">Addendum: if an object X (for whatever reason) stores, ref=
erences, or contains a pointer to another object Y, such that Y references =
X or stores a pointer to X, X was &quot;exported.&quot; If the move or copy=
 constructor of X exports the address of the move or copy-constructed objec=
t, X was &quot;exported&quot;. If X is &quot;exported&quot;, then copy elis=
ion of X is not guaranteed when X is returned.=C2=A0</div>

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On Wednesday, 30 May 2018 13:15:47 PDT Antonio Perez wrote:
> *Definition of export*
> A function exports an address or reference if that address or reference is
> (potentially) stored externally as a result of a side-effect of the
> function. If it is not known at compile time whether or not an export will
> occur, copy elision is not guaranteed. If a reference or address is passed
> to a function with external linkage, copy elision is not guaranteed. If a
> address or reference is passed to a function whose definition is visible to
> the compiler, then it is not exported if that function does not export it.

Remove the wording about external and internal linkage. Instead, talk about
constexpr and non-constexpr. You cannot require a compiler to perform the
analysis of what a local function will do.

--
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Software Architect - Intel Open Source Technology Center



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