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I'll take a wild guess this has already been asked, thought-of or written
about, but lets ask anyways. I'm curious why one cannot write the
implementation of non-template dependent member function in a compile unit,
in the hopes of improving compile times.

For example :

..h

template <class T>
struct test {
    int val() const;

private:
    int val { 42 };
    T t;
};


..cpp

// I do not depend on T, can implement in compile unit.
// Declaration semantics unimportant.
int test::val() const {
    return val;
}


I remember a cppcon talk from Ubisoft, where they bypass this issue by
inheriting a base class with non-template dependent behavior implemented in
their compile unit. Template dependent behavior is implemented in the
header of the derived class.
Why not offer this compilation behavior out-of-the-box?

Thanks & good day

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<div dir=3D"ltr"><div>I&#39;ll take a wild guess this has already been aske=
d, thought-of or written about, but lets ask anyways. I&#39;m curious why o=
ne cannot write the implementation of non-template dependent member functio=
n in a compile unit, in the hopes of improving compile times.<br></div><div=
><br></div><div>For example :</div><div><br></div><div>.h</div><div><br></d=
iv><div><div style=3D"background-color: rgb(250, 250, 250); border-color: r=
gb(187, 187, 187); border-style: solid; border-width: 1px; overflow-wrap: b=
reak-word;" class=3D"prettyprint"><code class=3D"prettyprint"><div class=3D=
"subprettyprint"><span style=3D"color: #008;" class=3D"styled-by-prettify">=
template</span><span style=3D"color: #000;" class=3D"styled-by-prettify"> <=
/span><span style=3D"color: #660;" class=3D"styled-by-prettify">&lt;</span>=
<span style=3D"color: #008;" class=3D"styled-by-prettify">class</span><span=
 style=3D"color: #000;" class=3D"styled-by-prettify"> T</span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">&gt;</span><span style=3D"co=
lor: #000;" class=3D"styled-by-prettify"><br></span><span style=3D"color: #=
008;" class=3D"styled-by-prettify">struct</span><span style=3D"color: #000;=
" class=3D"styled-by-prettify"> test </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 </span><span style=3D"color: #008;" cla=
ss=3D"styled-by-prettify">int</span><span style=3D"color: #000;" class=3D"s=
tyled-by-prettify"> val</span><span style=3D"color: #660;" class=3D"styled-=
by-prettify">()</span><span style=3D"color: #000;" class=3D"styled-by-prett=
ify"> </span><span style=3D"color: #008;" class=3D"styled-by-prettify">cons=
t</span><span style=3D"color: #660;" class=3D"styled-by-prettify">;</span><=
span style=3D"color: #000;" class=3D"styled-by-prettify"><br><br></span><sp=
an style=3D"color: #008;" class=3D"styled-by-prettify">private</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">int</span><span style=3D=
"color: #000;" class=3D"styled-by-prettify"> val </span><span style=3D"colo=
r: #660;" class=3D"styled-by-prettify">{</span><span style=3D"color: #000;"=
 class=3D"styled-by-prettify"> </span><span style=3D"color: #066;" class=3D=
"styled-by-prettify">42</span><span style=3D"color: #000;" class=3D"styled-=
by-prettify"> </span><span style=3D"color: #660;" class=3D"styled-by-pretti=
fy">};</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><br>=
=C2=A0 =C2=A0 T t</span><span style=3D"color: #660;" class=3D"styled-by-pre=
ttify">;</span><span style=3D"color: #000;" class=3D"styled-by-prettify"><b=
r></span><span style=3D"color: #660;" class=3D"styled-by-prettify">};</span=
></div></code></div><br><br></div><div></div><div>.cpp</div><div><br></div>=
<div style=3D"background-color: rgb(250, 250, 250); border-color: rgb(187, =
187, 187); border-style: solid; border-width: 1px; overflow-wrap: break-wor=
d;" class=3D"prettyprint"><code class=3D"prettyprint"><div class=3D"subpret=
typrint"><span style=3D"color: #800;" class=3D"styled-by-prettify">// I do =
not depend on T, can implement in compile unit.</span><span style=3D"color:=
 #000;" class=3D"styled-by-prettify"><br></span><span style=3D"color: #800;=
" class=3D"styled-by-prettify">// Declaration semantics unimportant.</span>=
<span style=3D"color: #000;" class=3D"styled-by-prettify"><br></span><span =
style=3D"color: #008;" class=3D"styled-by-prettify">int</span><span style=
=3D"color: #000;" class=3D"styled-by-prettify"> test</span><span style=3D"c=
olor: #660;" class=3D"styled-by-prettify">::</span><span style=3D"color: #0=
00;" class=3D"styled-by-prettify">val</span><span style=3D"color: #660;" cl=
ass=3D"styled-by-prettify">()</span><span style=3D"color: #000;" class=3D"s=
tyled-by-prettify"> </span><span style=3D"color: #008;" class=3D"styled-by-=
prettify">const</span><span style=3D"color: #000;" class=3D"styled-by-prett=
ify"> </span><span style=3D"color: #660;" class=3D"styled-by-prettify">{</s=
pan><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"><br></span><span style=
=3D"color: #660;" class=3D"styled-by-prettify">}</span></div></code></div><=
div><br></div><div><br></div><div>I remember a cppcon talk from Ubisoft, wh=
ere they bypass this issue by inheriting a base class with non-template dep=
endent behavior implemented in their compile unit. Template dependent behav=
ior is implemented in the header of the derived class.</div><div>Why not of=
fer this compilation behavior out-of-the-box?</div><div><br></div><div>Than=
ks &amp; good day<br></div></div>

<p></p>

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