<s>
In	O
the	O
context	O
of	O
the	O
C	B-Language
or	O
C++	B-Language
programming	I-Language
languages	I-Language
,	O
a	O
library	B-Library
is	O
called	O
header-only	B-Language
if	O
the	O
full	O
definitions	O
of	O
all	O
macros	O
,	O
functions	O
and	O
classes	O
comprising	O
the	O
library	B-Library
are	O
visible	O
to	O
the	O
compiler	B-Language
in	O
a	O
header	B-Language
file	I-Language
form	O
.	O
</s>
<s>
Header-only	B-Language
libraries	O
do	O
not	O
need	O
to	O
be	O
separately	O
compiled	B-Language
,	O
packaged	O
and	O
installed	O
in	O
order	O
to	O
be	O
used	O
.	O
</s>
<s>
All	O
that	O
is	O
required	O
is	O
to	O
point	O
the	O
compiler	B-Language
at	O
the	O
location	O
of	O
the	O
headers	O
,	O
and	O
then	O
the	O
header	B-Language
files	I-Language
into	O
the	O
application	O
source	O
.	O
</s>
<s>
Another	O
advantage	O
is	O
that	O
the	O
compiler	B-Language
's	O
optimizer	O
can	O
do	O
a	O
much	O
better	O
job	O
when	O
all	O
the	O
library	B-Library
's	O
source	O
code	O
is	O
available	O
.	O
</s>
<s>
Nonetheless	O
,	O
the	O
header-only	B-Language
form	O
is	O
popular	O
because	O
it	O
avoids	O
the	O
(	O
often	O
much	O
more	O
serious	O
)	O
problem	O
of	O
packaging	O
.	O
</s>
<s>
For	O
C++	B-Application
templates	I-Application
,	O
including	O
the	O
definitions	O
in	O
header	O
is	O
the	O
only	O
way	O
to	O
compile	B-Language
,	O
since	O
the	O
compiler	B-Language
needs	O
to	O
know	O
the	O
full	O
definition	O
of	O
the	O
templates	B-Application
in	O
order	O
to	O
instantiate	O
.	O
</s>
