<s>
libsigc++	B-Language
is	O
a	O
C++	B-Language
library	B-Library
for	O
typesafe	B-Language
callbacks	O
.	O
</s>
<s>
libsigc++	B-Language
implements	O
a	O
callback	O
system	O
for	O
use	O
in	O
abstract	O
interfaces	O
and	O
general	O
programming	O
.	O
</s>
<s>
libsigc++	B-Language
is	O
one	O
of	O
the	O
earliest	O
implementations	O
of	O
the	O
signals	B-Language
and	I-Language
slots	I-Language
concept	O
implemented	O
using	O
C++	B-Language
template	B-Language
metaprogramming	I-Language
.	O
</s>
<s>
It	O
was	O
created	O
as	O
an	O
alternative	O
to	O
the	O
use	O
of	O
a	O
meta	O
compiler	O
such	O
as	O
found	O
in	O
the	O
signals	B-Language
and	I-Language
slots	I-Language
implementation	O
in	O
Qt	B-Language
.	O
</s>
<s>
libsigc++	B-Language
originated	O
as	O
part	O
of	O
the	O
gtkmm	B-Language
project	O
in	O
1997	O
and	O
later	O
was	O
rewritten	O
to	O
be	O
a	O
standalone	O
library	B-Library
.	O
</s>
<s>
Functions	O
and	O
methods	O
are	O
then	O
wrapped	O
using	O
template	O
calls	O
to	O
produce	O
function	B-Language
objects	I-Language
(	O
functors	B-Language
)	O
which	O
can	O
be	O
bound	O
to	O
a	O
signal	O
.	O
</s>
<s>
Each	O
signal	O
can	O
be	O
connected	O
to	O
multiple	O
functors	B-Language
thus	O
creating	O
an	O
observer	B-Language
pattern	I-Language
through	O
which	O
a	O
message	O
can	O
be	O
distributed	O
to	O
multiple	O
anonymous	O
listener	O
objects	O
.	O
</s>
<s>
Reference	B-General_Concept
counting	I-General_Concept
based	O
object	O
lifespan	O
tracking	O
was	O
used	O
to	O
disconnect	O
the	O
functors	B-Language
from	O
signals	O
as	O
objects	O
are	O
deleted	O
.	O
</s>
<s>
The	O
use	O
of	O
templates	O
allowed	O
for	O
compile	O
time	O
typesafe	B-Language
verification	O
of	O
connections	O
.	O
</s>
<s>
The	O
addition	O
of	O
this	O
strict	O
compile	O
time	O
checking	O
required	O
the	O
addition	O
of	O
template	O
typecasting	O
adapters	O
which	O
convert	O
the	O
functor	B-Language
callback	O
profile	O
to	O
match	O
the	O
required	O
signal	O
pattern	O
.	O
</s>
<s>
libsigc++	B-Language
was	O
a	O
natural	O
expansion	O
of	O
the	O
C++	B-Language
standard	I-Language
library	I-Language
functors	B-Language
to	O
the	O
tracking	O
of	O
objects	O
necessary	O
to	O
implement	O
the	O
observer	B-Language
pattern	I-Language
.	O
</s>
<s>
It	O
inspired	O
multiple	O
C++	B-Language
template	O
based	O
signal	O
and	O
slot	O
implementations	O
including	O
the	O
signal	O
implementation	O
used	O
in	O
the	O
boost	B-Language
C++	I-Language
libraries	I-Language
.	O
</s>
<s>
libsigc++	B-Language
is	O
released	O
as	O
free	B-Application
software	I-Application
under	O
the	O
GNU	B-Application
Lesser	I-Application
General	I-Application
Public	I-Application
License	I-Application
(	O
LGPL	B-Application
)	O
.	O
</s>
