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>2. The Problem</H1
><P
>At some time you might have to load a library (and use its
functions) at runtime; this happens most often when you are
writing some kind of plug-in or module architecture for your
program.</P
><P
>In the C language, loading a library is very simple (calling
<TT
CLASS="function"
>dlopen</TT
>, <TT
CLASS="function"
>dlsym</TT
> and
<TT
CLASS="function"
>dlclose</TT
> is enough), with C++ this is a bit
more complicated. The difficulties of loading a C++ library
dynamically are partially due to <A
HREF="theproblem.html#mangling"
>name
mangling</A
>, and partially due to the fact that the
<TT
CLASS="function"
>dlopen</TT
> API was written with C in mind, thus
not offering a suitable way to load classes.</P
><P
>Before explaining how to load libraries in C++, let's better
analyze the problem by looking at name mangling in more
detail. I recommend you read the explanation of name mangling,
even if you're not interested in it because it will help you
understanding why problems occur and how to solve them.</P
><DIV
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><H2
CLASS="section"
><A
NAME="mangling"
></A
>2.1. Name Mangling</H2
><P
>In every C++ program (or library, or object file), all
non-static functions are represented in the binary file as
<EM
>symbols</EM
>. These symbols are special text
strings that uniquely identify a function in the program,
library, or object file.</P
><P
>In C, the symbol name is the same as the function name:
the symbol of <TT
CLASS="function"
>strcpy</TT
> will be
<TT
CLASS="computeroutput"
>strcpy</TT
>, and so on. This is
possible because in C no two non-static functions can have the
same name.</P
><P
>Because C++ allows overloading (different functions with
the same name but different arguments) and has many features C
does not — like classes, member functions, exception
specifications — it is not possible to simply use the
function name as the symbol name. To solve that, C++ uses
so-called <EM
>name mangling</EM
>, which transforms
the function name and all the necessary information (like the
number and size of the arguments) into some weird-looking
string which only the compiler knows about. The mangled name
of <TT
CLASS="function"
>foo</TT
> might look like
<TT
CLASS="computeroutput"
>foo@4%6^</TT
>, for example. Or it
might not even contain the word <SPAN
CLASS="QUOTE"
>"foo"</SPAN
>.</P
><P
> One of the problems with name mangling is that the C++
standard (currently [<SPAN
CLASS="citation"
>ISO14882</SPAN
>]) does not
define how names have to be mangled; thus every compiler
mangles names in its own way. Some compilers even change their
name mangling algorithm between different versions (notably
g++ 2.x and 3.x). Even if you worked out how your particular
compiler mangles names (and would thus be able to load
functions via <TT
CLASS="function"
>dlsym</TT
>), this would most
probably work with your compiler only, and might already be
broken with the next version.</P
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><H2
CLASS="section"
><A
NAME="AEN137"
></A
>2.2. Classes</H2
><P
>Another problem with the <TT
CLASS="function"
>dlopen</TT
> API
is the fact that it only supports loading
<EM
>functions</EM
>. But in C++ a library often
exposes a class which you would like to use in your
program. Obviously, to use that class you need to create an
instance of it, but that cannot be easily done.</P
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