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>Chapter 35. Extending <ACRONYM
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>35.15. Using C++ for Extensibility</A
></H1
><P
> It is possible to use a compiler in C++ mode to build
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> extensions by following these
guidelines:
<P
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><LI
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> All functions accessed by the backend must present a C interface
to the backend; these C functions can then call C++ functions.
For example, <TT
CLASS="LITERAL"
>extern C</TT
> linkage is required for
backend-accessed functions. This is also necessary for any
functions that are passed as pointers between the backend and
C++ code.
</P
></LI
><LI
><P
> Free memory using the appropriate deallocation method. For example,
most backend memory is allocated using <CODE
CLASS="FUNCTION"
>palloc()</CODE
>, so use
<CODE
CLASS="FUNCTION"
>pfree()</CODE
> to free it, i.e. using C++
<CODE
CLASS="FUNCTION"
>delete()</CODE
> in such cases will fail.
</P
></LI
><LI
><P
> Prevent exceptions from propagating into the C code (use a
catch-all block at the top level of all <TT
CLASS="LITERAL"
>extern C</TT
>
functions). This is necessary even if the C++ code does not
throw any exceptions because events like out-of-memory still
throw exceptions. Any exceptions must be caught and appropriate
errors passed back to the C interface. If possible, compile C++
with <TT
CLASS="OPTION"
>-fno-exceptions</TT
> to eliminate exceptions entirely;
in such cases, you must check for failures in your C++ code, e.g.
check for NULL returned by <CODE
CLASS="FUNCTION"
>new()</CODE
>.
</P
></LI
><LI
><P
> If calling backend functions from C++ code, be sure that the
C++ call stack contains only plain old data structures
(<ACRONYM
CLASS="ACRONYM"
>POD</ACRONYM
>). This is necessary because backend errors
generate a distant <CODE
CLASS="FUNCTION"
>longjmp()</CODE
> that does not properly
unroll a C++ call stack with non-POD objects.
</P
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><P>
</P
><P
> In summary, it is best to place C++ code behind a wall of
<TT
CLASS="LITERAL"
>extern C</TT
> functions that interface to the backend,
and avoid exception, memory, and call stack leakage.
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