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From: niklas@appli.se (Niklas Hallqvist)
Newsgroups: comp.lang.c++
Subject: I/O problem
Summary: How to save an objects binary image on a file?
Keywords: C++ I/O binary
Message-ID: <1179@appli.se>
Date: 3 Nov 90 11:26:27 GMT
Organization: Applitron Datasystem AB, GOTHENBURG, SWEDEN
Lines: 29


	Hello out there...

	Is there a way to do portable binary I/O on classes with virtual
functions?  Yes, I know there is, by letting the objects them self provide
a binary image via a virtual function.  But what I want is to write some-
thing like:

class A { private: int x; public: virtual int y(void) { return x; }};
void put(A* a) { write(1, &a, sizeof(a)); }
A* get(void) { A* a = new A; read(0, &a, sizeof(a)); return a; }

And what happens here?  Aha, the vtbl pointer gets saved along with the
binary image, and will be read in by other programs using the get function.
No harm in that, is it?  Well, YES!!!!  Another program would be differently
laid out in memory and the vtbl ptr would not be valid there.  Unpredictable
results will follow the first virtual call.  Now don't tell me you can't
do something like this without rewriting all the classes one want to be
able to store this way.  Summary: How would you do if you had a "black box"-
class (i.e no source, just relocatable object code) and wanted to do I/O,
keeping the objects full state (even private info)?

					Thanks, Niklas

-- 
Niklas Hallqvist	Phone: +46-(0)31-40 75 00
Applitron Datasystem	Fax:   +46-(0)31-83 39 50
Molndalsvagen 95	Email: niklas@appli.se
S-412 63  GOTEBORG, Sweden     mcsun!sunic!chalmers!appli!niklas