[Effective-cpp] Item 34: Understand How to Combine C++ and C in the Same Program.

[David Sykes]

Hi Jaspreet

You have joined nearly at the end of this project, which is probably a good 
time to get an idea of what is going on. The schedule can be found at 
http://www.paulgrenyer.co.uk/accu/mdevelopers/effectivecpp/index.htm

strdup might not pose any problems on windows platforms, but there are many 
other platforms that would need to conform. To be completely safe it is 
best, where possible, to avoid functions that could introduce some 
confusion.

David

-----Original Message-----
From:	Jaspreet Singh [SMTP:Jasingh at quark.co.in]
Sent:	06 June 2003 05:30
To:	effective-cpp at accu.org
Subject:	RE: [Effective-cpp] Item 34:  Understand How to Combine C++ and 
C	in the Same Program.

Hi
	In the "Dynamic Memory Allocation" section you are referring to
"strdup" function. in msdn i found _strdup function thats only for Win and
it specifically states that the memory is allocated using "malloc". So how
does this function pose any problem?
	Also, i am a new joinee in this list... Hi All... i noticed that
Scott Meyers is being discussed item-wise. Whats the schedule? item 31 was
discussed on 28th, item 32 on 29th, item 33 on 4th and item 34 today.. is
there a specific schedule you all follow? pls let me know so that i can 
grab
my own copy of ecpp and start with you guys !

Thanks,
Jaspreet

-----Original Message-----
From: David Sykes [mailto:davids at revolution.co.uk]
Sent: Friday, June 06, 2003 12:24 AM
To: 'effective-cpp at accu.org'
Subject: [Effective-cpp] Item 34: Understand How to Combine C++ and C in
the Same Program.


[I have tried to be concise here, but apologies if I cut too much]



It is perfectly possible to combine C++ code with C code, but once you have 

determined that the C++ and C compilers produce compatible object files
there are four things that you need to consider for the combination to
work.

Name Mangling
===========
Function overloading in C++ means having separate functions with the same
name, and to implement this and other features C++ compilers mangle the
function names into something unique for each combination of parameters. In 

order for a C program to reference a C++ function, and vice versa, without
knowing specifically how the name is mangled it is necessary to turn the
mangling off for this function. This is achieved using the extern C
directive:

extern "C" function ( type parameter ); // The name of this function will
not be mangled.

Multiple functions can be grouped, and the __cplusplus definition utilised
so that the directives will compile for C as well as C++

#ifdef __cplusplus
extern "C" {
#endif
  void function1 ( type parameter );
  void function2 ( type parameter );
  void function3 ( type parameter );
#ifdef __cplusplus
}
#endif


Initialisation of statics
================
In C++ statics and global objects can have constructors, and the code for
these constructors must be called before main begins. To implement this C++ 

compilers add a secret function call before main that deals with all the
construction. If main is in the C code then the initialisation function
will not be called, and the static and global objects will not be
constructed correctly. For this reason it is preferable for main to be in
the C++ code. The C version of main can be called realMain, and the C++
main calls this

extern "C" int realMain ( int argc , char **argv);
int main ( int argc , char **argv)
{
   return realMain ( argc , argv );
}

If you cannot write main in C++ you have a problem, and must find out how
to call the C++ initialisation code from C.

Dynamic Memory Allocation
====================
In general C++ uses new and delete, C uses malloc and free, and the two are 

completely incompatible. It is vital to rigorously separate the two.
Sometimes this is not straightforward, however, for example the function
strdup is generally available in C and C++ and produces a new pointer, but
how should it be deleted? It is best to avoid using any functions that can
cause this sort of confustion

Data Structure Compatibility
====================
Object definitions will not compile in C so it is not possible to directly
pass pointers to objects between C and C++. The rules governing the layout
of a struct in C++ are consistent with those of C, however, so it is
therefore safe to pass pointers to structures back and forth between C and
C++. Adding non virtual functions to the C++ version of the struct does not 

change the structure layout, so pointers to structs with non virtual
functions will transfer between C and C++. Adding virtual functions,
however, causes structs to use a different memory layout, and transfer is
no longer possible.
In general you should limit what passes between C and C++ to that which
compiles under the C compiler.




David

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