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

[David Sykes]

[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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