Combining C and C++ codes

[philipho]

Hi all,

I have an Opencv face detection code that is written in C. I am currently doing a face detection project that makes use of Euresys frame grabbers. The problem is the image acquisition codes for Euresys are all written in C++, and i have to pass the frames captured to the face detection program for real time face tracking.

Will it be feasible to combine C and C++ codes?

[Banfa]

Yes it is possible, but you will have a few problems to overcome.

1. C can not use classes (seems obvious but I thought I'd mention it).

2. C and C++ use different function calling conventions. C++ can be told use the C calling convention but C can not use the C++ calling convention.

You will often see code like this in a header file

#ifdef  __cplusplus
extern "C" {
#endif

// Header file definitions

#ifdef __cplusplus
}
#endif

This is used in a header file that contains the definitions for a C source file. All C++ compilers define __cplusplus when compiling C++ source files so basically what these directives do is tell the C++ compiler to use C calling convention for anything between the { and }. This allows the C++ and C csource code to call these functions.

There are many tricks to pull with this, for instance you can declare a function in a C++ file with C calling convention, then C code can call that function but because it is in a C++ file it can use C++ features, like access to classes.

I have worked on several projects that have mixed C and C++ code, it just takes a little care.