Boost.DateTime from C?

Question

Is it possible to use the Boost.DateTime library from C, and if so are there any examples available (preferably covering the build process using gcc-type tools)? I've searched the Boost documentation and the internet in general, and it seems theoretically possible, but have not found any clear answer one way or the other.

Answer 1

No. Boost is a C++ library and Boost.DateTime is no exception. If you look at any of the DateTime headers you'll see C++ features everywhere, from classes to templates to namespaces and much more - none of which are supported in C.

You can, however, create a wrapper library in C++ which exposes the functionality you need as pure C functions, then call those from C.

Answer 2

C and C++ are highly interoperable; calling C code from C++ is trivial; calling C++ code from C however is more restricted because the interface must have C linkage and contain only types available in C and cannot use C++ specific features such as default arguments or function overloading for example.

To use C++ code in C requires that the interface has a extern "C" linkage specification, which is itself not valid C, so the specification in any header file must be conditional e.g.:

// interop.h
#if !defined INTEROP_H
#define INTEROP_H

#if defined __cplusplus
extern "C"
{
#endif

// C interface here
void somefunction() ;

#if defined __cplusplus
}
#endif

#endif // INTEROP_H

Then the implementation of the interface itself would be C++ code:

// interop.cpp
#include "interop.h"

void somefunction()
{
    // C++ code here callable from C through somefunction()
}

But the interface is callable from C code:

// notcpp.c
#include "interop.h"
int main()
{
    somefunction() ;
}

This approach is fine where the interface is simple, so for example if you use the Boost Date/Time library for some very specific procedure the results of which can be represented in C, then that might be appropriate if implementing equivalent functionality in C alone would be prohibitive. But creating C wrappers for entire C++ classes is time consuming error prone, inflexible and and ultimately pointless - it would be generally simpler to compile your C code as C++ then the interoperability barrier disappears as you can then introduce C++ specific code directly into your existing codebase (though it can never thereafter be compiled as C). C++ is multi-paradigm and includes the C library within its own C++ library, and most third-party and OS C library headers already include the C++ conditional C linkage wrapper, so you need not rewrite all your code in the OOP style in order to use C++ if you only want to take advantage of the larger library ecosystem.