Question

In a C++ project, including .h files of C source files will cause many errors because of different standards between C and C++.
How to use C source files in a C++ project (or in main.cpp)?

Answer 1

For the maximum reliability:

• Compile the C source with a C compiler.
• Compile the C++ source with a C++ compiler
• Preferably, write the main() function in C++.
• Link the program with a C++ compiler.

Make sure that the C headers are either themselves aware of C++ or that the C++ code includes the C headers inside an extern "C" { ... } block.

Either (C header file cheader.h):

#ifndef CHEADER_H_INCLUDED
#define CHEADER_H_INCLUDED

#ifdef __cplusplus
extern "C" {
#endif

...main contents of header...

#ifdef __cplusplus
}
#endif

#endif /* CHEADER_H_INCLUDED */ 

or (C++ source code):

extern "C" {
#include "cheader.h"
}

Modern C style is very close to the common subset of the C and C++ languages. However, arbitrary C code is not C++ code for any of a very large number of reasons, and simply calling the C source files C++ source files (by changing the extension, or simply by compiling with the C++ compiler) is not guaranteed to be successful. In general, it is easier to compile C as C and C++ as C++ and then link the resulting object files with the C++ compiler (to ensure the correct support libraries are invoked).

However, if the MSVC compiler is saying that programs using MFC have to be written solely in C++ (MFC requires C++ compilation (use a .cpp suffix) is the reported error), then you may have no choice but to ensure that your C code is compilable as C++ code. That means you’ll have to cast the return values from malloc() et al; you have to worry about other places where you do not use a cast to convert a void * into some other pointer type; you have to worry about sizeof('a') == 4 in C and sizeof('a') == 1 in C++; you have to ensure that every function is declared before it is used; you have to ensure your C code does not use any C++ keywords (typename, class in particular; also inline sometimes — but the complete list is quite large).

In some circles, you’d have to worry about the use of features in C99 that are not in C++2003 or C++2011, such as flexible array members, designated initializers, compound literals, variable-length arrays, and so on. However, if the C code is for MSVC, then that probably isn’t going to be a problem; those features are not supported by the MSVC C compiler (it only supports C89, not C99).

FWIW: I have a script to hunt down C++ keywords. It contains the following comment:

# http://en.cppreference.com/w/cpp/keywords
# plus JL annotations
# and                               C (<iso646.h>)
# and_eq                            C (<iso646.h>)
# alignas (C++11 feature)
# alignof (C++11 feature)
# asm                               C (core)
# auto(1)                           C (core)
# bitand                            C (<iso646.h>)
# bitor                             C (<iso646.h>)
# bool                              C99 (<stdbool.h>)
# break                             C (core)
# case                              C (core)
# catch
# char                              C (core)
# char16_t (C++11 feature)
# char32_t (C++11 feature)
# class
# compl                             C (<iso646.h>)
# const                             C (core)
# constexpr (C++11 feature)
# const_cast
# continue                          C (core)
# decltype (C++11 feature)
# default(1)                        C (core)
# delete(1)
# double                            C (core)
# dynamic_cast
# else                              C (core)
# enum                              C (core)
# explicit
# export
# extern                            C (core)
# false                             C99 (<stdbool.h>)
# float                             C (core)
# for                               C (core)
# friend
# goto                              C (core)
# if                                C (core)
# inline                            C (core)
# int                               C (core)
# long                              C (core)
# mutable
# namespace
# new
# noexcept (C++11 feature)
# not                               C (<iso646.h>)
# not_eq                            C (<iso646.h>)
# nullptr (C++11 feature)
# operator
# or                                C (<iso646.h>)
# or_eq                             C (<iso646.h>)
# private
# protected
# public
# register                          C (core)
# reinterpret_cast
# return                            C (core)
# short                             C (core)
# signed                            C (core)
# sizeof                            C (core)
# static                            C (core)
# static_assert (C++11 feature)
# static_cast
# struct                            C (core)
# switch                            C (core)
# template
# this
# thread_local (C++11 feature)
# throw
# true                              C99 (<stdbool.h>)
# try
# typedef                           C (core)
# typeid
# typename
# union                             C (core)
# unsigned                          C (core)
# using(1)
# virtual
# void                              C (core)
# volatile                          C (core)
# wchar_t                           C (core)
# while                             C (core)
# xor                               C (<iso646.h>)
# xor_eq                            C (<iso646.h>)

The (1) suffixes is a footnote at CPP Reference:

• (1) — meaning changed in C++11