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Trying to just compile existing C code as C++ will likely cause many errors and may have potential functional differences (not likely, but possible).
https://stackoverflow.com/questions/5228056/can-i-use-c-compiler-to-compile-c-source-code
http://www.geeksforgeeks.org/write-c-program-wont-compiler-c/
http://www.geeksforgeeks.org/write-c-program-produce-different-result-c/
C/C++ interop is well supported. We can compile C files as C into object files and C as C++ into object files and link them without issue.
To access C functions from C++, C++ code will need to include the C headers (wrapping in extern "C" { }).
To access C++ functions from C, C++ needs to define the function inside extern "C" { }.
The extern "C" { } forces C++ to use C naming convention in object file (instead of doing C++ name mangling), which allows the C++ and C objects to link properly.
So we have to make sure that any header file used by both C and C++ is compatible with both. For existing C files the big sticking points will be use of C++ keywords. Renaming can clear most of this up, but bool typedefs is another story.
In C++ bool is a keyword and built in type. In C we just typedef it. Can we somehow make C bool and C++ bool 100% compatible, such that we can guarantee the size is the same and we can pass bool values between C and C++ code without worrying about strange or undefined behavior? Answer: I think so…
#ifndef BOOLTYPE_H #define BOOLTYPE_H /* http://www.pixelbeat.org/programming/gcc/static_assert.html */ #define ASSERT_CONCAT_(a, b) a##b #define ASSERT_CONCAT(a, b) ASSERT_CONCAT_(a, b) /* These can't be used after statements in c89. */ #ifdef __COUNTER__ #define STATIC_ASSERT(e,m) \ enum { ASSERT_CONCAT(static_assert_, __COUNTER__) = 1/static_cast<int>(!!(e)) } #else /* This can't be used twice on the same line so ensure if using in headers * that the headers are not included twice (by wrapping in #ifndef...#endif) * Note it doesn't cause an issue when used on same line of separate modules * compiled with gcc -combine -fwhole-program. */ #define STATIC_ASSERT(e,m) \ ;enum { ASSERT_CONCAT(assert_line_, __LINE__) = 1/(int)(!!(e)) } #endif typedef unsigned char C_bool_t; #define C_TRUE 1 #define C_FALSE 0 #if !defined(__cplusplus) typedef C_bool_t bool; #define true C_TRUE #define false C_FALSE #else /* verify compatibility between C and C++ booleans */ STATIC_ASSERT(sizeof(C_bool_t) == sizeof(bool), C_CPP_bool_size_mismatch); STATIC_ASSERT(C_TRUE == true, C_CPP_true_equiv); STATIC_ASSERT(C_FALSE == false, C_CPP_false_equiv); STATIC_ASSERT(!C_TRUE == C_FALSE, C_TRUE_not); STATIC_ASSERT(!C_TRUE == false, C_TRUE_not); STATIC_ASSERT(!true == C_FALSE, CPP_true_not); STATIC_ASSERT(!true == false, CPP_true_not); STATIC_ASSERT(!C_FALSE == C_TRUE, C_FALSE_not); STATIC_ASSERT(!C_FALSE == true, C_FALSE_not); STATIC_ASSERT(!false == C_TRUE, CPP_false_not); STATIC_ASSERT(!false == true, CPP_false_not); #endif // !defined(__cplusplus) /* Values specifically for use with C_bool_t */ #define TRUE true #define FALSE false #undef ASSERT_CONCAT_ #undef ASSERT_CONCAT #undef STATIC_ASSERT #endif
https://en.wikipedia.org/wiki/Compatibility_of_C_and_C%2B%2B
https://stackoverflow.com/questions/8284167/nested-structures-in-c-and-c