Testing C include file for C++ compatibility without a C++ caller?

Newsgroup: comp.lang.c++

Subject: Testing C include file for C++ compatibility without a C++ caller?

From: mathog <dmathog@...>

Date: Thu, 17 Oct 2013 10:09:54 -0700



I have a C package that until recently was callable by C++ programs

without generating warnings. Everything is properly wrapped up in



#ifdef __cplusplus

extern "C" {

#endif



which I always assumed precluded the possibility of a cleanly compiling

C module throwing warnings and errors when compiled by a C++ compiler.

However, recent versions of g++ throw warnings like this when some C++

modules include AND use one of the include files in the package:



../../src/extension/internal/metafile-print.cpp:132:13: warning:

narrowing conversion of ‘((color >> 16) & 255u)’ from ‘uint32_t

{aka unsigned int}’ to ‘uint8_t {aka unsigned char}’ inside { } is

ill-formed in C++11 [-Wnarrowing]



These trace back to this line in the include file:



#define U_RGBA(r,g,b,a) (U_COLORREF){r,g,b,a}



which must be changed to this to eliminate the warning:



#define U_RGBA(r,g,b,a)

(U_COLORREF){(uint8_t)(r), (uint8_t)(g), (uint8_t)(b), (uint8_t)(a)}



Where U_COLORREF is defined earlier in that include file as:



typedef struct {

uint8_t Red; //!< Red color (0-255)

uint8_t Green; //!< Green color (0-255)

uint8_t Blue; //!< Blue color (0-255)

uint8_t Reserved; //!< Not used

} U_COLORREF, *PU_COLORREF;



The problem is that so far the warning only shows up when the U_RGBA is

actually USED in a C++ module, apparently with a 4 byte unsigned

(holding a value in the range 0-255) employed as one of the color values.



I have the c++ compiler line that when applied to the cpp file generates

the error message, but when run exactly the same way, but just on the

include file, like:



g++ .... -c include.h



there are no warnings.



Is it possible to test for this sort of issue using just the C++

compiler and the include file? (Using gcc or g++).



Thank you,



David Mathog

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