Re: Making a struct field constant, when this struct is imported to a particular Cpp files

[Axel Freyn <axel-freyn at gmx dot de>]

Hi Ansis,

first: you should use gcc-help@gcc.gnu.org for questions like this...


On Thu, Nov 12, 2009 at 05:44:58AM -0800, ansis atteka wrote:
> I have a struct in a header file. And I would like to have some of this
> struct member fields to be constant, in case if this header file is included
> from some particular Cpp files. Motivation for this is to avoid some silly
> programmer errors by accidentally writing to this variable, when developer
> is not supposed to do that. I guess that it is possible to catch these
> errors already at the compile time with a const keyword.
> 
> I already have approach, but I am not quite sure whether it is
> safe(especially regarding the compiler optimizations). Below are uplaoded
> files.
> 
> In this case util.cpp is not allowed to edit S.y contents, while rw.cpp is
> allowed to do that. File h.hpp contains the struct which is customized at
> the preprocessor execution time by looking at the READONLY define.
> 
> If this is not safe, are there any other approaches? I am afraid that
> compiler might optimize out something. Assumption is that other threads will
> not touch this member. 
> 
> Regards,
> Ansis
> 
> http://old.nabble.com/file/p26318895/Makefile Makefile 
> http://old.nabble.com/file/p26318895/rw.cpp rw.cpp 
> http://old.nabble.com/file/p26318895/h.hpp h.hpp 
> http://old.nabble.com/file/p26318895/util.cpp util.cpp 
> http://old.nabble.com/file/p26318895/util.hpp util.hpp 
> 
Yes, this approach is save. However, some minor remarks:

 - I'm not completely sure whether the position of "-DONLY_READ" is
   important for some versions of gcc ??? - it is safer, when you write
   it as argument before the first file:
   g++ -O3 -DONLY_READ -c util.cpp -o util.o 


 - instead of using "-DONLY_READ" in the makefile it might be easier to
   use "#define ONLY_READ" inside the cpp-File - before including h.hpp:

   #define ONLY_READ
   #include "util.hpp"

   That creates exactly the same code.

 - The code might be easier to understand if you write:
     ONLY_READ int *y;
   instead of 
     #ifdef ONLY_READ
     	const int * y;
     #endif
     #ifndef ONLY_READ
     	int * y;
     #endif
   inside h.hpp, and
   "#define ONLY_READ const"
   in all cpp-Files where you want it to be constant (util.cpp)
   and
   "#define ONLY_READ "
   in all cpp-Files, where you want read-access (rw.cpp). These defines
   have to be before the #include-Command.

 - if you use "ONLY_READ int *y;", you can also try
   
   #ifndef ONLY_READ
   #define ONLY_READ
   #endif
   ONLY_READ int *y;
   
   in the header-file h.hpp. This allows to include without "#define
   ONLY_READ":
   - if "ONLY_READ" is not defined, you get read-Access
   - if "ONLY_READ" is defined as "const", you have constant access.


Axel