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

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

[ansis atteka]

Greetings,

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 


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

[Axel Freyn]

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  
	

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

[Dave Korn]

Axel Freyn wrote:
> Hi Ansis,
> 
> first: you should use gcc-help@gcc.gnu.org for questions like this...

  Indeed.  However, from the gcc@ point of view:

> Yes, this approach is save. 

  No, not remotely, and everyone on this list already knows why (or ought to
after a few moments thought, or else they're probably on the wrong list), so
I'll just point it out here for the record.  Anyone stumbling across this in
the archives can go check the reply on gcc-help(*) if they want a detailed
explanation.

    cheers,
      DaveK
-- 
(*) - http://gcc.gnu.org/ml/gcc-help/2009-11/msg00144.html