Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c++ > #82173

Re: Global const objects missing from object file

Show all headers | View raw

Paavo Helde <myfirstname@osa.pri.ee> writes:

> 10.10.2021 09:55 Steve Keller kirjutas:
>
>> Paavo Helde <myfirstname@osa.pri.ee> writes:
>>
>>> Such global const definitions normally appear in a common header file
>>> and would just cause duplicate linker symbols if they had external
>>> linkage.  Now they can be just optimized away in translation units
>>> which do not use them.
>>>
>>> I believe the original motivation was to have an easy replacement of C
>>> macros in header files, i.e. instead of
>>>
>>> #define a 42
>>>
>>> one can easily use
>>>
>>> const int a = 42;
>>>
>>> which would function almost exactly like a #define, plus it honors C++
>>> namespaces and cannot be undefined.
>>
>> If you write such "global" const objects into a header file they are
>> in fact not global, because the implicit 'static' makes them local to
>> file translation unit and this can even cause duplication:
>>
>>      $ cat foo.hh
>>      const int a = 42;
>>           const int *bar();
>>      $ cat x.cc
>>      #include "foo.hh"
>>           const int *bar() { return &a; }
>>      $ cat y.cc
>>      #include <iostream>
>>           #include "foo.hh"
>>           const int *foo() { return &a; }
>>           int main()
>>      {
>>          std::cout << (void *)foo() << '\n' << (void *)bar() << '\n';
>>      }
>>      $ g++ -Os -o foo x.cc y.cc
>>      $ ./foo
>>      0x56027d731008
>>      0x56027d731004
>>
>> For a small 'int' one may not care but for larger constant objects
>> this may be really bad.
>
> The unused const objects are routinely optimized away be the compiler,
> as you saw by yourself ("object file was empty").
>
> It's true that for larger and more complicated objects this might not
> work so well.  So don't do that.  Put the object in a single TU and
> provide functions to access it.  (Making the object extern is not safe
> in general because of the "Static Initialization Order Fiasco".)
>
>> To avoid it, I think you still have to write the defintion into only
>> one C++ source file using 'extern'
>>
>>      extern const int = 42;
>>
>> and put
>>
>>      extern const int a;
>>
>> into the header file.  I still don't see the advantage for C++ to
>> differ from C here.
>
> In some sense the do not differ.  The const globals in C++ are
> primarily meant for replacing C #define.  Each C #define is TU-specific
> and gets fully preprocessed by the preprocessor, no other TU-s are
> involved.

It appears you have sidestepped the central question here.  What a
definition like 'const int foo = 7;' (without any mention anywhere
of 'extern') does in C++ is, AFAICT, exactly the same as a similar
definition with static, namely 'static const int foo = 7;'.  If
these two definitions are exactly the same (and I believe they are),
why change the meaning of the version that doesn't say 'static'?
Why introduce what appears to be a gratuitous incompatibility with C
when there was already a perfectly good construct that could be used
(and works in C++ now) for defining a local constant?

Back to comp.lang.c++ | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Global const objects missing from object file Steve Keller <keller.steve@gmx.de> - 2021-10-08 22:47 +0200
  Re: Global const objects missing from object file red floyd <no.spam.here@its.invalid> - 2021-10-08 15:09 -0700
    Re: Global const objects missing from object file Steve Keller <keller.steve@gmx.de> - 2021-10-10 08:28 +0200
      Re: Global const objects missing from object file Paavo Helde <myfirstname@osa.pri.ee> - 2021-10-10 12:03 +0300
      Re: Global const objects missing from object file David Brown <david.brown@hesbynett.no> - 2021-10-10 12:29 +0200
  Re: Global const objects missing from object file Paavo Helde <myfirstname@osa.pri.ee> - 2021-10-09 10:47 +0300
    Re: Global const objects missing from object file scott@slp53.sl.home (Scott Lurndal) - 2021-10-09 14:32 +0000
      Re: Global const objects missing from object file Paavo Helde <myfirstname@osa.pri.ee> - 2021-10-09 21:39 +0300
    Re: Global const objects missing from object file Steve Keller <keller.steve@gmx.de> - 2021-10-10 08:55 +0200
      Re: Global const objects missing from object file Bo Persson <bo@bo-persson.se> - 2021-10-10 10:34 +0200
        Re: Global const objects missing from object file Steve Keller <keller.steve@gmx.de> - 2021-10-10 15:55 +0200
          Re: Global const objects missing from object file Paavo Helde <myfirstname@osa.pri.ee> - 2021-10-10 17:28 +0300
            Re: Global const objects missing from object file Steve Keller <keller.steve@gmx.de> - 2021-10-10 20:42 +0200
              Re: Global const objects missing from object file Paavo Helde <myfirstname@osa.pri.ee> - 2021-10-10 22:44 +0300
      Re: Global const objects missing from object file Paavo Helde <myfirstname@osa.pri.ee> - 2021-10-10 12:14 +0300
        Re: Global const objects missing from object file Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-10-29 06:29 -0700
          Re: Global const objects missing from object file Bo Persson <bo@bo-persson.se> - 2021-10-29 15:55 +0200
            Re: Global const objects missing from object file Öö Tiib <ootiib@hot.ee> - 2021-10-29 22:20 -0700
            Re: Global const objects missing from object file Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-11-07 04:51 -0800
          Re: Global const objects missing from object file scott@slp53.sl.home (Scott Lurndal) - 2021-10-29 14:44 +0000
            Re: Global const objects missing from object file Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-11-07 05:08 -0800
              Re: Global const objects missing from object file scott@slp53.sl.home (Scott Lurndal) - 2021-11-07 14:21 +0000
      Re: Global const objects missing from object file David Brown <david.brown@hesbynett.no> - 2021-10-10 12:33 +0200
  Re: Global const objects missing from object file "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2021-10-09 15:05 +0200
  Re: Global const objects missing from object file Juha Nieminen <nospam@thanks.invalid> - 2021-10-09 17:40 +0000

csiph-web