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Re: Global const objects missing from object file

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Steve Keller <keller.steve@gmx.de> wrote:
> If I remove the 'const' in the definition and in the extern
> declaration it works as expected.  Also if add an 'extern' to the
> definition in x.cc so that it is
> 
> extern const int a = 42;
> 
> the object 'a' is created in x.o.  Can someone explain this?

This is one key difference between C and C++.

In C, const variables at the global namespace level have external linkage
by default. In other words, they are implicitly 'extern'.

In C++, const variables at the namespace level (including the global
namespace) have internal linkage by default. In other words, they are
implicitly 'static'.

In C, if you want a const variable in the global namespace to have
internal linkage, you need to explicitly specify 'static' in its
declaration.

In C++, if you want a const variable at a namespace level (including
the global namespace) to have external linkage, you need to explicitly
specify 'extern' in its declaration.

(In C++ this is true for *any* type, not just basic types.)

Previously this was a minor nuisance in C++ because a 'const' variable
in a header would be duplicated in each object file that used it, if
the compiler couldn't optimize it away (ie. "inline" it). If you wanted
only one instance of that const variable, you had to 'extern' it manually.

C++17 added support for "inline const" variables, which does this for
you automatically (in the same way as 'inline' does with functions).

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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

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