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From: Bo Persson <bo@bo-persson.se>
Newsgroups: comp.lang.c++
Subject: Re: Global const objects missing from object file
Date: Sun, 10 Oct 2021 10:34:08 +0200
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Xref: csiph.com comp.lang.c++:81959

On 2021-10-10 at 08:55, Steve Keller wrote:
> 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.
> 

It only causes duplication if you take the address and return a pointer. 
Why do that for a constant defined in a header?