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Re: static constants in a class

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On 09/15/11 09:43 AM, Urs Thuermann wrote:
> How should one define a constant, say of type int, in a class?  The
> only way that worked for me in all cases is to delcare the constant in
> the class in the header file and define its value in the
> implementation file:
>
> ---- foo.hh ----
>      class Foo {
>      public:
>      	static const int N;
>      	...
>      };
> ---- foo.cc ----
>      #include "foo.hh"
>      const int Foo::N = 10;
> ----------------
>
> But this will prevent many optimizations by the compiler, e.g. when I
> compile
>
> ---- bar.cc ----
>      void f() {
>          int array[N];

This shouldn't compile, C++ doesn't have VLAs (yet, and they are 
optional in the new standard).

>          for (int i = 0; i<  N; i++) { ... }

This isn't really an optimisation issue, it is unlikely the compiler 
would use an immediate compare in a loop construct.

> since the compiler doesn't know the value of N.  Is it possible to
> have the constant in foo.hh, i.e. visible in all translation units
> including foo.hh, without getting other problems?
>
> I tried two other ways, both having problems:
>
> 1.  class Foo {
>      public:
>          static const int N;
>          ...
>      };
>      const int Foo::N = 10;
>
>      If I include this into multiple translation units I get multiple
>      definitions of N and therefore a linker error.

Correct.

> 2.  class Foo {
>      public:
>          static const int N = 10;
>          ...
>      };
>
>      This seemed to work at first glance, since the compiler will
>      optimize most accesses to N away and use the value directly.
>
>      But when N is used where a reference is needed, e.g.

It will work as long as the address of N isn't required.

>          some_std_list.push_back(N);
>
>      my compiler (gcc) produces an access to N which however is not
>      defined anywhere so I get a linker error again.

Correct.

> The traditional C way would be to use #define, but I wanted to avoid
> that.  Is there way to have the constant definition in the header
> file?

Use the cleaner C way: an enum constant.

enum{ N = 10 };

-- 
Ian Collins

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Thread

static constants in a class Urs Thuermann <urs@isnogud.escape.de> - 2011-09-14 23:43 +0200
  Re: static constants in a class Ian Collins <ian-news@hotmail.com> - 2011-09-15 10:21 +1200
  Re: static constants in a class Leigh Johnston <leigh@i42.co.uk> - 2011-09-15 00:06 +0100
    Re: static constants in a class Leigh Johnston <leigh@i42.co.uk> - 2011-09-15 00:16 +0100
  Re: static constants in a class Juha Nieminen <nospam@thanks.invalid> - 2011-09-15 06:46 +0000
  Re: static constants in a class Marc <marc.glisse@gmail.com> - 2011-09-15 07:59 +0000
    Re: static constants in a class Ian Collins <ian-news@hotmail.com> - 2011-09-15 20:09 +1200
      Re: static constants in a class Marc <marc.glisse@gmail.com> - 2011-09-15 08:11 +0000
        Re: static constants in a class Ian Collins <ian-news@hotmail.com> - 2011-09-15 21:20 +1200
          Re: static constants in a class Marc <marc.glisse@gmail.com> - 2011-09-15 14:03 +0000
            Re: static constants in a class Ian Collins <ian-news@hotmail.com> - 2011-09-16 09:32 +1200
              Re: static constants in a class Marc <marc.glisse@gmail.com> - 2011-09-16 04:28 +0000
  Re: static constants in a class Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2011-09-15 10:50 +0200
    Re: static constants in a class Ian Collins <ian-news@hotmail.com> - 2011-09-15 21:17 +1200
      Re: static constants in a class Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2011-09-15 11:48 +0200
      Re: static constants in a class Christopher <cpisz@austin.rr.com> - 2011-09-15 14:24 -0700
        Re: static constants in a class Ian Collins <ian-news@hotmail.com> - 2011-09-16 09:32 +1200
          Re: static constants in a class Leigh Johnston <leigh@i42.co.uk> - 2011-09-15 22:54 +0100
  Re: static constants in a class TonyO <guinness.tony@gmail.com> - 2011-09-15 02:50 -0700

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