Inclusion of unused symbols in object files by compiler in C vs C++

Question

This might be a dumb question, but maybe someone can provide some insight.

I have some global variables defined in a header file (yes yes I know that\'s bad, but thi

Answer 1

Don't define variables in headers - that is evil.

Only ever declare variables in headers - with an explicit extern keyword.

The difference is that C++ expressly requires the one definition rule - there may only be one definition of any given variable with external linkage in a C++ program.

Strictly, the C standard has the same requirement. However, Appendix J of the standard lists a common extension of allowing multiple uninitialized definitions to be treated as one - it is called 'common definitions' because it is similar to the behaviour of COMMON blocks in (old school) Fortran (Fortran IV, aka Fortran 66, and Fortran 77, for example).

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Caveat: yes, if you know enough not to need to ask such questions, there can occasionally, very seldomly but just occasionally, be a reason for defining variables in a header. But such occasions are so few and far between that it is near enough accurate to say "don't define variables in headers".

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Christoph raises an interesting point about 'static const variables'. One weasel-wording way around the issue is to claim that "a constant isn't a variable". However, Christoph is right: in C++ in particular, static const 'variables' are used. In C, I thought such constants would tend to elicit 'unused' warnings from compilers; however, GCC 4.4.2 does not give any warnings when given this minimal code:

static const int x = 3;
extern int p(void);
int p(void)
{
    return(3);
}

It does not complain about the unused x even under '-Wall -Wextra'. So, it is OK to define constants in headers - as in 'static const SomeType constName = InitialValue;'. At least, if your compiler is GCC, though the code also compiles without warning under Sun Studio compiler with 'cc -v':

C compiler: /compilers/v12/SUNWspro/bin/cc
cc: Sun C 5.9 SunOS_sparc Patch 124867-09 2008/11/25
acomp: Sun C 5.9 SunOS_sparc Patch 124867-09 2008/11/25
iropt: Sun Compiler Common 12 SunOS_sparc Patch 124861-13 2009/03/10
cg: Sun Compiler Common 12 SunOS_sparc Patch 124861-13 2009/03/10
ld: Software Generation Utilities - Solaris Link Editors: 5.10-1.486