Doubt regarding extern usage

[solita]

Will this compile? I have 1 header file , and it is included in 2 source files.
1. What about re-declaration error ? (for arraysize... )
2. #include always = textual substitution ??? Or does it have some intelligence ?

my header file - consts.h:
const int arraysize = 15;

my sources file 1 - op1.cpp:

#include "consts.h"
char array1[arraysize];

my source file 2 - op2.cpp:

#include "consts.h"
char array2[arraysize];

[Banfa]

In C++ that is not a problem, a const automatically has static linkage scope so it is fine to put a const like that into a header file.

Additionally since C++ consts are much more constants than C consts (which are really just read only variables) the compiler is likely to be able to optimise away the variable arraysize away so that you don't end up with multiple copies of it. The cases where it can include places where you take a pointer to a reference to the constant.

[weaknessforcats]

The better solution is to define a const int that is sharable. That is, one that has external linkage:

//consts.cpp

extern const int arraysize = 15;


//consts.h
my header file - consts.h:
extern const int arraysize;

You should never define objects in header files. Every time the header is included you get another object. That may cause the link to fail and it may cause bloat.

[solita]

Do we need extern keyword in both cpp and .h file?.

[Banfa]

You need the extern in the cpp file because constant variables default to static linkage (unlike other variables that default to external linkage). To give the variable defined in the cpp file external linkage it needs to be explicitly stated.

Or the short version: Yes.