Question

I have a static library written in C++ and I have a structure describing data format, i.e.

struct Format{     long fmtId;     long dataChunkSize;     long headerSize;      Format(long, long, long);      bool operator==(Format const & other) const; }; 

Some of data formats are widely used, like {fmtId=0, dataChunkSize=128, headerSize=0} and {fmtId=0, dataChunkSize=256, headerSize=0}

Some data structure classes receive format in constructor. I’d like to have some sort of shortcuts for those widely used formats, like a couple of global Format members gFmt128, gFmt256 that I can pass by reference. I instantiate them in a .cpp file like

Format gFmt128(0, 128, 0);

and in .h there is

extern Format gFmt128;

also, I declare Format const & Format::Fmt128(){return gFmt128;} and try to use it in the main module.

But if I try and do it in the main module that uses the lib, the linker complains about unresolved external gFmt128.

How can I make my library ‘export’ those global vars, so I can use them from other modules?

Answer 1

Don’t use the static keyword on global declarations. Here is an article explain the visibility of variables with/without static. The static gives globals internal linkage, that is, only visible in the translation unit they are declared in.