Re: Linking errors after moving to SSQLSs, macro problem

[Warren Young]

Simon Pickles wrote:
> 
> My app was building and running without using SSQLS, but seeing the 
> simplicity and extra functionality they afford, I moved to SSQLS. 

A search for those errors on the list would have turned up the answer. 
We've certainly talked about it enough times.

The next version of the user manual covers it explicitly:

> It's convenient to define an SSQLS in a header file so you can use it in multiple
> modules. You run into a bit of a problem, though, because each SSQLS includes a few static
> data members to hold information common to all structures of that type. (The table name
> and the list of field names.) When you #include that header in more than one module, you
> get a multiply-defined symbol error at link time.
> 
> The way around this is to define the preprocessor macro MYSQLPP_SSQLS_NO_STATICS in
> all but one of the modules that use the header definining the SSQLS. When this macro is
> defined, it suppresses the static data members in any SSQLS defined thereaftear.
> 
> Imagine we have a file my_ssqls.h which includes a sql_create_N macro call to define
> an SSQLS, and that that SSQLS is used in at least two modules. One we'll call foo.cpp, and
> we'll say it's just a user of the SSQLS; it doesn't "own" it. Another of the modules,
> my_ssqls.cpp uses the SSQLS more heavily, so we've called it the owner of the SSQLS. If
> there aren't very many modules, this works nicely:
> 
> // File foo.cpp, which just uses the SSQLS, but doesn't "own" it:
> #define MYSQLPP_SSQLS_NO_STATICS
> #include "my_ssqls.h"
> 
> // File my_ssqls.cpp, which owns the SSQLS, so we just #include it directly
> #include "my_ssqls.h"
> 
> If there are many modules that need the SSQLS, adding all those #defines can be a
> pain. In that case, it's easier if you flip the above pattern on its head:
> 
> // File my_ssqls.h:
> #if !defined(EXPAND_MY_SSQLS_STATICS)
> #   define MYSQLPP_SSQLS_NO_STATICS
> #endif
> sql_create_X(Y, Z....) // the SSQLS definition
> 
> // File foo.cpp, a mere user of the SSQLS:
> #include "my_ssqls.h"
> 
> // File my_ssqls.cpp, which owns the SSQLS:
> #define EXPAND_MY_SSQLS_STATICS
> #include "my_ssqls.h"
> 
> Note that due to a compiler limitation, you can't use the MYSQLPP_SSQLS_NO_STATICS
> feature with Visual C++ 2003. As instructed in README.vc, you have to disable this feature
> in order to get the SSQLS header files to compile. Having done that, the SSQLS feature
> works fine as long as you can live with using each structure type in a single module.
> Visual C++ 2005 and newer don't suffer from this limitation.