use of #define (...) #undefine ?

[sanctus]

Hello,
I'm studying a program where at some point something is defined and little bit further undefined. What could be the use of this? Is it to allocate some memory to things and then not touching them until re-defined elsewhere?

I don't know if posting the code clears the things up, but anyway it is (source cmbeasy):

#define CACHESPLINE(a) Spline* a; double a##X, a##Y      /* a define for the spline variables for convenience: one refernce and two caching double variables */
 
 
 CACHESPLINE(Visibility);
 CACHESPLINE(DVisibility);
 CACHESPLINE(DDVisibility);
 CACHESPLINE(Tau2A);
 CACHESPLINE(Tau2ADot);
 CACHESPLINE(Tau2ADotToA);
 CACHESPLINE(Opac);
 CACHESPLINE(D2Kappa);
 CACHESPLINE(Expmmu);
 CACHESPLINE(FreeElectronFraction);
 
 CACHESPLINE(Tau2Rho);
 CACHESPLINE(Tau2Rho_g);
 CACHESPLINE(Tau2Rho_nu);
 CACHESPLINE(Tau2Rho_nuNR);
 CACHESPLINE(Tau2Rho_m);
 CACHESPLINE(Tau2Rho_b);
 CACHESPLINE(Tau2Rho_cdm);
 
 
 CACHESPLINE(Tau2RhoDot);
 CACHESPLINE(Tau2RhoDot_m); 
 CACHESPLINE(Tau2RhoDot_g);
 CACHESPLINE(Tau2RhoDot_nu);
 
 CACHESPLINE(Tau2GRho);
 CACHESPLINE(Tau2GRhoDot);
(...)
#undef CACHESPLINE
 

Thanks in advance

[gpraghuram]

Hi,
Basically the used after the #define CACHESPLINE will be expecting this and when it is not needed they are calling the #undef
Others acn add more info on this

Thanks
Raghuram

[sanctus]

But are the values stored somewhere after the undefine comand?

[Banfa]

The main reason to undef a symbol is if you want to use it again later for something else.

In your example do all those code lines appear in the same file?

I technique I have seen and used is to have a header file that uses a symbol like your CACHESPLINE to define various instances of an item. Then in the C/C++ files the synbol is defined to 1 thing and the header is included then the symbol is undefined, then the symbol is defined to something else and the header is included again and the symbol is undefined again.

In this way you can define all the data required about a set of things in 1 place (the header) and then declare, define and initialise it just by defining the symbol correctly.