Ada Gnat project which includes differently-named files for different build configurations

Question

I have a Gnat/Gprbuild project with several build configurations. I have a main source file and an secondary ads file which the main source file includes:

with Secondary_File; use Secondary_File;

The problem is that in each configuration, the secondary file has a different name. For example, it may be called Secondary_File_1.ads for one config and Secondary_File_2.ads for another. This makes it impossible to use the above with statement.

In C, I would do something like this:

#ifdef BUILD_CFG_1
#include "secondary_file_1.h"
#else
#include "secondary_file_2.h"
#endif

Is there a clever way to do something like this in ADA, using the Gprbuild system?

Answer 1

Many purists reject the idea of preprocessing, but it’s possible using GNAT.

You can include this in a GPR-based build environment by writing your source, e.g. main.adb, like so:

with Secondary_File_$NUMBER;
procedure Main is
begin
   null;
end Main;

(observe the $NUMBER) and the project file like so:

project Prj is

   for Main use ("main.adb");

   --  Configurations
   type Config_Type is ("config_1", "config_2");
   --  Which one? (default is "config_1")
   Config : Config_Type := external ("CONFIG", "config_1");

   package Compiler is
      case Config is
         when "config_1" =>
            for Switches ("main.adb") use ("-gnateDNUMBER=1");
         when "config_2" =>
            for Switches ("main.adb") use ("-gnateDNUMBER=2");
      end case;
   end Compiler;

end Prj;

Compiling gives

$ gprbuild -Pprj
Compile
   [Ada]          main.adb
main.adb:1:06: error: file "secondary_file_1.ads" not found
gprbuild: *** compilation phase failed

(the compilation looked for secondary_file_1.ads)

$ gprbuild -Pprj -XCONFIG=config_2
Compile
   [Ada]          main.adb
main.adb:1:06: error: file "secondary_file_2.ads" not found
gprbuild: *** compilation phase failed

(the compilation looked for secondary_file_2.ads)