11.2.3 Importing Other Projects

A compilation unit in a source file in one project may depend on compilation units in source files in other projects. To compile this unit under control of a project file, the dependent project must import the projects containing the needed source files. This effect is obtained using syntax similar to an Ada with clause, but where withed entities are strings that denote project files.

As an example, suppose that the two projects GUI_Proj and Comm_Proj are defined in the project files gui_proj.gpr and comm_proj.gpr in directories /gui and /comm, respectively. Suppose that the source files for GUI_Proj are gui.ads and gui.adb, and that the source files for Comm_Proj are comm.ads and comm.adb, where each set of files is located in its respective project file directory. Schematically:

     /gui
       gui_proj.gpr
       gui.ads
       gui.adb
     
     /comm
       comm_proj.gpr
       comm.ads
       comm.adb

We want to develop an application in directory /app that with the packages GUI and Comm, using the properties of the corresponding project files (e.g. the switch settings and object directory). Skeletal code for a main procedure might be something like the following:

     with GUI, Comm;
     procedure App_Main is
        ...
     begin
        ...
     end App_Main;

Here is a project file, app_proj.gpr, that achieves the desired effect:

     with "/gui/gui_proj", "/comm/comm_proj";
     project App_Proj is
        for Main use ("app_main");
     end App_Proj;

Building an executable is achieved through the command:

     gnatmake -P/app/app_proj

which will generate the app_main executable in the directory where app_proj.gpr resides.

If an imported project file uses the standard extension (gpr) then (as illustrated above) the with clause can omit the extension.

Our example specified an absolute path for each imported project file. Alternatively, the directory name of an imported object can be omitted if either

• The imported project file is in the same directory as the importing project file, or
• You have defined one or two environment variables that includes the directory containing the needed project file. The syntax of GPR_PROJECT_PATH and ADA_PROJECT_PATH is the same as the syntax of ADA_INCLUDE_PATH and ADA_OBJECTS_PATH: a list of directory names separated by colons (semicolons on Windows).

Thus, if we define ADA_PROJECT_PATH or GPR_PROJECT_PATH to include /gui and /comm, then our project file app_proj.gpr can be written as follows:

     with "gui_proj", "comm_proj";
     project App_Proj is
        for Main use ("app_main");
     end App_Proj;

Importing other projects can create ambiguities. For example, the same unit might be present in different imported projects, or it might be present in both the importing project and in an imported project. Both of these conditions are errors. Note that in the current version of the Project Manager, it is illegal to have an ambiguous unit even if the unit is never referenced by the importing project. This restriction may be relaxed in a future release.