Instead of supplying different project files for debug and release, we can
define a single project file that queries an external variable (set either
on the command line or via an environment variable) in order to
conditionally define the appropriate settings. Again, assume that the
source files pack.ads, pack.adb, and proc.adb are
located in directory /common. The following project file,
build.gpr, queries the external variable named
defines an object directory and switch settings based on whether
the value is
"deb" (debug) or
"rel" (release), and where
the default is
project Build is for Main use ("proc"); type Style_Type is ("deb", "rel"); Style : Style_Type := external ("STYLE", "deb"); case Style is when "deb" => for Object_Dir use "debug"; when "rel" => for Object_Dir use "release"; for Exec_Dir use "."; end case; package Builder is case Style is when "deb" => for Default_Switches ("Ada") use ("-g"); for Executable ("proc") use "proc1"; when others => null; end case; end Builder; package Compiler is case Style is when "deb" => for Default_Switches ("Ada") use ("-gnata", "-gnato", "-gnatE"); when "rel" => for Default_Switches ("Ada") use ("-O2"); end case; end Compiler; end Build;
Style_Type is an example of a string type, which is the project
file analog of an Ada enumeration type but whose components are string literals
rather than identifiers.
Style is declared as a variable of this type.
external("STYLE", "deb") is known as an
external reference; its first argument is the name of an
external variable, and the second argument is a default value to be
used if the external variable doesn't exist. You can define an external
variable on the command line via the -X switch,
or you can use an environment variable
as an external variable.
case construct is expanded by the Project Manager based on the
Style. Thus the command
gnatmake -P/common/build.gpr -XSTYLE=deb
is equivalent to the gnatmake invocation using the project file debug.gpr in the earlier example. So is the command
"deb" is the default for
gnatmake -P/common/build.gpr -XSTYLE=rel
is equivalent to the gnatmake invocation using the project file release.gpr in the earlier example.