qmake Advanced Usage

Variables

Many of the variables used in project files are special variables that qmake uses when generating Makefiles, such as DEFINES, SOURCES, and HEADERS. It is possible for you to create variables for your own use; qmake creates new variables with a given name when it encounters an assignment to that name. For example:

 MY_VARIABLE = value

There are no restricitions on what you do to your own variables, as qmake will ignore them unless it needs to evaluate them when processing a scope.

You can also assign the value of a current variable to another variable by prefixing $$ to the variable name. For example:

 MY_DEFINES = $$DEFINES

Now the MY_DEFINES variable contains what is in the DEFINES variable at this point in the project file. This is also equivalent to:

 MY_DEFINES = $${DEFINES}

The second notation allows you to append the contents of the variable to another value without separating the two with a space. For example, the following will ensure that the final executable will be given a name that includes the project template being used:

 TARGET = myproject_$${TEMPLATE}

Variables can be used to store the contents of environment variables. These can be evaluated at the time that qmake is run, or included in the generated Makefile for evaluation when the project is built.

To obtain the contents of an environment value when qmake is run, use the $$(...) operator:

 DESTDIR = $$(PWD)
 message(The project will be installed in $$DESTDIR)

In the above assignment, the value of the PWD environment variable is read when the project file is processed.

To obtain the contents of an environment value at the time when the generated Makefile is processed, use the $(...) operator:

 DESTDIR = $$(PWD)
 message(The project will be installed in $$DESTDIR)

 DESTDIR = $(PWD)
 message(The project will be installed in the value of PWD)
 message(when the Makefile is processed.)

In the above assignment, the value of PWD is read immediately when the project file is processed, but $(PWD) is assigned to DESTDIR in the generated Makefile. This makes the build process more flexible as long as the environment variable is set correctly when the Makefile is processed.

The special $$[...] operator can be used to access various configuration options that were set when Qt was built:

 message(Qt version: $$[QT_VERSION])
 message(Qt is installed in $$[QT_INSTALL_PREFIX])
 message(Qt resources can be found in the following locations:)
 message(Documentation: $$[QT_INSTALL_DOCS])
 message(Header files: $$[QT_INSTALL_HEADERS])
 message(Libraries: $$[QT_INSTALL_LIBS])
 message(Binary files (executables): $$[QT_INSTALL_BINS])
 message(Plugins: $$[QT_INSTALL_PLUGINS])
 message(Data files: $$[QT_INSTALL_DATA])
 message(Translation files: $$[QT_INSTALL_TRANSLATIONS])
 message(Settings: $$[QT_INSTALL_SETTINGS])
 message(Examples: $$[QT_INSTALL_EXAMPLES])
 message(Demonstrations: $$[QT_INSTALL_DEMOS])

The variables accessible with this operator are typically used to enable third party plugins and components to be integrated with Qt. For example, a Qt Designer plugin can be installed alongside Qt Designer's built-in plugins if the following declaration is made in its project file:

 target.path = $$[QT_INSTALL_PLUGINS]/designer
 INSTALLS += target

 HEADERS = model.h
 HEADERS += $$OTHER_HEADERS
 HEADERS = $$unique(HEADERS)

 defineReplace(functionName){
     #function code
 }

 defineReplace(headersAndSources) {
     variable = $$1
     names = $$eval($$variable)
     headers =
     sources =

     for(name, names) {
         header = $${name}.h
         exists($$header) {
             headers += $$header
         }
         source = $${name}.cpp
         exists($$source) {
             sources += $$source
         }
     }
     return($$headers $$sources)
 }

 count(options, 2) {
     message(Both release and debug specified.)
 }

 defineTest(allFiles) {
     files = $$ARGS

     for(file, files) {
         !exists($$file) {
             return(false)
         }
     }
     return(true)
 }

 CONFIG += myfeatures