Using Precompiled Headers
Precompiled headers are a performance feature supported by some compilers to compile a stable body of code, and store the compiled state of the code in a binary file. During subsequent compilations, the compiler will load the stored state, and continue compiling the specified file. Each subsequent compilation is faster because the stable code does not need to be recompiled.
qmake
supports the use of precompiled headers (PCH) on some platforms and build environments, including:
- Windows
- nmake
- Dsp projects (VC 6.0)
- Vcproj projects (VC 7.0 & 7.1)
- Mac OS X
- Makefile
- Xcode
- Unix
- GCC 3.4 and above
Adding Precompiled Headers to Your Project
Contents of the Precompiled Header File
The precompiled header must contain code which is stable and static throughout your project. A typical PCH might look like this:
Example: stable.h
// Add C includes here #if defined __cplusplus // Add C++ includes here #include <stdlib> #include <iostream> #include <vector> #include <QApplication> // Qt includes #include <QPushButton> #include <QLabel> #include "thirdparty/include/libmain.h" #include "my_stable_class.h" ... #endif
Note that a precompiled header file needs to separate C includes from C++ includes, since the precompiled header file for C files may not contain C++ code.
Project Options
To make your project use PCH, you only need to define the PRECOMPILED_HEADER
variable in your project file:
PRECOMPILED_HEADER = stable.h
qmake
will handle the rest, to ensure the creation and use of the precompiled header file. You do not need to include the precompiled header file in HEADERS
, as qmake
will do this if the configuration supports PCH.
The MSVC and g++ specs targeting Windows (and Windows CE) enable precompile_header
by default.
Using this option, you may trigger conditional blocks in your project file to add settings when using precompiled headers. For example:
precompile_header:!isEmpty(PRECOMPILED_HEADER) { DEFINES += USING_PCH }
Notes on Possible Issues
On some platforms, the file name suffix for precompiled header files is the same as that for other object files. For example, the following declarations may cause two different object files with the same name to be generated:
PRECOMPILED_HEADER = window.h SOURCES = window.cpp
To avoid potential conflicts like these, it is good practice to ensure that header files that will be precompiled are given distinctive names.
Example Project
You can find the following source code in the examples/qmake/precompile
directory in the Qt distribution:
mydialog.ui
<ui version="4.0" > <author></author> <comment></comment> <exportmacro></exportmacro> <class>MyDialog</class> <widget class="QDialog" name="MyDialog" > <property name="geometry" > <rect> <x>0</x> <y>0</y> <width>401</width> <height>70</height> </rect> </property> <property name="windowTitle" > <string>Mach 2!</string> </property> <layout class="QVBoxLayout" > <property name="margin" > <number>9</number> </property> <property name="spacing" > <number>6</number> </property> <item> <widget class="QLabel" name="aLabel" > <property name="text" > <string>Join the life in the fastlane; - PCH enable your project today! -</string> </property> </widget> </item> <item> <widget class="QPushButton" name="aButton" > <property name="text" > <string>&Quit</string> </property> <property name="shortcut" > <string>Alt+Q</string> </property> </widget> </item> </layout> </widget> <pixmapfunction>qPixmapFromMimeSource</pixmapfunction> <resources/> <connections/> </ui>
stable.h
/* Add C includes here */ #if defined __cplusplus /* Add C++ includes here */ # include <iostream> # include <QApplication> # include <QPushButton> # include <QLabel> #endif
myobject.h
#include <QObject> class MyObject : public QObject { public: MyObject(); ~MyObject(); };
myobject.cpp
#include <iostream> #include <QDebug> #include <QObject> #include "myobject.h" MyObject::MyObject() : QObject() { std::cout << "MyObject::MyObject()\n"; }
util.cpp
void util_function_does_nothing() { // Nothing here... int x = 0; ++x; }
main.cpp
#include <QApplication> #include <QPushButton> #include <QLabel> #include "myobject.h" #include "mydialog.h" int main(int argc, char **argv) { QApplication app(argc, argv); MyObject obj; MyDialog dialog; dialog.connect(dialog.aButton, SIGNAL(clicked()), SLOT(close())); dialog.show(); return app.exec(); }
precompile.pro
TEMPLATE = app LANGUAGE = C++ CONFIG += console precompile_header # Use Precompiled headers (PCH) PRECOMPILED_HEADER = stable.h HEADERS = stable.h \ mydialog.h \ myobject.h SOURCES = main.cpp \ mydialog.cpp \ myobject.cpp \ util.cpp FORMS = mydialog.ui
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