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## Using Precompiled Headers

### About 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

### Adding PCH to your project

#### Contents of the precompiled header file

The precompiled header must contain code which is stable and static throughout your project. When compiling Qt programs, it may be wise to add an include to qt.h in your precompiled header, to significantly increase compilation speed. A typical PCH might look like this:

##### stable.h

            /* Add C includes here */

            #if defined __cplusplus
            /* Add C++ includes here */
            #include <stdlib>
            #include <iostream>
            #include <vector>
            #include <qt.h> // Includes all of Qt
            #include "thirdparty/include/libmain.h"
            #include "my_stable_class.h"
            ...
            #endif

Note that a precompiled header file needs to separate C includes from CPP includes, since C files and CPP files cannot share the same precompiled header file.

#### Project options

To make your project use PCH, the only thing you need to change in your project settings (.pro), is to include the PRECOMPH option:

            PRECOMPH = stable.h

qmake will handle the rest, to ensure the creation and use of the precompiled header file.

##### Advanced Windows option

This is used on Windows only

On the Windows platform, it is common to generate the precompiled header file, based on a specified `.cpp` file. This is because MSVC also lets you precompile functions and whole classes, to further increase compilation speed.

You can specify your own `.cpp` file like this:

            PRECOMPCPP = stable.cpp

Note that with qmake, you don't have to specify this `.cpp` file, since qmake will generate one for you, which will include the header file specified in `PRECOMPH`. Do not use this feature for multiplatform projects.

### Example project

You can find the following source code in the qt/qmake/examples/precompile directory:

mydialog.ui

        <!DOCTYPE UI><UI version="3.3" stdsetdef="1">
        <class>MyDialog</class>
        <widget class="QDialog">
            <property name="name">
                <cstring>MyDialog</cstring>
            </property>
            <property name="caption">
                <string>Mach 2!</string>
            </property>
            <vbox>
                <widget class="QLabel">
                    <property name="name">
                        <cstring>aLabel</cstring>
                    </property>
                    <property name="text">
                        <string>Join the life in the fastlane; - PCH enable your project today! -</string>
                    </property>
                </widget>
                <widget class="QPushButton">
                    <property name="name">
                        <cstring>aButton</cstring>
                    </property>
                    <property name="text">
                        <string>&amp;Quit</string>
                    </property>
                    <property name="accel">
                        <string>Alt+Q</string>
                    </property>
                </widget>
            </vbox>
        </widget>
        </UI>

stable.h

        /* Add C includes here */

        #if defined __cplusplus
        /* Add C++ includes here */

        # include <iostream>
        # include <qapplication.h>
        # include <qpushbutton.h>
        # include <qlabel.h>
        #endif

myobject.h

        #include <qobject.h>

        class MyObject : public QObject
        {
        public:
            MyObject();
            ~MyObject();
        };

myobject.cpp

        #include <iostream>
        #include <qobject.h>
        #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.h>
        #include <qpushbutton.h>
        #include <qlabel.h>
        #include "myobject.h"
        #include "mydialog.h"

        int main(int argc, char **argv)
        {
            QApplication app(argc, argv);

            MyObject obj;
            MyDialog dia;
            app.setMainWidget( &dia );
            dia.connect( dia.aButton, SIGNAL(clicked()), SLOT(close()) );
            dia.show();

            return app.exec();
        }

precompile.pro

        #############################################
        #
        # Example for using Precompiled Headers
        #
        #############################################
        TEMPLATE  = app
        LANGUAGE  = C++
        CONFIG   += console

        # Use Precompiled headers (PCH)
        PRECOMPH  = stable.h

        HEADERS  += stable.h \
                    myobject.h
        SOURCES  += main.cpp \
                    myobject.cpp \
                    util.cpp
        FORMS     = mydialog.ui

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