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\chapter Using Precompiled Headers
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\target About
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\section1 About Precompiled Headers
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\index About Precompiled Headers
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\index Using Precompiled Headers
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\index Precompiled Headers
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\index PCH
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Precompiled headers are a performance feature supported by some
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compilers to compile a stable body of code, and store the compiled
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state of the code in a binary file. During subsequent compilations,
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the compiler will load the stored state, and continue compiling the
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specified file. Each subsequent compilation is faster because the
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stable code does not need to be recompiled.
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\e qmake supports the use of precompiled headers (PCH) on some
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platforms and build environments, including:
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\list
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\i Windows
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\list
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\i nmake
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\i Dsp projects (VC 6.0)
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\i Vcproj projects (VC 7.0 \& 7.1)
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\endlist
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\i Mac OS X
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\list
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\i Makefile
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\i Xcode
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\i GCC 3.3 and up
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\endlist
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\i Unix
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\list
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\i GCC 3.4 and up
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\endlist
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\endlist
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\target ADD_PCH
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\section1 Adding PCH to your project
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\target PCH_CONTENTS
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\section2 Contents of the precompiled header file
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The precompiled header must contain code which is \e stable
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and \e static throughout your project. A typical PCH might look
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like this:
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\section3 stable.h
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\code
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/* Add C includes here */
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#if defined __cplusplus
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/* Add C++ includes here */
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#include <stdlib>
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#include <iostream>
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#include <vector>
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#include <ntqapplication.h> // Qt includes
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#include <ntqpushbutton.h>
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#include <ntqlabel.h>
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#include "thirdparty/include/libmain.h"
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#include "my_stable_class.h"
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...
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#endif
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\endcode
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Note that a precompiled header file needs to separate C includes from
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CPP includes, since the precompiled header file for C files may not
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contain C++ code.
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\target PROJECT_OPTIONS
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\section2 Project options
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To make your project use PCH, the only thing you need to change in
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your project settings (.pro), is to include the PRECOMPILED_HEADER option:
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\code
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PRECOMPILED_HEADER = stable.h
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\endcode
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\e qmake will handle the rest, to ensure the creation and use of the
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precompiled header file. You do not need to include the precompiled
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header file in HEADERS, as qmake will do this if the configuration
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supports PCH.
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All platforms that support precompiled headers have the configuration
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option \Bold precompile_header set. Using this option, you may trigger
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conditional blocks in your .pro file, to add settings when using PCH.
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For example:
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\code
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precompile_header:!isEmpty(PRECOMPILED_HEADER) {
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DEFINES += USING_PCH
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}
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\endcode
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\target EXAMPLE_PROJECT
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\section1 Example project
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You can find the following source code in the
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\e{qt/qmake/examples/precompile} directory:
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\Bold mydialog.ui
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\quotefile precompile/mydialog.ui
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\skipto <!
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\printuntil </UI>
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\Bold stable.h
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\quotefile precompile/stable.h
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\skipto /*
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\printuntil #endif
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\Bold myobject.h
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\quotefile precompile/myobject.h
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\skipto #include
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\printuntil }
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\Bold myobject.cpp
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\quotefile precompile/myobject.cpp
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\skipto #include
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\printuntil }
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\Bold util.cpp
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\quotefile precompile/util.cpp
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\skipto void
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\printuntil }
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\Bold main.cpp
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\quotefile precompile/main.cpp
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\skipto #include
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\printuntil }
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\Bold precompile.pro
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\quotefile precompile/precompile.pro
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\skipto #
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\printuntil .ui
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