CMake precompiled header support via custom PCH compiler extension
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README.md

CMake Precompiled Headers

This module defines extra CXXPCH compiler that compiles .h into .pch/.gch.

For convenience it defines

target_precompiled_header(target [...] header
                          [REUSE other_target]
                          [TYPE type])

Uses given header as precompiled header for given target.

Optionally it may share compiled header object with other target, so it is precompiled just once.

Also header may be given different type that default c++-header.

Supported & tested platforms

  1. Windows with MSVC, tested on VS2015
  2. OSX with Clang, GCC, tested on OSX 10.10 & Xcode 6.1
  3. Linux with GCC, tested on Ubuntu 14.04 LTS & GCC 4.8

Example

cmake_minimum_required(VERSION 2.6)

list(APPEND CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake/CMakePCHCompiler)

project(pchtest CXX CXXPCH)

add_library(engine SHARED src/engine.cpp src/library.cpp)
target_precompiled_header(engine src/prefix.h)

add_executable(demo src/demo.cpp)
target_link_libraries(demo engine)
target_precompiled_header(demo src/prefix.h REUSE engine)

What it is about?

CMake does not support precompiled headers by default. There are several modules providing precompiled header support, but all of them use custom commands and complicated wrappers to achieve their goals.

This module is somehow different. It defines a meta C++ compiler that simply just patches compiler command template for precompiled header case.

Next it treats precompiled header file as source file for CXXPCH that makes CMake use CXXPCH patched instead origin CXX compiler template. This ensures that all CXX language flags and specific settings such as these populated by add_definitions are also applied to precompiled header.

Passing same flags during precompiled header and other source files compilation is very important. It is simply impossible to catch all flags, such as these defined after calling target_precompiled_header or these using CMake internal variables such as add_definitions, using custom commands. This is the reason for such implementation.

This module is also transparent to source code. There is absolutely no need to change you source files. Only requirement is a precompiled header .h file added to given target via target_precompiled_header function.

Nevertheless this is not an ideal solution. In perfect world it is CMake that should handle precompiled headers generation internally, based on given compiler command templates. However this may be good start to request native support using simple API of:

target_precompiled_header(<target> <path/to/precompiled_header.h>)
target_precompiled_header(<target1> <target2>
                          <path/to/precompiled_header.h>)
target_precompiled_header(<target> <path/to/precompiled_header.h> REUSE
                          <other_target_to_reuse_precompiled_header_from>)

How does it work?

First, we define new compilers CPCH and CXXPCH using CMAKE_<LANG>_* variables. These compilers copy run templates and options from existing C and CXX compilers respectively.

Next we provide target_precompiled_header function that enabled precompiled header on given target.

Pre-compiler header is build in new target.pch subtarget using:

add_library(${target}.pch OBJECT ${header})

This is done on purpose because of few reasons:

  1. CMake does not allow to insert source file to existing target once it has been defined.

  2. Even if it was possible, we could not ensure precompiled header is built first in main target, but adding it as subtarget we can.

  3. We cannot prevent header.pch, which is output of CPCH/CXXPCH compiler from being linked when it is in part of main target, but if we put it into OBJECT library, then by definition we skip linking process. Also we take the result object to be a recompiled header for main target.

License

Copyright (c) 2014-2015 Adam Strzelecki. All rights reserved.

This code is licensed under the MIT License:

The MIT License

License for the specific language governing rights and limitations under Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.