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67 lines
1.7 KiB
Markdown
67 lines
1.7 KiB
Markdown
+++
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title = "Creating precompiled headers with cmake"
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description = "A brief introduction to target_precompile_headers"
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date = 2020-11-12
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[taxonomies]
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categories = ["cmake"]
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+++
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## What are precompiled headers?
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They are a partially processed version of header files, this speeds up compilation because it doesn't have to repeatedly parse the original header.
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## How to use it
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The way to do it is to pass the header files you want precompiled to the `target_precompile_headers` command.
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Imagine this folder structure:
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```
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.
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├── CMakeLists.txt
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└── src
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├── header.h
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└── main.cpp
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1 directory, 3 files
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```
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Now we create a very basic CMakeLists.txt:
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```cmake
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cmake_minimum_required(VERSION 3.16)
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project(MyProject)
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set(HEADER_FILES
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src/header.h)
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set(SOURCE_FILES
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src/main.cpp
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${HEADER_FILES}
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)
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include_directories(src)
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add_executable(MyProject ${SOURCE_FILES})
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```
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Notice we require a minimum cmake version of 3.16, this is due to `target_precompile_headers` being added in that version.
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We add the command after the `add_executable` instruction to the private scope.
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It's recommended to use the private scope to prevent precompiled headers appearing in an installed target, consumers should decide whether they want to use precompiled headers or not.
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```cmake
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target_precompile_headers(MyProject PRIVATE ${HEADER_FILES})
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```
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When compiling you can see now that it indeed compiles the headers:
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```
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Scanning dependencies of target MyProject
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[ 33%] Building CXX object CMakeFiles/MyProject.dir/cmake_pch.hxx.gch
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[ 66%] Building CXX object CMakeFiles/MyProject.dir/src/main.cpp.o
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[100%] Linking CXX executable MyProject
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[100%] Built target MyProject
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```
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