54 lines
2.5 KiB
Markdown
54 lines
2.5 KiB
Markdown
# Communication with your code
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## Configure File
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CMake allows you to access CMake variables from your code using `configure_file`. This command copies a file (traditionally ending in `.in` from one place to another, substituting all CMake variables it finds. If you want to avoid replacing existing `${}` syntax in your input file, use the `@ONLY` keyword. There's also a `COPY_ONLY` keyword if you are just using this as a replacement for `file(COPY`.
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This functionality is used quite frequently; for example, on `Version.h.in`:
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#### Version.h.in
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```cpp
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#pragma once
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#define MY_VERSION_MAJOR @PROJECT_VERSION_MAJOR@
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#define MY_VERSION_MINOR @PROJECT_VERSION_MINOR@
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#define MY_VERSION_PATCH @PROJECT_VERSION_PATCH@
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#define MY_VERSION_TWEAK @PROJECT_VERSION_TWEAK@
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#define MY_VERSION "@PROJECT_VERSION@"
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```
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#### CMake lines:
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```cmake
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configure_file (
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"${PROJECT_SOURCE_DIR}/include/My/Version.h.in"
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"${PROJECT_BINARY_DIR}/include/My/Version.h"
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)
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```
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You should include the binary include directory as well when building your project. If you want to put any true/false variables in a header, CMake has C specific `#cmakedefine` and `#cmakedefine01` replacements to make appropriate define lines.
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You can also (and often do) use this to produce `.cmake` files, such as the configure files (see [installing](https://cliutils.gitlab.io/modern-cmake/chapters/install/installing.html)).
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## Reading files
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The other direction can be done too; you can read in something (like a version) from your source files. If you have a header only library that you'd like to make available with or without CMake, for example, then this would be the best way to handle a version. This would look something like this:
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```cmake
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# Assuming the canonical version is listed in a single line
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# This would be in several parts if picking up from MAJOR, MINOR, etc.
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set(VERSION_REGEX "#define MY_VERSION[ \t]+\"(.+)\"")
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# Read in the line containing the version
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file(STRINGS "${CMAKE_CURRENT_SOURCE_DIR}/include/My/Version.hpp"
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VERSION_STRING REGEX ${VERSION_REGEX})
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# Pick out just the version
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string(REGEX REPLACE ${VERSION_REGEX} "\\1" VERSION_STRING "${VERSION_STRING}")
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# Automatically getting PROJECT_VERSION_MAJOR, My_VERSION_MAJOR, etc.
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project(My LANGUAGES CXX VERSION ${VERSION_STRING})
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```
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Above, `file(STRINGS file_name variable_name REGEX regex)` picks lines that match a regex; and the same regex is used to then pick out the parentheses capture group with the version part. Replace is used with back substitution to output only that one group.
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