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`.
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.
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)).
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:
```cmake
# Assuming the canonical version is listed in a single line
# This would be in several parts if picking up from MAJOR, MINOR, etc.
# Automatically getting PROJECT_VERSION_MAJOR, My_VERSION_MAJOR, etc.
project(My LANGUAGES CXX VERSION ${VERSION_STRING})
```
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.
