Communication with your code

Communication with your code#

Configure File#

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.

This functionality is used quite frequently; for example, on Version.h.in:

Version.h.in#

#pragma once

#define MY_VERSION_MAJOR @PROJECT_VERSION_MAJOR@
#define MY_VERSION_MINOR @PROJECT_VERSION_MINOR@
#define MY_VERSION_PATCH @PROJECT_VERSION_PATCH@
#define MY_VERSION_TWEAK @PROJECT_VERSION_TWEAK@
#define MY_VERSION "@PROJECT_VERSION@"

CMake lines:#

configure_file (
    "${PROJECT_SOURCE_DIR}/include/My/Version.h.in"
    "${PROJECT_BINARY_DIR}/include/My/Version.h"
)

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).

Reading files#

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:

# Assuming the canonical version is listed in a single line
# This would be in several parts if picking up from MAJOR, MINOR, etc.
set(VERSION_REGEX "#define MY_VERSION[ \t]+\"(.+)\"")

# Read in the line containing the version
file(STRINGS "${CMAKE_CURRENT_SOURCE_DIR}/include/My/Version.hpp"
    VERSION_STRING REGEX ${VERSION_REGEX})

# Pick out just the version
string(REGEX REPLACE ${VERSION_REGEX} "\\1" VERSION_STRING "${VERSION_STRING}")

# 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.

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