How it works

We have introduced two variables: USE_LIBRARY and BUILD_SHARED_LIBS. Both of them have been set to OFF. As detailed in the CMake language documentation, true or false values can be expressed in a number of ways:

  • A logical variable is true if it is set to any of the following: 1, ON, YES, TRUE, Y, or a non-zero number.
  • A logical variable is false if it is set to any of the following: 0, OFF, NO, FALSE, N, IGNORE, NOTFOUND, an empty string, or it ends in the suffix -NOTFOUND.

The USE_LIBRARY variable will toggle between the first and the second behavior. BUILD_SHARED_LIBS is a global flag offered by CMake. Remember that the add_library command can be invoked without passing the STATIC/SHARED/OBJECT argument. This is because, internally, the BUILD_SHARED_LIBS global variable is looked up; if false or undefined, a static library will be generated.

This example shows that it is possible to introduce conditionals to control the execution flow in CMake. However, the current setup does not allow the toggles to be set from outside, that is, without modifying CMakeLists.txt by hand. In principle, we want to be able to expose all toggles to the user, so that configuration can be tweaked without modifying the code for the build system. We will show how to do that in a moment.

The () in else() and endif() may surprise you when starting to read and write CMake code. The historical reason for these is the ability to indicate the scope. For instance, it is possible instead to use if(USE_LIBRARY) ... else(USE_LIBRARY) ... endif(USE_LIBRARY) if this helps the reader. This is a matter of taste.
When introducing the _sources variable, we have indicated to readers of this code that this is a local variable that should not be used outside the current scope by prefixing it with an underscore.
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