CMake/Policies

The CMake Policy mechanism provides backwards compatibility as a first-class feature.

Motivation

CMake is an evolving project. The developers strive to support existing projects as much as possible as changes are made. Unfortunately there are some cases where it is not possible to fix bugs and preserve backwards compatibility at the same time. We give some examples here.

Interface of ADD_DEFINITIONS

Consider the add_definitions command:

 add_definitions(-DFOO)

When originally introduced the command was intended only to add simple definitions. Its implementation was simply to pass its arguments on to the compiler's command line. Since CMake supports configured header files using the configure_file command it is not necessary to pass complicated definitions on compile command lines. However, some project authors tried to do so anyway with code like

 add_definitions("-DFOO=\"some string\"")

but found that it did not work. The string

 -DFOO="some string"

would appear on the command line and the compiler would receive a definition equivalent to

 #define FOO some string

Some authors proceeded to work around the problem by adding escape sequences manually:

 add_definitions("-DFOO=\"\\\"some string\\\"\"")

The escape sequences work for some native build tools (such as Unix Makefiles) but not others. The proper way to deal with this issue was to fix the implementation in CMake to actually produce the correct escape sequences for each native build tool automatically.

Unfortunately introducing the fix would break existing projects that add their own escape sequences because the escapes themselves would be escaped. In order to support such projects no fix was introduced for years. This allowed many more projects to continue to suffer from the problem and add their own work-arounds which must now also be supported.

This problem with add_definitions is an example of a class of problems: how are we to fix an interface without breaking work-arounds for the very problem being fixed? The policy mechanism is a solution to this problem.

Magic Link Directories

When using CMake 2.4 or below projects may write this (wrong) code and it works by accident:

  add_executable(myexe myexe.c)
  target_link_libraries(myexe /path/to/libA.so B)

where "B" is meant to link "/path/to/libB.so". This code is incorrect because it asks CMake to link to B but does not provide the proper linker search path for it. The correct code would be

  link_directories(/path/to)
  add_executable(myexe myexe.c)
  target_link_libraries(myexe /path/to/libA.so B)

or even better

  add_executable(myexe myexe.c)
  target_link_libraries(myexe /path/to/libA.so /path/to/libB.so)

CMake 2.4 implemented the link to library A partly by adding -L/path/to to the linker command line. This allowed library B to be found even though no linker search path was provided for it. CMake 2.6 implements linking to library A by passing /path/to/libA.so directly to the linker as a path. This leaves out the -L/path/to which may prevent library B from being found.

While the code above leading to this problem is technically wrong it worked with a previous CMake release and needs to be supported. Therefore CMake 2.6 has support for passing the directories containing libraries whose full paths are known as linker search paths even though they are not needed for correct user code. Full compatibility would require us to support this behavior by default forever. That would allow new projects to be written with the same bug.

This problem is an example of a class of problems: how are we to fix an implementation without breaking projects depending on undocumented details of the original implementation? The policy mechanism is a solution to this problem.

Design Goals

The design goals for the CMake Policy mechanism were as follows:

1. Existing projects should build with versions of CMake newer than that used by the project authors
   • Users should not need to edit code to get the projects to build
   • Warnings may be issued but the projects should build
2. Correctness of new interfaces or bugs fixed in old ones should not be inhibited by compatibility requirements
   • Any reduction in correctness of the latest interface is not fair to new projects
3. Every change to CMake that may require changes to project code should be documented
   • Each change should also have a unique identifier that can be referenced by warning and error messages
4. We must be able to eventually remove code implementing compatibility with ancient CMake versions
   • Such removal is necessary to keep the code clean and allow internal refactoring
   • After such removal attempts to build projects written for ancient versions must fail with an informative message