CMake Recommendations

This section includes several recommendations for how to wield CMake. Some of them are embodied in BLT, others are broader suggestions for CMake bliss.

Disable In-source Builds

BLT Enforces This

In-source builds clutter source code with temporary build files and prevent other out-of-source builds from being created. Disabling in-source builds avoids clutter and accidental checkins of temporary build files.

Avoid using Globs to Identify Source Files

Globs are evaluated at CMake configure time - not build time. This means CMake will not detect new source files when they are added to the file system unless there are other changes that trigger CMake to reconfigure.

The CMake documentation also warns against this.

Use Arguments instead of Options in CMake Macros and Functions

CMAKE_PARSE_ARGUMENTS allows Macros or Functions to support options. Options are enabled by passing them by name when calling a Macro or Function. Because of this, wrapping an existing Macro or Function in a way that passes through options requires if tests and multiple copies of the call. For example:

    my_function(arg1 arg2 arg3 OPTION)
    my_function(arg1 arg2 arg3)

Adding more options compounds the logic to achieve these type of calls.

To simplify calling logic, we recommend using an argument instead of an option.

    set(arg4_value ON)

my_function(arg1 arg2 arg3 ${arg4_value})

Prefer Explicit Paths to Locate Third-party Dependencies

Require passing explicit paths (ex: ZZZ_DIR) for third-party dependency locations. This avoids surprises with incompatible installs sprinkled in various system locations. If you are using off-the-shelf FindZZZ logic, also consider adding CMake checks to verify that FindZZZ logic actually found the dependencies at the location specified.

Error at Configure Time for Third-party Dependency Problems

Emit a configure error if an explicitly identified third-party dependency is not found or an incorrect version is found. If an explicit path to a dependency is given (ex: ZZZ_DIR) it should be valid or result in a CMake configure error.

In contrast, if you only issue a warning and automatically disable a feature when a third-party dependency is bad, the warning often goes unnoticed and may not be caught until folks using your software are surprised. Emitting a configure error stops CMake and draws attention to the fact that something is wrong. Optional dependencies are still supported by including them only if an explicit path to the dependency is provided (ex: ZZZ_DIR).

Add Headers as Source Files to Targets

BLT Macros Support This

This ensures headers are tracked as dependencies and are included in the projects created by CMake’s IDE generators, like Xcode or Eclipse.

Always Support make install

This allows CMake to do the right thing based on CMAKE_INSTALL_PREFIX, and also helps support CPack create release packages. This is especially important for libraries. In addition to targets, header files require an explicit install command.

Here is an example that installs a target and its headers:

# Install Targets for example lib
install(FILES ${example_headers} DESTINATION include)
install(TARGETS example
  EXPORT example