# UNITY_BUILD

Added in version 3.16.

When this property is set to true, the target source files will be combined into batches for faster compilation. This is done by creating a (set of) unity sources which `#include` the original sources, then compiling these unity sources instead of the originals. This is known as a Unity or Jumbo build.

CMake provides different algorithms for selecting which sources are grouped together into a bucket. Algorithm selection is decided by the `UNITY_BUILD_MODE` target property, which has the following acceptable values:

- `BATCH` When in this mode CMake determines which files are grouped together. The `UNITY_BUILD_BATCH_SIZE` property controls the upper limit on how many sources can be combined per unity source file.

- `GROUP` When in this mode each target explicitly specifies how to group source files. Each source file that has the same `UNITY_GROUP` value will be grouped together. Any sources that don't have this property will be compiled individually. The `UNITY_BUILD_BATCH_SIZE` property is ignored when using this mode.

If no explicit `UNITY_BUILD_MODE` has been specified, CMake will default to `BATCH`.

Unity builds are supported for the following languages:

`C`

  Added in version 3.16.

`CXX`

  Added in version 3.16.

`OBJC`

  Added in version 3.29.

`OBJCXX`

  Added in version 3.29.

For targets that mix source files from more than one language, CMake separates the languages such that each generated unity source file only contains sources for a single language.

This property is initialized by the value of the `CMAKE_UNITY_BUILD` variable when a target is created.

Note

Projects should not directly set the `UNITY_BUILD` property or its associated `CMAKE_UNITY_BUILD` variable to true. Depending on the capabilities of the build machine and compiler used, it might or might not be appropriate to enable unity builds. Therefore, this feature should be under developer control, which would normally be through the developer choosing whether or not to set the `CMAKE_UNITY_BUILD` variable on the `cmake(1)` command line or some other equivalent method. However, it IS recommended to set the `UNITY_BUILD` target property to false if it is known that enabling unity builds for the target can lead to problems.

## ODR (One definition rule) errors

When multiple source files are included into one source file, as is done for unity builds, it can potentially lead to ODR errors. CMake provides a number of measures to help address such problems:

- Any source file that has a non-empty `COMPILE_OPTIONS`, `COMPILE_DEFINITIONS`, `COMPILE_FLAGS`, or `INCLUDE_DIRECTORIES` source property will not be combined into a unity source.

- Any source file which is scanned for C++ module sources via `CXX_SCAN_FOR_MODULES`, `CXX_SCAN_FOR_MODULES`, or membership of a `CXX_MODULES` file set will not be combined into a unity source. See `cmake-cxxmodules(7)` for details.

- Projects can prevent an individual source file from being combined into a unity source by setting its `SKIP_UNITY_BUILD_INCLUSION` source property to true. This can be a more effective way to prevent problems with specific files than disabling unity builds for an entire target.

- Projects can set `UNITY_BUILD_UNIQUE_ID` to cause a valid C-identifier to be generated which is unique per file in a unity build. This can be used to avoid problems with anonymous namespaces in unity builds.

- The `UNITY_BUILD_CODE_BEFORE_INCLUDE` and `UNITY_BUILD_CODE_AFTER_INCLUDE` target properties can be used to inject code into the unity source files before and after every `#include` statement.

- The order of source files added to the target via commands like `add_library()`, `add_executable()` or `target_sources()` will be preserved in the generated unity source files. This can be used to manually enforce a specific grouping based on the `UNITY_BUILD_BATCH_SIZE` target property.