UNITY_BUILD_UNIQUE_ID

The name of a valid C-identifier which is set to a unique per-file value during unity builds.

When this property is populated and when UNITY_BUILD is true, the property value is used to define a compiler definition of the specified name. The value of the defined symbol is unspecified, but it is unique per file path.

Given:

set_target_properties(myTarget PROPERTIES
  UNITY_BUILD "ON"
  UNITY_BUILD_UNIQUE_ID "MY_UNITY_ID"
)

the MY_UNITY_ID symbol is defined to a unique per-file value.

One known use case for this identifier is to disambiguate the variables in an anonymous namespace in a limited scope. Anonymous namespaces present a problem for unity builds because they are used to ensure that certain variables and declarations are scoped to a translation unit which is approximated by a single source file. When source files are combined in a unity build file, those variables in different files are combined in a single translation unit and the names clash. This property can be used to avoid that with code like the following:

// Needed for when unity builds are disabled
#ifndef MY_UNITY_ID
#define MY_UNITY_ID
#endif

namespace { namespace MY_UNITY_ID {
  // The name 'i' clashes (or could clash) with other
  // variables in other anonymous namespaces
  int i = 42;
}}

int use_var()
{
  return MY_UNITY_ID::i;
}

The pseudononymous namespace is used within a truly anonymous namespace. On many platforms, this maintains the invariant that the symbols within do not get external linkage when performing a unity build.