# Question about unity build and interface library / ODR

**URL:** https://discourse.cmake.org/t/question-about-unity-build-and-interface-library-odr/6279
**Category:** Usage
**Created:** [August 16, 2022, 8:00am UTC](https://discourse.cmake.org/t/question-about-unity-build-and-interface-library-odr/6279 "2022-08-16T08:00:13Z")
**Posts on this page:** 3
**Page:** 1

<div class="post-metadata">

### Author: ![Milerius](https://discourse.cmake.org/user_avatar/discourse.cmake.org/milerius/32/2682_2.png) [@Milerius](https://discourse.cmake.org/u/Milerius)
#### Post date: [August 16, 2022, 8:00am UTC](https://discourse.cmake.org/t/question-about-unity-build-and-interface-library-odr/6279/1 "2022-08-16T08:00:13Z")

</div>

Let’s take the following example:

```nohighlight
add_library(tw_base INTERFACE)
target_sources(tw_base INTERFACE Data.cpp)
target_include_directories(tw_base INTERFACE ${CMAKE_CURRENT_SOURCE_DIR})
set_target_properties(tw_base PROPERTIES PUBLIC_HEADER Data.h)

add_library(tw::base ALIAS tw_base)

if (TW_UNIT_TESTS)
    add_library(tw_base_tests INTERFACE)
    target_sources(tw_base_tests INTERFACE tests/DataTests.cpp)
    target_link_libraries(tw_base_tests INTERFACE tw::base)
    add_library(tw::base_tests ALIAS tw_base_tests)
endif()

```

```nohighlight
add_library(MyAwesomeStaticLib sources...)
target_link_libraries(MyAwesomeStaticLib PUBLIC tw::base)

```

Later in tests folder:

```nohighlight
add_executable(tests ${test_sources})
target_link_libraries(tests gtest_main MyAwesomeStaticLib tw::base_tests)

```

Why Data.cpp is included in multiple unity files (tests sources and lib sources) ? I would like Data.cpp to be compiled in unity build only for `MyAwesomeStaticLib`

I would like Data.cpp to be compiled only on MyAwesomeStaticLib and not the unit test target, Any hints on how to do that properly?

---

<div class="post-metadata">

### Author: ![McMartin](https://discourse.cmake.org/user_avatar/discourse.cmake.org/mcmartin/32/150_2.png) [@McMartin](https://discourse.cmake.org/u/McMartin)
#### Post date: [August 16, 2022, 5:27pm UTC](https://discourse.cmake.org/t/question-about-unity-build-and-interface-library-odr/6279/2 "2022-08-16T17:27:30Z")

</div>

Why are `tw_base` and `tw_base_tests` defined as `INTERFACE` libraries? Why not `SHARED` or `STATIC`?

If you want `Data.cpp` to be compiled only for `MyAwesomeStaticLib`, why is it added to another target (`tw_base`)?

---

<div class="post-metadata">

### Author: ![vdamewood](https://discourse.cmake.org/letter_avatar_proxy/v4/letter/v/7ab992/32.png) [@vdamewood](https://discourse.cmake.org/u/vdamewood)
#### Post date: [August 16, 2022, 8:37pm UTC](https://discourse.cmake.org/t/question-about-unity-build-and-interface-library-odr/6279/3 "2022-08-16T20:37:23Z")

</div>

You can get rid of the double building of `Data.c` if you have `tw_base_tests` depend on `MyAwesomeStaticLib` and not on `tw::base` directly, and if you have `MyAwesomeStaticLib` depend on `tw::base` as a `PRIVATE` dependency, and not a `PUBLIC` one.

When you have a source file in an `INTERFACE` library, then that source file will be added to anything that depends on it. And when you have a `PUBLIC` dependency like that, then anything that was inherited from the dependency will also propagate to anything that depends on the target, so everything that depends on `MyAwesomeStaticLib` will also get built with `Data.c`.