Can there be an impact on build time when include/link libraries PUBLIC or PRIVATE?

Question

As explained here, in modern cmake you should keep include/link targets private. Your CMakeList.txt should somewhat look like this:

add_executable(myexe main.cpp)
target_include_directories(myexe PRIVATE ${mylib_include_dir})
target_link_libraries(myexe PRIVATE mylib)

Can it make a difference, regarding build time, if I use PRIVATE or PUBLIC?

Answer 1

The CMake part mainly reflects your C++ module architecture.

E.g. if you have two libraries and one (B) contains a public class that derives from a class in another one (A), then the header containing class B will include class A. And this will be reflected in a PUBLIC CMake dependency declaration between both.

Optimizing your code/architecture by reducing dependencies has a lot of positive effects incl. compilation time. For discussion on the effects see e.g.:

• Effective C++ "35. Minimize compilation dependencies between files". Is it still valid today?
• What techniques can be used to speed up C++ compilation times?

One of the general topics I'm referring to here is often titled "Reduce C++ Build Times by Reducing Header Dependencies".

Reference

• How to speed up Compile Time of my CMake enabled C++ Project?
• CMake target_link_libraries Interface Dependencies
• [CMake] Difference between PRIVATE and PUBLIC with target_link_libraries