CMake: Public VS Private VS Interface

Introduction

CMake is one of the most convenient building tools for C/C++ projects. When it comes to target_include_directories and target_link_libraries, there are several keywords, PUBLIC, PRIVATE, and INTERFACE, that I got confused about from time to time even if I have read the related official documentations. So when I was building my C/C++ projects using CMake, I often just use PUBLIC everywhere or leave the keyword blank (CMake will then use PUBLIC by default), the libraries and executables built from the projects would work in most of the scenarios. However, it is certainly not best practice.

Today, I read Kuba Sejdak’s blog post “Modern CMake is Like Inheritance” and I found his interpretation on the CMake keywords PUBLIC, PRIVATE, and INTERFACE inspiring. So in this blog post, I would like to discuss some of my thoughts on these CMake keywords from the perspective of “inheritance”.

C++ Inheritance

Access Specifiers

In C++ object oriented programming, there are three types of access specifiers for classes.

Access Specifier	Description
public	Members are accessible from outside the class.
protected	Members cannot be accessed from outside the class. However, they can be accessed in inherited classes.
private	Members cannot be accessed (or viewed) from outside the class.

Alternatively, this could be described using the following simplified table.

Access Specifier	Same Class	Derived Class	Outside Class
public	Yes	Yes	Yes
protected	Yes	Yes	No
private	Yes	No	No

Inheritance Types

When it comes to class inheritance, there are also three types of inheritances.

Inheritance Type	Description
public	Public members of the base class become public members of the derived class and protected members of the base class become protected members of the derived class. A base class’s private members are never accessible directly from a derived class, but can be accessed through calls to the public and protected members of the base class.
protected	Public and protected members of the base class become protected members of the derived class.
private	Public and protected members of the base class become private members of the derived class.

Alternatively, this could be described using the following simplified table.

Inheritance Type	base: public member	base: protected member	base: private member
public	derived: public member	derived: protected member	-
protected	derived: protected member	derived: protected member	-
private	derived: private member	derived: private member	-

CMake Inheritance

CMake uses somewhat similar inheritance concepts to C++, especially for the C++ public and private access specifiers and inheritance types. The CMake keywords PUBLIC, PRIVATE, and INTERFACE used in target_include_directories and target_link_libraries, in my opinion, are mixtures of access specifier and inheritance type from C++.

Include Inheritance

In CMake, for any target, in the preprocessing stage, it comes with a INCLUDE_DIRECTORIES and a INTERFACE_INCLUDE_DIRECTORIES for searching the header files building. target_include_directories will populate all the directories to INCLUDE_DIRECTORIES and/or INTERFACE_INCLUDE_DIRECTORIES depending on the keyword <PRIVATE|PUBLIC|INTERFACE> we specified. The INCLUDE_DIRECTORIES will be used for the current target only and the INTERFACE_INCLUDE_DIRECTORIES will be appended to the INCLUDE_DIRECTORIES of any other target which has dependencies on the current target. With such settings, the configurations of INCLUDE_DIRECTORIES and INTERFACE_INCLUDE_DIRECTORIES for all building targets are easy to compute and scale up even for multiple hierarchical layers of building dependencies and many building targets.

Include Inheritance	Description
PUBLIC	All the directories following PUBLIC will be used for the current target and the other targets that have dependencies on the current target, i.e., appending the directories to INCLUDE_DIRECTORIES and INTERFACE_INCLUDE_DIRECTORIES.
PRIVATE	All the include directories following PRIVATE will be used for the current target only, i.e., appending the directories to INCLUDE_DIRECTORIES.
INTERFACE	All the include directories following INTERFACE will NOT be used for the current target but will be accessible for the other targets that have dependencies on the current target, i.e., appending the directories to INTERFACE_INCLUDE_DIRECTORIES.

Note that when we do target_link_libraries(<target> <PRIVATE|PUBLIC|INTERFACE> <item>), the dependent <item>, if built in the same CMake project, would append the INTERFACE_INCLUDE_DIRECTORIES of <item> to the INCLUDE_DIRECTORIES of <target>. By controlling the INTERFACE_INCLUDE_DIRECTORIES, we could eliminate some unwanted or conflicting declarations from <item> to the <target>.

For example, the fruit library has INCLUDE_DIRECTORIES of fruit_h, tree_h, and INTERFACE_INCLUDE_DIRECTORIES of fruit_h. If there is a apple library that is linked with the fruit library, the apple library would also have the fruit_h in its INCLUDE_DIRECTORIES as well. We could equivalently say, the apple library’s include directory inherited the fruit_h of the fruit library.

Link Inheritance

Similarly, for any target, in the linking stage, we would need to decide, given the item to be linked, whether we have to put the item in the link dependencies, or the link interface, or both, in the compiled target. Here the link dependencies means the item has some implementations that the target would use, and it is linked to the item, so that whenever we call the functions or methods corresponding to those implementations it will always be mapped correctly to the implementations in item via the link, whereas the link interface means the target becomes an interface for linking the item for other targets which have dependencies on the target, and the target does not have to use item at all.

Link Type	Description
PUBLIC	All the objects following PUBLIC will be used for linking to the current target and providing the interface to the other targets that have dependencies on the current target.
PRIVATE	All the objects following PRIVATE will only be used for linking to the current target.
INTERFACE	All the objects following INTERFACE will only be used for providing the interface to the other targets that have dependencies on the current target.

For example, if the fruit library has the implementation of functions, such as size and color, and the apple library has a function apple_size which called the size from the fruit library and was PRIVATE linked with the fruit library. We could create an executable eat_apple that calls apple_size by PUBLIC or PRIVATE linking with the apple library. However, if we want to create an executable eat_apple that calls the size and color from the fruit library, only linking with the apple library will cause building error, since the fruit library was not part of the interface in the apple library, and is thus inaccessible to eat_apple. To make the apple library to inherit the size and color from the fruit library, we have to make the linking of the apple library to the the fruit library PUBLIC instead of PRIVATE.

Conclusions

The CMake builds a hierarchical project via the include interface or link interface. The “inheritance” mechanism in C++ is built upon the include interface or link interface.

FAQ

How to Understand CMake Interface?

In my understanding, CMake interface is just like a telephone switch station in old times.

If A wants to call B and there is no direct telephone cable connection between A and B, A has to call a telephone switch station that has connection to B and the personal in the telephone switch station will connect A and B by jointing the cable of A and the cable of B together. If the telephone switch station does not know there is a B, it is impossible to get A and B connected. So CMake interface is simply a registration in the telephone switch station. When there is a dependency in CMake targets, targets from different levels of hierarchy are connected via interfaces, for both include and link.

What are the Key Points for This Blog Post?

PRIVATE only cares about himself and does not allow inheritance. INTERFACE only cares about others and allows inheritance. PUBLIC cares about everyone and allows inheritance.

Is PUBLIC, PRIVATE, INTERFACE Part of the GCC/G++ Compiler?

No. Compilers, such as gcc and g++, do not have such mechanism. CMake invented those keywords for user to create a building graph that has very clear and explicit dependencies. The building graph translates to normal building commands using gcc and g++.

References

• Modern CMake is Like Inheritance