Library linking in C++, dependence between three libraries

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This might be a weird / beginners question for someone. However I have the following situation.

In my code, I am creating three libraries, lets say A,B and C. In library B i am linking library A, so something like target_link_libraries(... A).

My C library is linking against B. So I have: B -> A C -> B.

Should it then be possible, that I use methods, enums whatever defined in library A also in C? Although C is not linking A explicitly? I was using an enum defined in library A in C, A::someClass::someEnum and it compiled and showed correct value entries.

circular-dependency dependencies cmake c++
2 Answers

Should it then be possible, that I use methods, enums whatever defined in library A also in C?

Does your C library uses these methods/enums/etc. via including a header from the library A or from the library B?

If C directly includes the header from the library A, then it should be linked with the library A explicitly:

# A is a private linkage of B.
target_link_libraries(B PRIVATE A)
# C needs to be linked with A
target_link_libraries(C B A)

If C includes the header from the library B, which in turn includes the header from the library A, then A library should be a part of the B interface:

# B links with A and this linkage is an interface of B
target_link_libraries(B PUBLIC A)
# C may link with B only
target_link_libraries(C B)

These approaches assume "modern" (target-oriented) CMake to be used. But using these approaches one would get most scalable solution.

user_34407450

Remember that linkage dependency and compilation dependency are not the same thing.

It's quite possible that the sources of library C use definitions from headers associated with library A... irrespective of whether C depends on A when linking.

Now, in CMake, we use target_link_libraries() not only for linking. This CMake command makes sure that the library's headers are available to the dependent target, for compilation, and that its libraries are available for linking.

It's also possible for library C to require library A for linking - to have the definitions of, say, functions or variables from library A. Libraries can theoretically have circular link dependencies (which means that you need to either link against the entire circle, or no part of it).

user_15930770
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