Welcome to include/grpc/impl/codegen
Why is this directory here?
This directory exists so that generated C++ code can include selected files upon
which it depends without having to depend on the entire gRPC C++ library. This
directory thus exists to support include/grpcpp/impl/codegen. This constraint
is particularly relevant for users of bazel, particularly if they use the
multi-lingual proto_library target type. Generated code that uses this target
only depends on the gRPC C++ targets associated with these header files, not the
entire gRPC C++ codebase since that would make the build time of these types of
targets excessively large (particularly when they are not even C++ specific).
What should user code do?
User code should not include anything from this directory. Only generated code
and gRPC library code should include contents from this directory. C++ user code
should instead include contents from the main grpcpp directory or its
accessible subcomponents like grpcpp/support. It is possible that we may
remove this directory altogether if the motivations for its existence are no
longer strong enough (e.g., if the gRPC C++ library no longer has a need for an
impl/codegen directory of its own).