I think that using pass-by-value struct will bring more headache for language bindings developers, because this complicates ABI and not every language's foreign function interface supports passing structs by value. (I think that passing structs by value is not part of C language, but of C++ (or am I wrong?), so it is problematic to support) I would support having both kinds of accessors: "simple" ones (option "a") and convenient for C (option "b"). Just the same as with other kinds of functions, e.g., g_object_get and g_object_get_property. One part of interface for convenience of C programming, one part for convenience of other languages.
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