Yes, but doesn't std::unique_ptr introduce compiler / toolchain constraints? I.e. if the class declaration was in a header file of a library, as a user of the library you'd be bound to the same toolchain as the library, no?
Considering this is C++ rather than C and it doesn't have a portable ABI to begin with officially you'd have to use the same toolchain regardless.
In practice you get a strong amount of compatibility between Clang and GCC at the compiler level (since Clang basically copies GCC's ABI). But you also get the same fudging at the stdlib level with std::unique_ptr because it's header-only and is a zero overhead abstraction for a raw pointer (i.e. at the ABI level a normal unique_ptr is the same as a pointer except you can't move it around in a register).
