lifetime of an object managed by const std::unique_ptr

Question

I see below notes in std::unique_ptr reference:

Only non-const unique_ptr can transfer the ownership of the managed object to another unique_ptr. The lifetime of an object managed by const std::unique_ptr is limited to the scope in which the pointer was created.

Is there anyone who can explain it with an example? I could not figure it out why.

Answer 1

You simply can not move from a const std::unique_ptr and you can't use other modifying member functions - swap, release and reset either (these are logically non-const qualified, cannot be called on a const instance).

Transferring ownership implies resetting the old owner to non-owning state, thus modifying it.

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^{const std::unique_ptr will manage at most one object during its lifetime (starting from its initialization).
In case of std::unique_ptr const&, you won't be able to transfer ownership from the referenced (even non-const) std::unique_ptr through this reference (const correctness).}

Answer 2

The reset, release, swap, and move assignment functions are non-const member functions and therefore cannot be used with a const instance of the std::unique_ptr class. Therefore, a const std::unique_ptr has no way of being modified and is forced to own the pointer until it goes out of scope.