This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++17 status.
Section: 184.108.40.206.2 [func.wrap.func.con] Status: C++17 Submitter: Geoffrey Romer Opened: 2016-12-15 Last modified: 2020-09-06
View other active issues in [func.wrap.func.con].
View all other issues in [func.wrap.func.con].
View all issues with C++17 status.
Effects: If !f, *this has no target; otherwise, move constructs the target of f into the target of *this, leaving f in a valid state with an unspecified value.
By my reading, this wording requires the move constructor of std::function to construct an entirely new target object. This is silly: in cases where the target is held in separately allocated memory (i.e. where the target doesn't fit in std::function's internal buffer, if any), std::function's move constructor can be implemented by simply transferring ownership of the existing target object (which is a simple pointer assignment), so this requirement forces an unnecessary constructor invocation and probably an unnecessary allocation (the latter can be avoided with something like double-buffering, but ew). Fixing this would technically be a visible change, but I have a hard time imagining reasonable code that would be broken by it, especially since both libstdc++ and libc++ already do the sensible thing, constructing a new target only if the target is held in an internal buffer, and otherwise assigning pointers.
This wording is relative to N4618.
Edit 220.127.116.11.2 [func.wrap.func.con]/5 as indicated:
Drafting note: The "equivalent to ... before the construction" wording is based on the wording for MoveConstructible.
Effects: If !f, *this has no target; otherwise, move constructs the target of f into the target of *this, leaving fin a valid state with an unspecified value.