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2850. std::function move constructor does unnecessary work

Section: 22.10.17.3.2 [func.wrap.func.con] Status: C++17 Submitter: Geoffrey Romer Opened: 2016-12-15 Last modified: 2020-09-06

Priority: 0

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Discussion:

Consider [func.wrap.func.con]/p5: 

function(function&& f);

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.

[2017-01-27 Telecon]

Priority 0

Proposed resolution:

This wording is relative to N4618.

  1. Edit 22.10.17.3.2 [func.wrap.func.con]/5 as indicated:

    Drafting note: The "equivalent to ... before the construction" wording is based on the wording for MoveConstructible. 

    function(function&& f);

    -5- EffectsPostconditions: If !f, *this has no target; otherwise, move constructs the target of f into the target of *this, leaving fthe target of *this is equivalent to the target of f before the construction, and f is in a valid state with an unspecified value.