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### 2565. `std::function`'s move constructor should guarantee nothrow for `reference_wrapper`s and function pointers

Section: 20.14.17.3.2 [func.wrap.func.con] Status: C++17 Submitter: Tim Song Opened: 2015-12-05 Last modified: 2017-07-30

Priority: 0

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Discussion:

20.14.17.3.2 [func.wrap.func.con]/5 guarantees that copying a `std::function` whose "target is a callable object passed via `reference_wrapper` or a function pointer" does not throw exceptions, but the standard doesn't provide this guarantee for the move constructor, which makes scant sense.

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

> [Drafting note: The inserted paragraph is a copy of 20.14.17.3.2 [func.wrap.func.con]/5, only changing "copy constructor" to "copy or move constructor". It does not attempt to fix the issue identified in LWG 2370, whose P/R will likely need updating if this wording is adopted.]

1. Insert after 20.14.17.3.2 [func.wrap.func.con]/6:

   >     function(function&& f);
   >     template <class A> function(allocator_arg_t, const A& a, function&& f);

   > > -6- 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.

   > > -?- Throws: Shall not throw exceptions if `f`'s target is a callable object passed via `reference_wrapper` or a function pointer. Otherwise, may throw `bad_alloc` or any exception thrown by the copy or move constructor of the stored callable object. [Note: Implementations are encouraged to avoid the use of dynamically allocated memory for small callable objects, for example, where `f`'s target is an object holding only a pointer or reference to an object and a member function pointer. — end note]