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### 1287. `std::function` requires `CopyConstructible` target object

Section: 20.14.17.3.2 [func.wrap.func.con] Status: C++11 Submitter: Jonathan Wakely Opened: 2009-12-13 Last modified: 2016-01-28

Priority: Not Prioritized

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Discussion:

I think `std::function` should require `CopyConstructible` for the target object.

I initially thought that `MoveConstructible` was enough, but it's not. If `F` is move-only then function's copy constructor cannot be called, but because function uses type erasure, `F` is not known and so the copy constructor cannot be disabled via `enable_if`. One option would be to throw an exception if you try to copy a function with a non-copyable target type, but I think that would be a terrible idea.

So although the constructors require that the target be initialised by `std::move(f)`, that's only an optimisation, and a copy constructor is required.

[ 2009-12-24 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

Proposed resolution:

Add to 20.14.17.3.2 [func.wrap.func.con] paragraph 9:

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

> > 9 Requires: `F` shall be `CopyConstructible`. `f` shall be callable for argument types `ArgTypes` and return type `R`. The copy constructor and destructor of `A` shall not throw exceptions.