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1070. Ambiguous move overloads in function

Section: 22.10.17.3[func.wrap.func]Status:C++11Submitter: Howard HinnantOpened: 2009-03-19Last modified: 2016-01-28

Priority:Not Prioritized

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Discussion:

The synopsis in 22.10.17.3[func.wrap.func] says:

template<Returnable R, CopyConstructible... ArgTypes> class function<R(ArgTypes...)>{    ...    template<class F>       requires CopyConstructible<F> && Callable<F, ArgTypes...>             && Convertible<Callable<F, ArgTypes...>::result_type, R>       function(F);     template<class F>       requires CopyConstructible<F> && Callable<F, ArgTypes...>             && Convertible<Callable<F, ArgTypes...>::result_type, R>       function(F&&);    ...    template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F);     template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F&&);    ...    template<class F>       requires CopyConstructible<F> && Callable<F, ArgTypes..>             && Convertible<Callable<F, ArgTypes...>::result_type       function& operator=(F);     template<class F>       requires CopyConstructible<F> && Callable<F, ArgTypes...>             && Convertible<Callable<F, ArgTypes...>::result_type, R>       function& operator=(F&&);    ...};

Each of the 3 pairs above are ambiguous. We need only one of each pair, and wecould do it with either one. If we choose theF&& version weneed to bringdecay into the definition to get the pass-by-value behavior.In the proposed wording I've gotten lazy and just used the pass-by-value signature.

[2009-05-01 Daniel adds:]

1024⁽ⁱ⁾ modifies the second removed constructor.

[Batavia (2009-05):]

We briefly discussed whether we ought support moveable function objects,but decided that should be a separate issue if someone cares to propose it.

Move to Tentatively Ready.

Proposed resolution:

Change the synopsis of 22.10.17.3[func.wrap.func], and remove the associated definitions in22.10.17.3.2[func.wrap.func.con]:

template<Returnable R, CopyConstructible... ArgTypes> class function<R(ArgTypes...)>{    ...    template<class F>       requires CopyConstructible<F> && Callable<F, ArgTypes...>             && Convertible<Callable<F, ArgTypes...>::result_type, R>       function(F);template<class F>       requires CopyConstructible<F> && Callable<F, ArgTypes...>             && Convertible<Callable<F, ArgTypes...>::result_type, R>       function(F&&);    ...    template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F);template<class F, Allocator Alloc> function(allocator_arg_t, const Alloc&, F&&);    ...    template<class F>       requires CopyConstructible<F> && Callable<F, ArgTypes..>             && Convertible<Callable<F, ArgTypes...>::result_type       function& operator=(F);template<class F>       requires CopyConstructible<F> && Callable<F, ArgTypes...>             && Convertible<Callable<F, ArgTypes...>::result_type, R>       function& operator=(F&&);    ...};