std::function<R(Args...)>::assign
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std::function 
template< class F, class Alloc >
void assign( F&& f, const Alloc& alloc );
 
 (since C++11) void assign( F&& f, const Alloc& alloc );
(removed in C++17)
Initializes the target with f. The alloc is used to allocate memory for any internal data structures that the function might use.
Equivalent to function(std::allocator_arg, alloc, std::forward <F>(f)).swap(*this);.
Contents
[edit] Parameters
 f
 -
 callable function to initialize the target with
 alloc
 -
 allocator to use to allocate memory for the internal data structures
[edit] Return value
(none)
[edit] Exceptions
May throw implementation-defined exceptions.