std::allocator_arg
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                    |   Defined in header  
<memory>
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|   constexpr std::allocator_arg_t allocator_arg = std::allocator_arg_t(); 
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(since C++11) | |
std::allocator_arg is a constant of type std::allocator_arg_t used to disambiguate, at call site, the overloads of the constructors and member functions of allocator-aware objects, such as std::tuple, std::function, std::promise, and std::packaged_task.
[edit] See also
|    (C++11) 
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   tag type used to select allocator-aware constructor overloads  (class)  | 
|    (C++11) 
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   checks if the specified type supports uses-allocator construction   (class template)  |