std::experimental::pmr::set_default_resource - cppreference.com
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If r is not null, sets the default memory resource pointer to r; otherwise, sets the default memory resource pointer to new_delete_resource().
The default memory resource pointer is used by certain facilities when an explicit memory resource is not supplied. The initial default memory resource pointer is the return value of new_delete_resource().
This function is thread-safe. Every call to set_default_resource synchronizes with (see std::memory_order) the subsequent set_default_resource and get_default_resource calls.
Return value
Returns the previous value of the default memory resource pointer.