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The function <<ref_get_null_resource>> returns a global instance of the null resource. This resource offers a simple invariant: all calls to allocate will throw the exception `std::bad_alloc`. An instance of the null resource can be used to make parsing guarantee that allocations from the heap are never made. This is explored in more detail in a later section.
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Allocator Propagation
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The containers <<ref_array>>, <<ref_object>>, and <<ref_value>> all propagate the memory resource they were constructed with to child elements:
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This propagation acts recursively, containers within containers will all have the resource propagated. Once a container is constructed, its memory resource can never be changed.
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Resource Lifetime
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It is important to note that <<ref_storage_ptr>> supports both shared-ownership and reference lifetime models. Construction from a memory resource pointer does not transfer ownership:
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When using a memory resource in this fashion, including the case where a storage pointer or container is constructed from a {ref_polymorphic_allocator}, the caller must ensure that the lifetime of the resource is extended until it is no longer referenced by any variables; otherwise, undefined behavior is possible.
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Shared ownership is achieved using the function <<ref_make_shared_resource>>, which creates a new, reference-counted memory resource using a dynamic memory allocation and returns it as a <<ref_storage_ptr>>:
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When a storage pointer is constructed this way, the lifetime of the referenced memory resource is extended until all variables which reference it are destroyed.
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User-Defined Resource
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To implement custom memory allocation strategies, derive your class from {ref_memory_resource} and implement the functions `do_allocate`, `do_deallocate`, and `do_is_equal` as seen in this example below, which logs each operation it performs to the console:
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