Exit Zen
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{ref_polymorphic_allocator}
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An {req_Allocator} which uses a reference to a {ref_memory_resource} to
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perform allocations.
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<<ref_static_resource>>
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A memory resource that uses a single caller provided buffer. No dynamic
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allocations are used. This is fast for parsing but not suited for performing modifications.
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<<ref_storage_ptr>>
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A smart pointer through which a {ref_memory_resource} is managed and
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accessed. |===
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Default Memory Resource
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The default memory resource uses the global `operator new` and `operator delete` to allocate memory. This resource is not reference counted and has a non-trivial deallocate function. All default-constructed <<ref_storage_ptr>> objects reference the same memory resource:
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Default-constructed library containers use the default memory resource:
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The default memory resource is well suited for general usage. It offers reasonable performance for parsing, and conservative memory usage for modification of the contents of containers.
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This memory resource is not guaranteed to be the same as the result of `boost::container::pmr::get_default_resource`. It also cannot be changed with `boost::container::pmr::set_default_resource`.
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Monotonic Resource
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Consider the pattern of memory allocation during parsing: when an array, object, or string is encountered the parser accumulates elements in its temporary storage area. When all of the elements are known, a single memory allocation is requested from the resource when constructing the value. Thus, parsing only allocates and constructs containers at their final size. Memory is not reallocated; that is, a memory buffer never needs to grow by allocating a new larger buffer and deallocating the previous buffer.
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The <<ref_monotonic_resource>> optimizes this memory allocation pattern by allocating increasingly large blocks of global memory internally and parceling those blocks out in smaller pieces to fulfill allocation requests. It has a trivial deallocate function. The monotonic resource does not actually deallocate memory until the resource is destroyed. Thus, it is ideally suited for the use-case where JSON is parsed, and the resulting value is then inspected but not modified.
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The resource to use when constructing values may be specified in calls to <<ref_parse>> as shown here:
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Or, to parse into a value with shared ownership of the memory resource:
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A monotonic resource may be optionally constructed with an initial buffer to use first, before going to the heap. This allows the caller to use stack space and avoid dynamic allocations for most parsed JSON, falling back to dynamic allocation from the heap if the input JSON is larger than average, as shown here:
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