These libraries are not comparable. The output of the simdjson parser is a read-only structure. In other words, it can’t be changed, and the only way to create one is by parsing a JSON string. On the other hand, Boost.JSON allows you to modify the container holding the parsed JSON, or even build a JSON document from scratch through the container interface.

Using standard allocators would require that value be declared as a class template, which would impose an additional compilation burden. By avoiding the template, most of the function definitions in the library can be excluded from the headers and emitted in a separate static or dynamic library.

polymorphic_allocator treats the memory resource as a reference with respect to ownership. Boost.JSON uses a reference counted smart pointer container to simplify the lifetime management of memory resources. In addition to being reference counted, storage_ptr can function as an uncounted reference wrapper around a memory_resource.

The string provided by the library uses the storage_ptr allocator model, has the same interface on all C++ versions, and has an optimized class layout to keep the size of JSON values small. string also implements an improved interface that replaces extraneous overloads with ones that use string_view.