Direct Conversion
For large inputs parsing into the library’s containers followed by conversion
via value_to (or vice versa value_from followed by
serialization from a value) might be prohibitively expensive. For these
cases the library provides components that allow parsing directly into and
serializing directly from user-provided objects.
The drawback of this approach is that fully custom type representations are not supported, only the library-provided conversions are. Also all objects that should be populated by parsing have to be default constructible types. This includes not only the top-level object, but e.g. elements of containers, members of described `struct`s, and alternatives of variants.
That being said, if your types are default-constructible and you don’t need the
customisability allowed by value_to and value_from, then you
can get a significant performance boost with direct conversions.
Direct parsing is performed by the parse_into family of functions. The
library provides overloads that take either string_view or
std::istream, and can report errors either via throwing exceptions or setting
an error code.
std::map< std::string, std::vector<int> > vectors;
string_view input = R"( { "even": [2,4,6], "odd": [1,3,5] } )";
parse_into(vectors, input);If you need to combine incremental parsing with direct parsing, you can resort
to parser_for. parser_for<T> is an instantiation of
basic_parser that parses into an object of type T, and is what
parse_into uses under the hood.
Direct serialization doesn’t require any special components and works with the
refular serializer and serialize.