A simple C++20 JSON parser and serializer

Not necessarily fast, but fairly simple and spec-compliant.

Requires a C++20-ready compiler and header files (notably the <format> header). You may have to tweak the Makefile to get it compiled.

Makefile targets

• make test uses the test files from https://github.com/nst/JSONTestSuite to check spec compliance.

• make out/validator builds the out/validator binary. It takes a JSON file path as a command line argument, tries to parse the file, and returns a 0/2 status code depending on the result. Any parsing or encoding error is printed to stderr. This is used by test.py to run the test suite. (Status code 1 is skipped to allow distinguishing parser errors from crashes.)

• make out/formatter builds the out/formatter binary. It takes a JSON file path as a command line argument, parses the file, then serializes it back to standard output. This can be used to manually check round trips.

Implementation choices

Parser:

• Has a maximum nesting depth (see MAX_DEPTH in json.tpp). This could be avoided at the cost of code complexity.
• Numbers are parsed into a double if they contain a period or exponent, and into an int64_t otherwise. Out-of-range values are rejected by the parser.
• Only input encoded with UTF-8 without BOM is supported. No attempt at encoding detection is done.
• Strings that describe invalid Unicode (a lone UTF-16 surrogate, for example) are rejected by the parser.

Serializer:

• No superfluous whitespace is added.
• Characters outside of the printable ASCII range in strings are always turned into a Unicode escape sequence, even when they could theoretically be output as-is. This means the output of the serializer is always valid ASCII.
• Doubles are currently serialized with 17 digits of precision systematically. This is always enough to round trip, but may not be very human-readable. For example, 0.1 becomes 0.10000000000000001.
• Doubles that have a simple integer value are serialized with .0 appended to them. This avoids them being parsed back as an integer.

The part of the test suite dealing with "implementation-defined" behavior can be run by setting the CHECK_IMPL_DEFINED constant to True in test.py. All such tests currently result in a parse error, except for tests/i_structure_500_nested_arrays.json, since the nesting limit is higher than 500.