examples/feature-tour.json puts every FSON extension in one place.

/* FSON feature tour.
   Every syntax extension over plain JSON appears in this file;
   see docs/notation.md for the grammar. /* Nested block comments
   are allowed, like this one. */ Back at the outer level. */
{
  // 1. Bare-name keys: letters and digits separated by single underscores.
  bare_key_1: "no quotes needed",

  // 2. Quoted keys for anything else.
  "key with spaces": true,

  // 3. Disabled members: '--' switches a member off without deleting it.
  --switched_off: "still stored, invisible to queries",
  --"also works on quoted keys": 42,

  // 4. All JSON value types.
  values: {
    text: "string with escapes: \" \ \n \t €",
    integer: -42,
    fraction: 3.50,
    exponent: 6.02e+23,
    truthy: true,
    falsy: false,
    nothing: null,
    list: [1, 2, 3],
    nested: {}
  },

  // 5. Comments survive a read-modify-write cycle (try fson-check -i).
  documented: "leading comment above" // trailing comment beside
  /* dangling comments before the closing brace are preserved too */
}

The tour

#FeaturePage
header /* … /* … */ … */a nested block comment — one comment, not a syntax errorComments
1bare_key_1: — a bare nameKeys & bare names
2"key with spaces": — quote anything that is not a bare nameKeys & bare names
3--switched_off: / --"also works…": — disabled members, invisible to queries, preserved verbatimDisabled members
4€ in a string; 3.50 keeps its trailing zero; 6.02e+23; null; [] / {}Strings, Numbers, Values
5leading, trailing and dangling comment positionsComments

The round-trip claim, checked

fson-check -i feature-tour.json
git diff feature-tour.json      # no change

3.50 stays 3.50 (not 3.5), € is decoded and re-escaped as €… actually written back as the raw UTF-8 €, every comment stays where it was, and --switched_off is still present with its prefix.

Reading a disabled member

auto r = Fson::load( "feature-tour.json" );
r.document->getString( "switched_off", "N/A" );   // "N/A" — disabled, invisible

Object& root = *r.document->getRootObject();
for( std::size_t k = 0; k < root.memberCount(); ++k )
  if( root.getMember( k ).getKey().getName() == "switched_off" )
    /* found it via the member API */;