fix(cpp): initialize class members from JSON Schema default

[schani]

Bug

For a JSON Schema property with a default, the C++ generator ignored it entirely: the generated class member was declared with no initializer, leaving it uninitialized (garbage) when a class is default-constructed.

Repro (--lang cpp --src-lang schema) with:

{
  "type": "object",
  "properties": { "id": { "type": "integer", "default": 0 } },
  "required": ["id"],
  "additionalProperties": false
}

Before:

private:
int64_t id;

After:

private:
int64_t id = 0;

Root cause

JSONSchemaInput.ts had no attribute producer that captured a schema's default value onto the corresponding quicktype Type, so no downstream renderer ever had access to it. The C++ renderer's emitMember also had no notion of a default-value initializer.

Fix

• Added packages/quicktype-core/src/attributes/DefaultValue.ts: a new TypeAttributeKind<DefaultValue> (boolean | number | string | null) plus a defaultValueAttributeProducer that reads schema.default and attaches it to the produced number/string/boolean/null type attributes.
• Wired the new producer into JSONSchemaInput.ts's attribute-producer list, and extended JSONSchemaAttributes with forBoolean/forNull so those primitive kinds can carry attributes the way forNumber/forString already did.
• CPlusPlusRenderer.ts: emitMember now accepts an optional default-value initializer Sourcelike and emits Type name = default; when present. A new cppDefaultValue helper turns the attribute into the right C++ literal (numeric literal, true/false, nullptr, a quoted/escaped string literal, or EnumType::Case for enum defaults), and both class-member and getter/setter emission paths pass it through.

Scope: this focuses on the confirmed scalar-property repro from the issue (and covers enum defaults too, since the plumbing generalized naturally). It only affects JSON-Schema-sourced default values reaching the C++ generator; no other language's default handling was touched.

Test coverage

Added test/inputs/schema/default-value.schema (+ matching default-value.1.json sample) — a schema with a required integer property that has default: 0. JSON Schema inputs under test/inputs/schema/ run automatically for every language's schema-<lang> fixture unless opted out via skipSchema, so this is picked up by schema-cplusplus (and all other schema fixtures) without further registration.

Verification

• npm run build passes cleanly.
• Manually re-ran the repro from the issue: generated C++ now shows int64_t id = 0; instead of the uninitialized int64_t id;.
• Ran the full schema-cplusplus fixture locally: QUICKTEST=true FIXTURE=schema-cplusplus script/test — all 68 tests pass, including the new default-value.schema case.
• Did not re-run every other language's schema fixture locally; CI will validate that the new schema input and JSONSchemaAttributes change don't regress other generators.

Fixes #1535

🤖 Generated with Claude Code