c++ warning unused parameter ‘j’ for empty object

[elionne]

I have this minimal json schema:

{
  "type": "object",
  "additionalProperties": false
}

which render as following code:

// ...
    struct Nop {};

    inline void from_json(const json & j, Nop& x) {}

    inline void to_json(json & j, const Nop & x) {
        j = json::object();
    }

The compiler raise warning about j or x are not used. And since I use the -Werror option, it turns into an error.

I can use #pragma GCC diagnostic … to change compiler warning behavior, but it is not satisfying and not portable. It will be better if the generated code does not create any warning.

[schani]

Triage note (systematic backlog sweep, assisted by Claude): confirmed reproducible on current master (v23.3.0, 68ecb284).

Root cause: for an empty JSON Schema object (additionalProperties: false, no properties), quicktype emits an empty class with from_json/to_json free functions whose parameters go unused:

inline void from_json(const json & j, Verify& x) {
}

inline void to_json(json & j, const Verify & x) {
    j = json::object();
}

from_json doesn't touch j or x at all; to_json doesn't touch x. With -Wextra/-Wunused-parameter (commonly combined with -Werror), this fails the build.

Source: packages/quicktype-core/src/language/CPlusPlus/CPlusPlusRenderer.ts, emitClassFunctions (from from_json around line 1367, to_json around line 1558) — when a class has no properties, the emitted bodies don't reference all their parameters.

Assessment: severity low — it's warning-only and needs a specific combination (-Wextra/-Wunused-parameter plus -Werror, plus an empty object schema) to actually break a build, but it does break strict -Werror builds as reported. Fix difficulty easy — for classes with no properties, either drop the parameter names or add (void)j;/(void)x; at those emit sites in emitClassFunctions.

Reproduction:

quicktype --lang cpp --src-lang schema empty.schema.json -o out.hpp

{
  "type": "object",
  "additionalProperties": false
}

Generated out.hpp contains from_json/to_json with unused j/x parameters, which trips -Wunused-parameter under -Werror.