C++ Error : cannot convert from 'fromType' to 'std::optional<std::string>

[deandt100]

Hi,

I am getting the following compile error when optional properties are used alongside wstring (C++17)

Error

'return': cannot convert from 'fromType' to 'std::optional<std::string>'

Schema (from example on quicktype.io)

{
  "id": "http://json-schema.org/geo",
  "$schema": "http://json-schema.org/draft-06/schema#",
  "description": "A geographical coordinate",
  "type": "object",
  "properties": {
    "latitude": {
      "type": "number"
    },
    "longitude": {
      "type": "number"
    }
  }
}

Options used

quicktype schema.json -o testy.h --namespace quicktype  -s schema --lang c++ --wstring use-wstring --no-boost --include-location global-include --code-format with-struct  --hide-null-optional

Code :

#include <string>
#include <iostream>
#include <sstream>
#include "generated/testy.h"
#include "testy.h"

int main()
{
    quicktype::coordinate request;
    auto json = quicktype::wdump(static_cast<nlohmann::json>(request));
    std::wcout << json;
}

I believe this can be fixed by adding the following convert overloads to the generated Utf16_Utf8 class :

static std::optional<std::string> convert(tag<std::optional<std::wstring>>, tag< std::optional<std::string>>, std::optional<std::wstring> str) 
{
    return str.has_value() ? std::optional{ convert(tag<std::wstring>(), tag<std::string>(), str.value()) } : std::nullopt;
}

static std::optional<std::wstring> convert(tag<std::optional<std::string>>, tag< std::optional<std::wstring>>, std::optional<std::string> str) 
{
    return str.has_value() ? std::optional{ convert(tag<std::string>(), tag<std::wstring>(), str.value()) } : std::nullopt;
}

[schani]

Triage sweep finding (systematic backlog triage, assisted by Claude) — confirmed reproducible on current master (v23.3.0, 68ecb284, g++ 13.3.0, nlohmann/json 3.11.3).

Root cause: with --wstring use-wstring --no-boost, quicktype's C++ output for an optional string property fails to compile. The generated from_json/to_json code calls Utf16_Utf8<std::optional<std::string>, std::optional<std::wstring>>::convert(...), but the Utf16_Utf8 helper (generated in emitHelperFunctions, packages/quicktype-core/src/language/CPlusPlus/CPlusPlusRenderer.ts, class Utf16_Utf8 starting at line 3388) only has overloads for shared_ptr, vector, and map — not for std::optional/stack values. The call falls through to the generic convert(tag<TF>, tag<TT>, fromType) overload (line ~3393 region, generic fallback around line 98 of the generated header), which just returns fromType unchanged, so the compiler rejects the implicit conversion from optional<string> to optional<wstring>.

Confirmed actual compiler output:

testy2.h: In instantiation of 'static toType quicktype::Utf16_Utf8<fromType, toType>::convert(fromType) [with fromType = std::optional<std::string>; toType = std::optional<std::wstring>]':
testy2.h:165:94:   required from here
testy2.h:98:62: error: could not convert '...convert(...)' from 'optional<basic_string<char>>' to 'optional<basic_string<wchar_t>>'

Assessment: severity medium (non-compiling generated output, but scoped to the non-default use-wstring option combined with optional string properties). Fix difficulty easy: add std::optional overloads to Utf16_Utf8 mirroring the existing shared_ptr overload (~15 lines); the reporter's suggested patch in the issue does this.

Reproduction:

node dist/index.js schema2.json -o testy2.h --namespace quicktype -s schema --lang c++ \
  --wstring use-wstring --no-boost --include-location global-include \
  --code-format with-struct --hide-null-optional
g++ -std=c++17 -I. -c main2.cpp

schema2.json:

{
  "$schema": "http://json-schema.org/draft-06/schema#",
  "type": "object",
  "properties": {
    "name": { "type": "string" },
    "latitude": { "type": "number" }
  }
}

main2.cpp:

#include <string>
#include <iostream>
#include <sstream>
#include "testy2.h"

int main()
{
    quicktype::Testy request;
    auto json = quicktype::wdump(static_cast<nlohmann::json>(request));
    std::wcout << json;
}

(also needs nlohmann/json.hpp on the include path). Neither property is required, so name becomes std::optional<std::string>, triggering the failing Utf16_Utf8 conversion.