[BUG] [cpp-httplib-server] Throw exception if optional property not present in json payload

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

Many client generators omit optional properties (those not listed in required) when they are unset, which is spec-compliant — the JSON node for these properties is then absent. In this case the deserialization code generated by the cpp-httplib-server generator crashes instead of handling the missing node.

openapi-generator version

v7.23.0

OpenAPI declaration file content or url

openapi: "3.0.0"
info:
  title: Enum Bug Reproduction
  version: "1.0.0"


paths:
  /example:
    post:
      operationId: postExample
      parameters:
        - in: query
          name: status
          schema:
            $ref: '#/components/schemas/PostBody'
      responses:
        '200':
          description: OK

components:
  schemas:
    PostBody:
      type: object
      properties:
        status:
          type: string
        data:
          type: string
      required:
        - status

Generation Details

generation script:

#!/usr/bin/env pwsh
param()

$ErrorActionPreference = 'Stop'

$SpecDir = Resolve-Path (Join-Path $PSScriptRoot 'openapi')
$OutputDir = Resolve-Path (Join-Path $PSScriptRoot 'source')

Write-Output "[generate-models] Spec:   $SpecDir\openapi.yaml"
Write-Output "[generate-models] Output: $OutputDir"

Write-Output "[generate-models] Using Docker..."
docker run --rm `
    -v "${SpecDir}:/spec" `
    -v "${OutputDir}:/output" `
    "openapitools/openapi-generator-cli:latest" generate `
    -i /spec/openapi.yaml `
    -g cpp-httplib-server `
    -o /output `
    --global-property 'models,modelDocs=false,modelTests=false' `
    --additional-properties='enumClassPrefix=true'

if ($LASTEXITCODE -ne 0) {
    throw "[generate-models] ERROR: Generation failed."
}

Write-Output "[generate-models] Done."

Steps to reproduce

    std::string jsonString = "{\"status\":\"accepted\"}";
    nlohmann::json jsonObject = nlohmann::json::parse(jsonString);
    models::PostBody deserializedBody = jsonObject.get<models::PostBody>();

Related issues/PRs

Suggest a fix

Probably instead of NLOHMANN_DEFINE_TYPE_INTRUSIVE it would be necessary to implement explicit friend methods. maybe something like this:

    friend void to_json(nlohmann::json& j, const {{vendorExtensions.modelClassName}}& o)
    {
        {{#vars}}j["{{baseName}}"] = o.{{nameInCamelCase}};
        {{/vars}}
    }

    friend void from_json(const nlohmann::json& j, {{vendorExtensions.modelClassName}}& o)
    {
        {{#requiredVars}}j.at("{{baseName}}").get_to(o.{{nameInCamelCase}});
        {{/requiredVars}}
        {{#optionalVars}}if (j.contains("{{baseName}}") && !j.at("{{baseName}}").is_null())
        {
            j.at("{{baseName}}").get_to(o.{{nameInCamelCase}});
        }
        {{/optionalVars}}
    }

Start with modules/openapi-generator/src/main/resources/cpp-httplib-server/model-header.mustache, especially the NLOHMANN_DEFINE_TYPE_INTRUSIVE usage. Generate the provided PostBody model and reproduce deserialization of JSON containing only the required status property. Done means absent optional properties no longer throw while required properties remain enforced.