fix(cpp-httplib-server): generate top-level enum models

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/CppHttplibServerCodegen.java

@@ -1543,6 +1543,17 @@ public String getTypeDeclaration(Schema p) {
         return typeDecl;
     }
 
+    // untyped enum schemas (no explicit `type: string`) are still string-backed if their values are strings
+    @SuppressWarnings("rawtypes")
+    private boolean isStringEnumSchema(Schema schema) {
+        if (ModelUtils.isStringSchema(schema)) {
+            return true;
+        }
+        List<?> enumValues = schema.getEnum();
+        return enumValues != null && !enumValues.isEmpty()
+                && enumValues.stream().allMatch(value -> value instanceof String);
+    }
+
     @Override
     @SuppressWarnings({"rawtypes", "unchecked"})
     public CodegenModel fromModel(String name, Schema schema) {
@@ -1560,6 +1571,9 @@ public CodegenModel fromModel(String name, Schema schema) {
         if (model != null) {
             model.name = toModelName(modelName);
             model.classname = model.name;
+            if (model.isEnum) {
+                model.vendorExtensions.put("isStringEnum", isStringEnumSchema(schema));
+            }
 
             if (ModelUtils.hasAllOf(schema) && this.openAPI != null) {
                 int refCount = 0;

modules/openapi-generator/src/main/resources/cpp-httplib-server/README.mustache

@@ -97,6 +97,18 @@ The server will start on `http://0.0.0.0:8080` with:
 #### {{vendorExtensions.modelNamespace}}::{{vendorExtensions.modelClassName}}
 
 ```cpp
+{{#isEnum}}
+// Select an enum value
+auto model = {{vendorExtensions.modelNamespace}}::{{vendorExtensions.modelClassName}}::{{#allowableValues}}{{#enumVars}}{{#-first}}{{name}}{{/-first}}{{/enumVars}}{{/allowableValues}};
+
+// Serialize to JSON via the generated to_json free function
+nlohmann::json json = model;
+std::string jsonString = json.dump();
+
+// Deserialize from JSON via the generated from_json free function
+auto parsedModel = nlohmann::json::parse(jsonString).get<{{vendorExtensions.modelNamespace}}::{{vendorExtensions.modelClassName}}>();
+{{/isEnum}}
+{{^isEnum}}
 // Create a model
 auto model = {{vendorExtensions.modelNamespace}}::{{vendorExtensions.modelClassName}}();
 {{#vars}}
@@ -109,6 +121,7 @@ std::string jsonString = json.dump();
 
 // Deserialize from JSON
 auto parsedModel = {{vendorExtensions.modelNamespace}}::{{vendorExtensions.modelClassName}}::fromJson(nlohmann::json::parse(jsonString));
+{{/isEnum}}
 ```
 {{/model}}
 {{/models}}

modules/openapi-generator/src/main/resources/cpp-httplib-server/model-header.mustache

@@ -16,6 +16,38 @@
 
 namespace {{vendorExtensions.modelNamespace}} {
 
+{{#isEnum}}
+enum class {{vendorExtensions.modelClassName}} {
+    {{#allowableValues}}{{#enumVars}}
+    {{name}}{{^-last}},{{/-last}}
+    {{/enumVars}}{{/allowableValues}}
+};
+
+inline void to_json(nlohmann::json& j, const {{vendorExtensions.modelClassName}}& value)
+{
+    switch (value)
+    {
+        {{#allowableValues}}{{#enumVars}}
+        case {{vendorExtensions.modelClassName}}::{{name}}: j = {{#vendorExtensions.isStringEnum}}"{{{value}}}"{{/vendorExtensions.isStringEnum}}{{^vendorExtensions.isStringEnum}}{{value}}{{/vendorExtensions.isStringEnum}}; break;
+        {{/enumVars}}{{/allowableValues}}
+        default: throw nlohmann::json::type_error::create(302, "Invalid value for {{vendorExtensions.modelClassName}}", &j);
+    }
+}
+
+inline void from_json(const nlohmann::json& j, {{vendorExtensions.modelClassName}}& value)
+{
+    {{#allowableValues}}{{#enumVars}}
+    if (j == {{#vendorExtensions.isStringEnum}}"{{{value}}}"{{/vendorExtensions.isStringEnum}}{{^vendorExtensions.isStringEnum}}{{value}}{{/vendorExtensions.isStringEnum}})
+    {
+        value = {{vendorExtensions.modelClassName}}::{{name}};
+        return;
+    }
+    {{/enumVars}}{{/allowableValues}}
+    throw nlohmann::json::type_error::create(302, "Invalid value for {{vendorExtensions.modelClassName}}", &j);
+}
+
+{{/isEnum}}
+{{^isEnum}}
 {{#vendorExtensions.isOneOfSchema}}
 // oneOf schema - type alias for std::variant
 using {{vendorExtensions.modelClassName}} = std::variant<
@@ -250,6 +282,7 @@ private:
 {{/vendorExtensions.isAnyOfSchema}}
 {{/vendorExtensions.isOneOfSchema}}
 
+{{/isEnum}}
 } // namespace {{vendorExtensions.modelNamespace}}
 
 {{/model}}

modules/openapi-generator/src/main/resources/cpp-httplib-server/model-source.mustache

@@ -3,6 +3,11 @@
 
 {{#models}}
 {{#model}}
+{{#isEnum}}
+#include "{{vendorExtensions.modelClassName}}.h"
+
+{{/isEnum}}
+{{^isEnum}}
 {{^vendorExtensions.isOneOfSchema}}
 {{^vendorExtensions.isAnyOfSchema}}
 #include "{{vendorExtensions.modelClassName}}.h"
@@ -79,5 +84,6 @@ std::string {{classname}}::{{enumName}}ToString({{classname}}::{{enumName}} valu
 {{^vendorExtensions.isOneOfSchema}}{{^vendorExtensions.isAnyOfSchema}}
 } // namespace {{vendorExtensions.modelNamespace}}
 {{/vendorExtensions.isAnyOfSchema}}{{/vendorExtensions.isOneOfSchema}}
+{{/isEnum}}
 {{/model}}
 {{/models}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/cpphttplibserver/CppHttplibServerCodegenModelTest.java

@@ -19,10 +19,15 @@
 import org.openapitools.codegen.languages.CppHttplibServerCodegen;
 import org.openapitools.codegen.model.ModelMap;
 import org.openapitools.codegen.model.ModelsMap;
+import io.swagger.v3.oas.models.OpenAPI;
 import io.swagger.v3.oas.models.media.*;
 import org.testng.Assert;
 import org.testng.annotations.Test;
 
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
@@ -35,13 +40,17 @@ public class CppHttplibServerCodegenModelTest {
      * Wraps a single model the way {@link org.openapitools.codegen.DefaultGenerator}
      * does before calling {@code postProcessAllModels}, so tests can exercise the full
      * enum vendor-extension pipeline (identifier + original-value derivation), not just
-     * the intermediate state produced by {@code fromModel}.
+     * the intermediate state produced by {@code fromModel}. This mirrors
+     * {@code DefaultGenerator}'s {@code processModels}, which runs {@code postProcessModels}
+     * (and, for C++, {@code postProcessModelsEnum} with it) before {@code postProcessAllModels}
+     * is ever invoked.
      */
-    private Map<String, ModelsMap> wrapForPostProcessAllModels(String name, CodegenModel model) {
+    private Map<String, ModelsMap> wrapForPostProcessAllModels(CppHttplibServerCodegen codegen, String name, CodegenModel model) {
         final ModelMap modelMap = new ModelMap();
         modelMap.setModel(model);
         final ModelsMap modelsMap = new ModelsMap();
         modelsMap.setModels(Collections.singletonList(modelMap));
+        codegen.postProcessModels(modelsMap);
         final HashMap<String, ModelsMap> allModels = new HashMap<>();
         allModels.put(name, modelsMap);
         return allModels;
@@ -173,7 +182,7 @@ public void arrayOfEnumsDeclaresValidUpperCaseIdentifiersTest() {
 
         final CodegenModel model = codegen.fromModel("ModelWithColorArray", schema);
         final CodegenModel processedModel = codegen.postProcessAllModels(
-                wrapForPostProcessAllModels("ModelWithColorArray", model))
+                wrapForPostProcessAllModels(codegen, "ModelWithColorArray", model))
                 .get("ModelWithColorArray").getModels().get(0).getModel();
 
         CodegenProperty arrayProp = processedModel.vars.get(0);
@@ -221,7 +230,7 @@ public void numericEnumPropertyTest() {
         // postProcessAllModels, since that's the single place both the identifier and the
         // original spec value are derived together (see enumSerializationUsesOriginalSpecValueTest).
         final CodegenModel processedModel = codegen.postProcessAllModels(
-                wrapForPostProcessAllModels("UserStatusModel", model))
+                wrapForPostProcessAllModels(codegen, "UserStatusModel", model))
                 .get("UserStatusModel").getModels().get(0).getModel();
         CodegenProperty statusProp = processedModel.vars.get(0);
         Assert.assertTrue((boolean) statusProp.vendorExtensions.getOrDefault("isEnum", false));
@@ -244,7 +253,7 @@ public void enumSerializationUsesOriginalSpecValueTest() {
 
         final CodegenModel model = codegen.fromModel("Pet", schema);
         final CodegenModel processedModel = codegen.postProcessAllModels(
-                wrapForPostProcessAllModels("Pet", model))
+                wrapForPostProcessAllModels(codegen, "Pet", model))
                 .get("Pet").getModels().get(0).getModel();
 
         CodegenProperty statusProp = processedModel.vars.get(0);
@@ -274,15 +283,150 @@ public void enumModelTest() {
 
         final CodegenModel model = codegen.fromModel("Status", enumSchema);
 
-        // Note: The C++ httplib server generator may not process enum-only models
-        // in the same way as regular object models. The model might be null or empty.
-        if (model != null) {
-            Assert.assertEquals(model.name, "Status");
-            // Check if it's marked as an enum in vendor extensions
-            if (model.vendorExtensions.containsKey("x-is-enum")) {
-                Assert.assertEquals(model.vendorExtensions.get("x-is-enum"), true);
-            }
-        }
+        Assert.assertNotNull(model);
+        Assert.assertEquals(model.name, "Status");
+        Assert.assertTrue(model.isEnum, "top-level enum schemas must remain enum models");
+        Assert.assertEquals(model.vendorExtensions.get("isStringEnum"), true,
+                "string-backed top-level enums must serialize their values as JSON strings");
+        Assert.assertNotNull(model.allowableValues);
+        Assert.assertEquals(model.allowableValues.get("values"),
+                java.util.Arrays.asList("ACTIVE", "INACTIVE", "PENDING"));
+
+        final CodegenModel processedModel = codegen.postProcessAllModels(
+                wrapForPostProcessAllModels(codegen, "Status", model))
+                .get("Status").getModels().get(0).getModel();
+        Assert.assertNotNull(processedModel.vendorExtensions.get("modelClassName"));
+        Assert.assertEquals(((List<?>) processedModel.allowableValues.get("enumVars")).size(), 3);
+    }
+
+    @Test(description = "convert integer-backed top-level enum model")
+    public void integerBackedEnumModelTest() {
+        final CppHttplibServerCodegen codegen = new CppHttplibServerCodegen();
+        codegen.processOpts();
+
+        IntegerSchema enumSchema = new IntegerSchema();
+        enumSchema.setEnum(java.util.Arrays.asList(0, 1, 2));
+
+        final CodegenModel model = codegen.fromModel("NumericStatus", enumSchema);
+
+        Assert.assertNotNull(model);
+        Assert.assertTrue(model.isEnum, "top-level enum schemas must remain enum models");
+        // model-header.mustache's `{{^vendorExtensions.isStringEnum}}` branch renders these
+        // values unquoted, so the flag must be false for integer-backed top-level enums.
+        Assert.assertEquals(model.vendorExtensions.get("isStringEnum"), false,
+                "integer-backed top-level enums must serialize their values as raw JSON numbers");
+    }
+
+    @Test(description = "convert 3.1 nullable top-level enum model with string values")
+    public void nullableStringEnumModelTest() {
+        final CppHttplibServerCodegen codegen = new CppHttplibServerCodegen();
+        codegen.processOpts();
+
+        // OpenAPI 3.1 `type: [null, string]`. ModelUtils.getType() returns the *first* declared
+        // type, so ModelUtils.isStringSchema() is false here even though the enum is string-backed.
+        JsonSchema enumSchema = new JsonSchema();
+        enumSchema.addType("null");
+        enumSchema.addType("string");
+        enumSchema.setEnum(java.util.Arrays.asList("available", "pending", "sold"));
+
+        final CodegenModel model = codegen.fromModel("NullableStatus", enumSchema);
+
+        Assert.assertNotNull(model);
+        Assert.assertTrue(model.isEnum, "top-level enum schemas must remain enum models");
+        // Without the value-based fallback the template would emit `j = available;` instead of
+        // `j = "available";`, which does not compile.
+        Assert.assertEquals(model.vendorExtensions.get("isStringEnum"), true,
+                "string-valued top-level enums must serialize their values as JSON strings even when"
+                        + " the declared type is not plainly `string`");
+    }
+
+    @Test(description = "model-header.mustache must render quoted values for string-backed "
+            + "top-level enums and bare values for integer-backed ones; the other enum tests "
+            + "only assert the isStringEnum flag that feeds this template, not its output")
+    public void topLevelEnumHeadersRenderTypedJsonValuesTest() throws IOException {
+        final File output = Files.createTempDirectory("cpp-httplib-server-enums").toFile();
+        output.deleteOnExit();
+
+        StringSchema stringEnumSchema = new StringSchema();
+        stringEnumSchema.setEnum(java.util.Arrays.asList("active", "inactive", "pending"));
+        IntegerSchema integerEnumSchema = new IntegerSchema();
+        integerEnumSchema.setEnum(java.util.Arrays.asList(0, 1, 2));
+
+        final OpenAPI openAPI = TestUtils.createOpenAPI();
+        openAPI.getComponents().addSchemas("TopLevelStatus", stringEnumSchema);
+        openAPI.getComponents().addSchemas("TopLevelPriority", integerEnumSchema);
+
+        final CppHttplibServerCodegen codegen = new CppHttplibServerCodegen();
+        codegen.additionalProperties().put("modelNamespace", "models");
+        codegen.setOutputDir(output.getAbsolutePath());
+
+        final List<File> files = new DefaultGenerator()
+                .opts(new ClientOptInput().openAPI(openAPI).config(codegen))
+                .generate();
+        files.forEach(File::deleteOnExit);
+
+        // model-header.mustache guards the quoting with `{{#vendorExtensions.isStringEnum}}`, so a
+        // `type: string` enum must emit quoted values; bare ones would be undeclared identifiers.
+        final Path stringEnumHeader = output.toPath().resolve("models/TopLevelStatus.h");
+        TestUtils.assertFileContains(stringEnumHeader,
+                "case TopLevelStatus::ACTIVE: j = \"active\"; break;",
+                "case TopLevelStatus::INACTIVE: j = \"inactive\"; break;",
+                "case TopLevelStatus::PENDING: j = \"pending\"; break;",
+                "if (j == \"active\")");
+        TestUtils.assertFileNotContains(stringEnumHeader, "j = active;");
+        // An enumerator outside the declared set must fail loudly rather than leave `j` untouched,
+        // mirroring the throw that from_json already performs on an unrecognised JSON value.
+        TestUtils.assertFileContains(stringEnumHeader,
+                "default: throw nlohmann::json::type_error::create(302, "
+                        + "\"Invalid value for TopLevelStatus\", &j);");
+
+        // ...while the `{{^vendorExtensions.isStringEnum}}` branch must leave a `type: integer`
+        // enum's values bare, or they serialize as JSON strings instead of numbers.
+        final Path integerEnumHeader = output.toPath().resolve("models/TopLevelPriority.h");
+        TestUtils.assertFileContains(integerEnumHeader,
+                "case TopLevelPriority::_0: j = 0; break;",
+                "case TopLevelPriority::_1: j = 1; break;",
+                "case TopLevelPriority::_2: j = 2; break;",
+                "if (j == 0)");
+        TestUtils.assertFileNotContains(integerEnumHeader, "j = \"0\";");
+        TestUtils.assertFileContains(integerEnumHeader,
+                "default: throw nlohmann::json::type_error::create(302, "
+                        + "\"Invalid value for TopLevelPriority\", &j);");
+    }
+
+    @Test(description = "model-source.mustache's ToString and FromString for an enum property "
+            + "are plain C++ conversion helpers, not nlohmann (de)serializers, so an unrecognised "
+            + "value must not throw nlohmann::json::type_error: ToString falls back to an empty "
+            + "string and FromString throws plain std::invalid_argument")
+    public void propertyEnumConversionRejectsUnknownValuesTest() throws IOException {
+        final File output = Files.createTempDirectory("cpp-httplib-server-prop-enums").toFile();
+        output.deleteOnExit();
+
+        StringSchema statusSchema = new StringSchema();
+        statusSchema.setEnum(java.util.Arrays.asList("available", "pending", "sold"));
+        ObjectSchema petSchema = new ObjectSchema();
+        petSchema.addProperty("status", statusSchema);
+
+        final OpenAPI openAPI = TestUtils.createOpenAPI();
+        openAPI.getComponents().addSchemas("Pet", petSchema);
+
+        final CppHttplibServerCodegen codegen = new CppHttplibServerCodegen();
+        codegen.additionalProperties().put("modelNamespace", "models");
+        codegen.setOutputDir(output.getAbsolutePath());
+
+        final List<File> files = new DefaultGenerator()
+                .opts(new ClientOptInput().openAPI(openAPI).config(codegen))
+                .generate();
+        files.forEach(File::deleteOnExit);
+
+        // Property enums (de)serialize through StatusEnumToString/StatusEnumFromString rather
+        // than the switch/if-chain in model-header.mustache's top-level to_json/from_json, and
+        // those helpers don't depend on nlohmann::json at all.
+        final Path source = output.toPath().resolve("models/Pet.cpp");
+        TestUtils.assertFileContains(source,
+                "default: return {};",
+                "throw std::invalid_argument(\"Invalid enum value\");");
+        TestUtils.assertFileNotContains(source, "nlohmann::json::type_error::create(302, \"Invalid value for Pet::StatusEnum\")");
     }
 
     @Test(description = "convert model with nullable property")

modules/openapi-generator/src/test/resources/3_0/cpp-httplib-server/feature-test.json

@@ -1006,6 +1006,14 @@
           }
         }
       },
+      "TopLevelStatus": {
+        "type": "string",
+        "enum": ["active", "inactive", "pending"]
+      },
+      "TopLevelPriority": {
+        "type": "integer",
+        "enum": [0, 1, 2]
+      },
       "SimpleObject": {
         "type": "object",
         "required": ["id", "name"],

samples/server/petstore/cpp-httplib-server/feature-test/.openapi-generator/FILES

@@ -68,3 +68,7 @@ models/TestQueryParameters200Response.cpp
 models/TestQueryParameters200Response.h
 models/TestQueryParametersDeepObjectParameter.cpp
 models/TestQueryParametersDeepObjectParameter.h
+models/TopLevelPriority.cpp
+models/TopLevelPriority.h
+models/TopLevelStatus.cpp
+models/TopLevelStatus.h