[cpp-httplib-server] Generate and compile a spec without models (#25279)

A spec with no schemas never reaches postProcessAllModels, so the model
namespace stayed unset and postProcessOperationsWithModels threw a
NullPointerException. Default it in processOpts with the other namespaces.

The API source then still named the undeclared model namespace and relied on
a model header for nlohmann/json. Include the JSON header directly and emit
the using-directive only when the API uses a model.

Fixes #24854

Co-authored-by: ybai08 <271969568+ybai08@users.noreply.github.com>

Author: ybai08

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

@@ -944,6 +944,7 @@ public OperationsMap postProcessOperationsWithModels(OperationsMap objs, List<Mo
             ArrayList<String> sortedModels = new ArrayList<>(modelsUsed);
             Collections.sort(sortedModels);
             objs.put("modelsUsed", new ArrayList<>(sortedModels));
+            objs.put("hasModelsUsed", true);
             objs.put("includeVariantHeader", INCLUDE_VARIANT);
         }
         if (includeOptionalHeader) {
@@ -1038,12 +1039,8 @@ public Map<String, ModelsMap> postProcessAllModels(Map<String, ModelsMap> objs)
 
         Map<String, ModelsMap> processed = super.postProcessAllModels(objs);
 
-        // Compute modelNamespace ONCE and append className
+        // Compute modelNamespace ONCE (defaulted in processOpts) and append className
         String modelNamespaceBase = (String) additionalProperties.get(MODEL_NAMESPACE);
-        if (modelNamespaceBase == null || modelNamespaceBase.isEmpty()) {
-            modelNamespaceBase = MODEL_SUFFIX.toLowerCase(Locale.ROOT);
-            additionalProperties.put(MODEL_NAMESPACE, modelNamespaceBase);
-        }
 
         for (Map.Entry<String, ModelsMap> entry : processed.entrySet()) {
             ModelsMap modelsMap = entry.getValue();
@@ -2143,6 +2140,14 @@ public void processOpts() {
         }
         additionalProperties.put(API_NAMESPACE, apiNamespaceValue);
 
+        // Set model namespace here rather than in postProcessAllModels, which is
+        // never called for a spec without models; the operations still read it.
+        String modelNamespaceValue = (String) additionalProperties.get(MODEL_NAMESPACE);
+        if (modelNamespaceValue == null || modelNamespaceValue.isEmpty()) {
+            modelNamespaceValue = MODEL_SUFFIX.toLowerCase(Locale.ROOT);
+        }
+        additionalProperties.put(MODEL_NAMESPACE, modelNamespaceValue);
+
         // Handle addApiImplStubs option
         if (additionalProperties.containsKey(ADD_API_IMPL_STUBS)) {
             boolean addApiImplStubs = Boolean.parseBoolean(additionalProperties.get(ADD_API_IMPL_STUBS).toString());

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

@@ -3,7 +3,8 @@
 // System headers
 #include <algorithm>
 #include <cctype>
-#include <locale>{{#operations}}{{#operation}}{{#vendorExtensions}}{{#hasAnyRequestSchema}}{{#hasArrayParams}}{{#-first}}
+#include <locale>
+#include <nlohmann/json.hpp>{{#operations}}{{#operation}}{{#vendorExtensions}}{{#hasAnyRequestSchema}}{{#hasArrayParams}}{{#-first}}
 #include <sstream>{{/-first}}{{/hasArrayParams}}{{/hasAnyRequestSchema}}{{/vendorExtensions}}{{/operation}}{{/operations}}
 
 // Project headers
@@ -16,8 +17,10 @@ constexpr int {{constName}} = {{statusCode}};
 
 namespace {{apiNamespace}} {
 
+{{#hasModelsUsed}}
 using namespace {{modelNamespace}};
 
+{{/hasModelsUsed}}
 {{#operations}}
 {{#operation}}
 {{#vendorExtensions}}

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

@@ -17,7 +17,13 @@
 
 import static org.openapitools.codegen.utils.StringUtils.underscore;
 
+import java.io.File;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.List;
+
 import org.openapitools.codegen.*;
+import org.openapitools.codegen.config.CodegenConfigurator;
 import org.openapitools.codegen.languages.CppHttplibServerCodegen;
 import org.testng.Assert;
 import org.testng.annotations.Test;
@@ -144,4 +150,63 @@ public void testApiNamespaceHandling() {
 
         Assert.assertEquals(codegen.additionalProperties().get("apiNamespace"), "myapp::api");
     }
+
+    @Test
+    public void testDefaultModelNamespaceIsSetBeforeAnyModelIsProcessed() {
+        CppHttplibServerCodegen fresh = new CppHttplibServerCodegen();
+        fresh.processOpts();
+
+        Assert.assertEquals(fresh.additionalProperties().get("modelNamespace"), "models");
+    }
+
+    @Test
+    public void testSpecWithoutModelsGeneratesPrimitiveResponses() throws Exception {
+        // #24854: a spec with no schemas never reaches postProcessAllModels
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("cpp-httplib-server")
+                .setInputSpec("src/test/resources/3_1/cpp-httplib-server/primitive-responses-no-models.yaml")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        List<File> files = new DefaultGenerator().opts(configurator.toClientOptInput()).generate();
+        files.forEach(File::deleteOnExit);
+
+        Path apiHeader = files.stream()
+                .map(File::toPath)
+                .filter(path -> path.toString().endsWith("Api.h"))
+                .findFirst()
+                .orElseThrow(() -> new AssertionError("no API header generated: " + files));
+        TestUtils.assertFileContains(apiHeader,
+                "using TextGetResponse = std::string;",
+                "using FileGetResponse = std::string;");
+        // Without a model header the source must include the JSON library itself
+        // and must not name the (undeclared) model namespace.
+        Path apiSource = Path.of(apiHeader.toString().replaceAll("\\.h$", ".cpp"));
+        TestUtils.assertFileContains(apiSource, "#include <nlohmann/json.hpp>");
+        TestUtils.assertFileNotContains(apiSource, "using namespace models;");
+        for (File file : files) {
+            if (file.getName().endsWith(".h") || file.getName().endsWith(".cpp")) {
+                TestUtils.assertFileNotContains(file.toPath(), "String.h", "File.h");
+            }
+        }
+    }
+
+    @Test
+    public void testApiSourceUsesModelNamespaceWhenModelsAreUsed() throws Exception {
+        File output = Files.createTempDirectory("test").toFile();
+        output.deleteOnExit();
+
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("cpp-httplib-server")
+                .setInputSpec("src/test/resources/3_0/cpp-httplib-server/petstore.json")
+                .setOutputDir(output.getAbsolutePath().replace("\\", "/"));
+
+        List<File> files = new DefaultGenerator().opts(configurator.toClientOptInput()).generate();
+        files.forEach(File::deleteOnExit);
+
+        Path petApiSource = output.toPath().resolve("api/PetApi.cpp");
+        TestUtils.assertFileContains(petApiSource, "using namespace models;", "#include <nlohmann/json.hpp>");
+    }
 }

modules/openapi-generator/src/test/resources/3_1/cpp-httplib-server/primitive-responses-no-models.yaml

@@ -0,0 +1,26 @@
+openapi: 3.1.0
+info:
+  title: Primitive Response Test
+  version: 1.0.0
+paths:
+  /text:
+    get:
+      operationId: getText
+      responses:
+        '200':
+          description: Successful response
+          content:
+            text/plain:
+              schema:
+                type: string
+  /file:
+    get:
+      operationId: getFile
+      responses:
+        '200':
+          description: Binary response
+          content:
+            application/octet-stream:
+              schema:
+                type: string
+                format: binary

samples/server/petstore/cpp-httplib-server/feature-test/api/CompositionApi.cpp

@@ -8,6 +8,7 @@
 #include <algorithm>
 #include <cctype>
 #include <locale>
+#include <nlohmann/json.hpp>
 
 // Project headers
 #include "CompositionApi.h"

samples/server/petstore/cpp-httplib-server/feature-test/api/DatatypesApi.cpp

@@ -8,6 +8,7 @@
 #include <algorithm>
 #include <cctype>
 #include <locale>
+#include <nlohmann/json.hpp>
 
 // Project headers
 #include "DatatypesApi.h"

samples/server/petstore/cpp-httplib-server/feature-test/api/ParametersApi.cpp

@@ -8,6 +8,7 @@
 #include <algorithm>
 #include <cctype>
 #include <locale>
+#include <nlohmann/json.hpp>
 
 // Project headers
 #include "ParametersApi.h"

samples/server/petstore/cpp-httplib-server/feature-test/api/SecurityApi.cpp

@@ -8,6 +8,7 @@
 #include <algorithm>
 #include <cctype>
 #include <locale>
+#include <nlohmann/json.hpp>
 
 // Project headers
 #include "SecurityApi.h"

samples/server/petstore/cpp-httplib-server/petstore/api/PetApi.cpp

@@ -8,6 +8,7 @@
 #include <algorithm>
 #include <cctype>
 #include <locale>
+#include <nlohmann/json.hpp>
 
 // Project headers
 #include "PetApi.h"

samples/server/petstore/cpp-httplib-server/petstore/api/StoreApi.cpp

@@ -8,6 +8,7 @@
 #include <algorithm>
 #include <cctype>
 #include <locale>
+#include <nlohmann/json.hpp>
 
 // Project headers
 #include "StoreApi.h"