Fix drogon_ctl compilation with clang-cl (#2477)

Author: Greisby

drogon_ctl/create.cc

@@ -56,3 +56,14 @@ void create::handleCommand(std::vector<std::string> &parameters)
     parameters[0] = createObjName;
     exeCommand(parameters);
 }
+
+// Prevent clang-cl/lld-link from discarding DrObject<T>::alloc_ on Windows.
+// On COFF targets, clang places the CRT initializer for the template static
+// member in the same COMDAT group as the variable itself.  When no code in the
+// translation unit takes the address of alloc_ (clang inlines className()),
+// the entire COMDAT is eligible for elimination — and lld-link's /OPT:REF
+// removes it, so DrClassMap is never populated.
+// Explicit template instantiation forces a strong (non-COMDAT) definition,
+// which the linker must keep.  This is a no-op on MSVC, GCC, and ELF targets
+// (where .init_array entries are GC roots and are never discarded).
+template class drogon::DrObject<drogon_ctl::create>;

drogon_ctl/create_controller.cc

@@ -469,3 +469,6 @@ void create_controller::createARestfulController(const std::string &className,
     std::cout << "File name: " << ctlName << ".h and " << ctlName << ".cc"
               << std::endl;
 }
+
+// See create.cc for rationale.
+template class drogon::DrObject<drogon_ctl::create_controller>;

drogon_ctl/create_filter.cc

@@ -116,3 +116,6 @@ void create_filter::handleCommand(std::vector<std::string> &parameters)
         createFilterSourceFile(oSourceFile, className, fileName);
     }
 }
+
+// See create.cc for rationale.
+template class drogon::DrObject<drogon_ctl::create_filter>;

drogon_ctl/create_model.cc

@@ -1460,3 +1460,6 @@ void create_model::createRestfulAPIController(
                   << std::endl;
     }
 }
+
+// See create.cc for rationale.
+template class drogon::DrObject<drogon_ctl::create_model>;

drogon_ctl/create_plugin.cc

@@ -116,3 +116,6 @@ void create_plugin::handleCommand(std::vector<std::string> &parameters)
         createPluginSourceFile(oSourceFile, className, fileName);
     }
 }
+
+// See create.cc for rationale.
+template class drogon::DrObject<drogon_ctl::create_plugin>;

drogon_ctl/create_project.cc

@@ -141,3 +141,6 @@ void create_project::createProject(const std::string &projectName)
     std::ofstream testCmakeFile("test/CMakeLists.txt", std::ofstream::out);
     newTestCmakeFile(testCmakeFile, projectName);
 }
+
+// See create.cc for rationale.
+template class drogon::DrObject<drogon_ctl::create_project>;

drogon_ctl/create_view.cc

@@ -552,3 +552,6 @@ void create_view::newViewSourceFile(std::ofstream &file,
     file << "return templ->genText(data);\n";
     file << "}\n}\n";
 }
+
+// See create.cc for rationale.
+template class drogon::DrObject<drogon_ctl::create_view>;

drogon_ctl/help.cc

@@ -68,3 +68,6 @@ void help::handleCommand(std::vector<std::string> &parameters)
         }
     }
 }
+
+// See create.cc for rationale.
+template class drogon::DrObject<drogon_ctl::help>;

drogon_ctl/press.cc

@@ -470,3 +470,6 @@ void press::outputResults()
               << std::endl;
     exit(0);
 }
+
+// See create.cc for rationale.
+template class drogon::DrObject<drogon_ctl::press>;

drogon_ctl/version.cc

@@ -66,3 +66,6 @@ void version::handleCommand(std::vector<std::string> &parameters)
     std::cout << "  yaml-cpp: no\n";
 #endif
 }
+
+// See create.cc for rationale.
+template class drogon::DrObject<drogon_ctl::version>;