Add a special "root" value for [options]

docs/cmake-toml.md

@@ -100,9 +100,12 @@ include-after = ["cmake/after-subdir.cmake"]
 [options]
 MYPROJECT_BUILD_TESTS = false
 MYPROJECT_SPECIAL_OPTION = { value = true, help = "Docstring for this option." }
+MYPROJECT_BUILD_EXAMPLES = "root"
 ```
 
-Options correspond to [CMake cache variables](https://cmake.org/cmake/help/book/mastering-cmake/chapter/CMake%20Cache.html) that can be used to customize your project at configure-time. You can configure with `cmake -DMYPROJECT_BUILD_TESTS=ON` to enable the option. Every options automatically gets a corresponding [condition](#conditions).
+Options correspond to [CMake cache variables](https://cmake.org/cmake/help/book/mastering-cmake/chapter/CMake%20Cache.html) that can be used to customize your project at configure-time. You can configure with `cmake -DMYPROJECT_BUILD_TESTS=ON` to enable the option. Every option automatically gets a corresponding [condition](#conditions).
+
+The special value `root` can be used to set the option to `true` if the project is compiled as the root project (it will be `false` if someone is including your project via `[fetch-content]` or `[subdir]`).
 
 ## Variables
 

include/project_parser.hpp

@@ -25,7 +25,7 @@ struct Variable {
 struct Option {
     std::string name;
     std::string help;
-    bool value = false;
+    mpark::variant<bool, std::string> value;
 };
 
 struct Package {

src/cmake_generator.cpp

@@ -656,7 +656,13 @@ void generate_cmake(const char *path, const parser::Project *parent_project) {
     if (!project.options.empty()) {
         comment("Options");
         for (const auto &opt : project.options) {
-            cmd("option")(opt.name, RawArg(Command::quote(opt.help)), opt.value ? "ON" : "OFF");
+            std::string default_val;
+            if (opt.value.index() == 0) {
+                default_val = mpark::get<0>(opt.value) ? "ON" : "OFF";
+            } else {
+                default_val = mpark::get<1>(opt.value);
+            }
+            cmd("option")(opt.name, RawArg(Command::quote(opt.help)), default_val);
         }
         endl();
     }

src/project_parser.cpp

@@ -377,10 +377,33 @@ Project::Project(const Project *parent, const std::string &path, bool build) : p
             const auto &value = itr.second;
             if (value.is_boolean()) {
                 o.value = value.as_boolean();
+            } else if (value.is_string()) {
+                auto str = std::string(value.as_string());
+                if (str == "root") {
+                    o.value = std::string("${CMKR_ROOT_PROJECT}");
                 } else {
+                    throw_key_error("Unsupported option value '" + str + "'", str, value);
+                }
+            } else if (value.is_table()) {
                 auto &option = checker.create(value);
                 option.optional("help", o.help);
-                option.optional("value", o.value);
+                if (option.contains("value")) {
+                    const auto &ovalue = option.find("value");
+                    if (ovalue.is_boolean()) {
+                        o.value = ovalue.as_boolean();
+                    } else if (ovalue.is_string()) {
+                        auto str = std::string(ovalue.as_string());
+                        if (str == "root") {
+                            o.value = std::string("${CMKR_ROOT_PROJECT}");
+                        } else {
+                            throw_key_error("Unsupported option value '" + str + "'", str, value);
+                        }
+                    } else {
+                        throw_key_error(toml::concat_to_string("Unsupported value type: ", ovalue.type()), "value", value);
+                    }
+                }
+            } else {
+                throw_key_error(toml::concat_to_string("Unsupported value type: ", itr.second.type()), itr.first, itr.second);
             }
             options.push_back(o);
             conditions.emplace(o.name, o.name);