add singleton example

examples/singleton.cpp

@@ -0,0 +1,69 @@
+#define SOL_CHECK_ARGUMENTS 1
+#include <sol.hpp>
+
+#include <iostream>
+
+#include <memory>
+#include <mutex>
+
+struct SomeLib {
+private:
+	SomeLib() {}
+public:
+	static std::shared_ptr<SomeLib> getInstance();
+
+	int doSomething() const {
+		return 20;
+	}
+
+	// destructor must be public to work with 
+	// std::shared_ptr and friends
+	// if you need it to be private, you must implement
+	// a custom deleter with access to the private members
+	// (e.g., a deleter struct defined in this class)
+	~SomeLib() {}
+};
+
+std::shared_ptr<SomeLib> SomeLib::getInstance() {
+	static std::weak_ptr<SomeLib> instance;
+	static std::mutex m;
+
+	m.lock();
+	auto ret = instance.lock();
+	if (!ret) {
+		ret.reset(new SomeLib());
+		instance = ret;
+	}
+	m.unlock();
+
+	return ret;
+}
+
+int main(int, char*[]) {
+	std::cout << "=== singleton example ===" << std::endl;
+
+	sol::state lua;
+	lua.open_libraries(sol::lib::base);
+
+	lua.new_usertype<SomeLib>("SomeLib",
+		"new", sol::no_constructor,
+		"getInstance", &SomeLib::getInstance,
+		"doSomething", &SomeLib::doSomething
+		);
+
+	lua.script(R"(
+
+-- note we use the `.` here, not `:` (there's no self to access)
+local sli = SomeLib.getInstance()
+
+-- we use the `:` here because there is something to access
+local value = sli:doSomething()
+
+-- check
+print('sli:doSomething() returned:', value)
+assert(value == 20)
+)");
+
+	std::cout << std::endl;
+	return 0;
+}