Added `opaque_handle::[try_]get_type()`: Get an `std::type_info` if possible.

Added typing related tests to main.

Fortune for opaque_handle's current commit: Small blessing − 小吉

include/opaque.hh

@@ -13,6 +13,7 @@ struct opaque_exception : std::exception $_oe_msg("unspecified opaque_handle rel
 struct opaque_not_copyable : $_oe("opaque_handle referred to a type that was not copyable");
 struct opaque_handle_moved : $_oe("this opaque_handle was moved");
 struct opaque_bad_cast : $_oe("bad opaque_handle pointer cast");
+struct opaque_unknown_type : $_oe("handle refers to an unknown or incomplete type");
 
 struct opaque_handle;
 struct opaque_handle_impl;
@@ -124,6 +125,14 @@ struct opaque_handle final {
 		return _impl ? _impl->is_type(ty) : false;
 	}
 
+	constexpr const std::type_info* try_get_type() const noexcept {
+		return _impl ? _impl->get_type_info() : nullptr;
+	}
+
+	constexpr const std::type_info& get_type() const {
+		if(__builtin_expect(_impl == nullptr, false)) throw opaque_handle_moved{};
+		return *(_impl->get_type_info() ?: throw opaque_unknown_type{});
+	}
 	
 	/// ptr_cast(): use try_cast() for complete types, and unsafe_cast() for incomplete types.
 	template<typename T>

src/main.cpp

@@ -5,18 +5,101 @@
 
 #include <unistd.h>
 
+#include <cxxabi.h>
+
 #include "../include/opaque.hh"
 
-static void s_puts(const auto& string)
+struct demangle_failed : std::exception {
+	virtual int status() const noexcept = 0;
+	const char* what() const noexcept override {
+		auto s = status();
+		switch(s) {
+			case -1: return "A memory allocation failure occurred";
+			case -2: return "`mangled` was not a valid name under the C++ ABI mangling rules";
+			case -3: return "One of the arguments was invalid";
+			default: return "Unknown error";
+		}
+	}
+protected:
+	inline demangle_failed() : std::exception(){}
+};
+static std::string demangle_name(const char* mangled)
+{
+	struct demangle_error final : public demangle_failed
+	{
+		inline demangle_error(int s) : demangle_failed(), _status(s) {}
+
+		inline int status() const noexcept override final { return _status; }
+
+		int _status;
+	};
+
+	size_t len;
+	int status;
+	char * demangled = abi::__cxa_demangle(mangled, nullptr, &len, &status);
+
+	const auto c_str_to_string = [len](char* str)
+			noexcept(std::is_nothrow_constructible_v<std::string, const char*, size_t>) 
+	{
+		const auto _free = [&]() { free(static_cast<void*>(str)); };
+
+		if constexpr(std::is_nothrow_constructible_v<std::string, const char*, size_t>) {
+			std::string v{str, len};
+			_free();
+			return v;
+		} else {
+			try {
+				std::string v{str, len};
+				_free();
+				return v;
+			} catch(...) {
+				_free();
+				throw;
+			}
+		}
+	};
+
+	if(demangled)
+		return c_str_to_string(demangled);
+	else 
+		throw demangle_error(status);
+}
+
+inline static std::string demangle_name(const std::string& mangled)
+{
+	return demangle_name(mangled.c_str());
+}
+
+static inline void s_puts(const auto& string, FILE* to = nullptr)
 {
 	std::string_view str{string};
+	to = to ?: stdout;
+
 #ifdef DIRECT_WRITE
-	write(1, str.data(), str.size());
+	write(fileno(to), str.data(), str.size());
 #else
-	printf("%.*s", (int)str.size(), str.data());
+	fprintf(to, "%.*s", (int)str.size(), str.data());
 #endif
 }
 
+static void print_name(const std::type_info& type)
+{
+	const char* name = type.name();
+	s_puts("type holds: ");
+	try {
+		s_puts(demangle_name(name));
+		s_puts("\n");
+	} catch(demangle_failed& err) {
+		s_puts("<mangled> ");
+		s_puts(name);
+		s_puts("\n");
+		
+		s_puts("Failed to demangle name: ", stderr);
+		s_puts(err.what(), stderr);
+		s_puts("\n", stderr);
+	}
+}
+
 static void print_view(std::string_view ptr)
 {
 	//std::string_view ptr{string};
@@ -55,6 +138,10 @@ static void use_moh()
 	std::string* str = new std::string(__PRETTY_FUNCTION__);
 
 	constexpr auto v_print = [](const opaque_handle& v) {
+		try {
+			print_name(v.get_type());
+		} catch(const opaque_unknown_type&) { s_puts("type holds: <unknown>\n"); }
+
 		print(*v);
 		{
 			auto copied = v;
@@ -119,6 +206,15 @@ static void use_c_struct()
 	opaque_handle impossible{make_opaque_handle<opaque, opaque_empty_handler<opaque>>(nullptr)};
 	black_box(impossible);
 
+	s_puts("Testing <incomplete>.get_type() throwing... ");
+	try {
+		print_name(impossible.get_type());
+		s_puts(" FAILED (expected `opaque_unknown_type` exception)\n");
+		std::terminate();
+	} catch(const opaque_unknown_type&) {
+		s_puts(" OK\n");
+	}
+
 	const opaque_handle v{make_opaque_handle<type>(allocp, [](auto* ptr, auto op) noexcept {
 		if(ptr) {
 			switch(op) {
@@ -129,6 +225,10 @@ static void use_c_struct()
 		return static_cast<type*>(nullptr);
 	})};
 
+	try {
+		print_name(v.get_type());
+	} catch(const opaque_unknown_type&) { s_puts("type holds: <unknown at this point>\n"); }
+
 	s_puts("> ");
 	cs_print(v);
 	{
@@ -145,7 +245,27 @@ int main()
 	const 
 	opaque_handle v{make_opaque_object_handle<std::string>("Hello world")};
 	
-	puts("Auto-downcasting to `const std::string&`:");
+	{
+		using prim_t = int;
+		struct never_t{};
+		auto prim = make_opaque_object_handle<prim_t>(-115);
+		puts("Primitive type handling");
+		s_puts("(pre-mutation)");
+		{
+			prim_t& vref = prim.cast<prim_t>();
+			print(std::to_string(std::as_const(vref)));
+			vref *= -1;
+		}
+		s_puts("(post-mutation)");
+		print(std::to_string(std::as_const(prim).cast<prim_t>()));
+		print_name(prim.get_type());
+		s_puts(prim.is_type<prim_t>() ? "\t-> +v: yes, " : "\t-> +v: no, ");
+		s_puts(prim.is_type<never_t>() ? "!v: yes\n" : "!v: no\n");	
+	}
+
+	puts("\nAuto-downcasting to `const std::string&`:");
+	print_name(v.get_type());
+
 	print(*v);
 #if 1
 	s_puts("\nAttempting invalid safe cast... ");