Reflection in Exile

Reflection, specifically type introspection, is an immensely useful feature provided by many modern languages. By enabling a way to inspect the properties of types—for example, the types of the members of a data structure—introspection allows one to write much more general and effective generic code, enforce interfaces/contracts, and more.

Though most obviously found in dynamic languages, introspection is just as powerful in compiled, statically typed languages such as Go and JAI. Unfortunately, C++ does not support run-time reflection as a language feature, and template-based compile-time reflection is neither standardized, elegant, nor robust. Standards proposals have been put forward to add reflection features, but an implementation is still rather far off. Further, libraries aiming to provide run-time reflection typically require laborious setup for each type you wish to make reflect-able.

For exile, I wanted to use run-time reflection to power truly type-agnostic serialization and UI, but I did not want the code overhead of specifying meta-info for all of my types. Inspired by JAI’s model, I chose to implement a library-level reflection system via meta-programming. Combined with exile’s hot reloading system, I find the environment very conducive to iteration and prototyping—even in a very C-like dialect of C++.

Usage

The end result allowed me to write code such as this, to print any structure:
(Full Implementation)

template<typename T>
void print_struct(T value) { 
	
	uint8_t* addr = &value;
	_type_info* info = TYPEINFO(T);

	print(info->name);
	
	print('{');
	for(int i = 0; i < info->_struct.member_count; i++) {

		print(info->_struct.member_names[i]);
		print(" : ");

		uint8_t* member_addr = addr + info->_struct.member_offsets[i];
		_type_info* member_type = TYPEINFO_H(info->_struct.member_types[i]);

		print_type(member_addr, member_type);

		if(i < info->_struct.member_count - 1) {
			print(", ");
		}
	}
	print('}');
}

Usage:

struct inner {
	int i = 10;
	float f = 10.0f;
};

struct outer {
	string name = "Data!";
	inner data;
};

outer o;
print(o);

Result:

outer{name : "Data!", data : inner{i : 10, f : 10.0}}

Structure

The reflection system relies on a global (thread-local) table that contains all type information. This is a simple hash map that maps from type_id to _type_info, which is a discriminated union containing the type’s size, name, type (that is, integer, floating point, pointer, structure, etc), and associated information, such as member values and names.
(Full Implementation)

enum class Type : uint8_t {
	_void,
	_int,
	_float,
	_ptr,
	_struct,
	// ...array, enum, etc...
};

struct Type_void_info {};
struct Type_int_info {
	bool is_signed = false;
};
struct Type_float_info {};
struct Type_bool_info {};
struct Type_ptr_info {
	type_id to = 0;
};
struct Type_struct_info {
	string 		member_names[64];
	type_id		member_types[64]   = {};
	uint32_t 	member_offsets[64] = {};
	uint32_t 	member_count       = 0;
};

struct _type_info {
	
	Type type_type 	= Type::_void;
	type_id hash;

	uint64_t size   = 0;
	string name;

	union {
		Type_void_info	 _void;
		Type_int_info    _int;
		Type_float_info  _float;
		Type_bool_info   _bool;
		Type_ptr_info    _ptr;
		Type_struct_info _struct;
	};
};

Retrieving the meta-information for a type is straightforward: a type_id is easily derived from C++’s built-in typeid operator using hash_code(). Technically, hash_code() is not guaranteed to never collide, so std::type_index() is the more robust option. However, I have not had a problem using hash_code(). Note that because typeid is only ever used on compile-time known types (that is, not polymorphic objects), this code works properly with RTTI disabled, which I compile without.

// ...last example...

map<type_id,_type_info> type_table;

template<typename T>
struct _get_type_info { 
	
	static _type_info* get_type_info() {

		return type_table.try_get((type_id)typeid(T).hash_code());
	}
};

// get by compile-time type
#define TYPEINFO(...) _get_type_info< __VA_ARGS__ >::get_type_info()

// get by hash
#define TYPEINFO_H(h) type_table.try_get(h)

Providing the TYPEINFO() syntax requires a bit of macro trickery via varargs to group incorrectly comma-delinated template types and a static member function for arbitrary specialization.

Getting information for pointers is slightly more complicated: there’s no limit on how many different types of pointers that exist, so we can’t populate the table with all of them! Instead, we automatically generate and add a _type_info when a type of pointer is requested that we haven’t seen before. This is possible because using SFINAE, we can extract the pointed-to type of a request, and set our pointer _type_info to refer to the correct entry in the type table. Because getting the information for an underlying type is a recursive process, this automatically works for multiple indirections: requesting an int** will create an entry for int** that refers to a new entry int* which refers to int.

// ...last example...

template<typename T>
struct _get_type_info<T*> {

	static _type_info* get_type_info() {

		_type_info* info = type_table.try_get((type_id)typeid(T*).hash_code());
		if(info) return info;

		_type_info* to = TYPEINFO(T);

		_type_info ptr_t;
		
		ptr_t.type_type = Type::_ptr;
		ptr_t.hash 		= (type_id)typeid(T*).hash_code();

		ptr_t.size 		= sizeof(T*);
		ptr_t.name 		= to->name;
		
		ptr_t._ptr.to 	= to->hash;
		
		return type_table.insert(ptr_t.hash, ptr_t);
	}
};

Fully built out, along with some utility functions such as enum-stringification and any-width integer load/store, this system is all that is needed to preform very powerful introspection operations. For example, custom UI elements for any type:
(Full Implementation)

ImGui::EditAny("cam", &camera);

Metaprogramming

Now, the type table isn’t especially useful if you have to manually populate it with type information. This is where the metaprogramming comes in: instead of relying on a language feature, I wrote a separate program using libclang to automatically generate the code that populates the type table on startup.

libclang is a C binding for LLVM’s clang compiler frontend. While not as powerful as the full C++ interface in libtooling or plug-ins for clang itself, it provides enough access to the parsed and type-checked AST for this use case. (See limitations).

The basic structure of the meta-program is quite simple: use libclang to parse the C++ source, traverse the AST to pick out type definitions, and generate the code corresponding to each type. Several complications arise when dealing with dependent types (ordering) and instantiating templates (duplication/dependency info), but for the basic case of ordered C-style structs, everything works intuitively. For brevity, most of the code is not included here.
(Full Implementation)

auto index = clang_createIndex(0, 0);
auto unit = clang_parseTranslationUnit(index, "path/to/code", 0, 0,
                                       nullptr, 0, CXTranslationUnit_KeepGoing);

auto cursor = clang_getTranslationUnitCursor(unit);

clang_visitChildren(cursor,
[](CXCursor c, CXCursor parent, CXClientData client_data) {

	parse_type(c);

	return CXChildVisit_Recurse;

}, nullptr);

Once a list of types is built from the AST, code is output to a file that is included by the game. The build system typically invokes the meta-program right before compiling the source.
(Full Output)

// example output for the "any" struct

[]() -> void {
	_type_info this_type_info;
	
	this_type_info.type_type = Type::_struct;
	this_type_info.size = sizeof(any);
	this_type_info.name = "any"_;
	this_type_info.hash = (type_id)typeid(any).hash_code();
	
	this_type_info._struct.member_count = 2;
	
	this_type_info._struct.member_types[0] = TYPEINFO(type_id) ? TYPEINFO(type_id)->hash : 0;
	this_type_info._struct.member_names[0] = "id"_;
	this_type_info._struct.member_offsets[0] = offsetof(any,id);
	
	this_type_info._struct.member_types[1] = TYPEINFO(void *) ? TYPEINFO(void *)->hash : 0;
	this_type_info._struct.member_names[1] = "value"_;
	this_type_info._struct.member_offsets[1] = offsetof(any,value);
	
	type_table.insert(this_type_info.hash, this_type_info, false);
}();

Finally, when the reflection system starts up, it invokes the function output by the meta-program, properly filling out the type table.

Limitations

In July 2019, the meta-program was rewritten to robustly handle nested templates and circular dependencies.

My implementation of the meta-program with libclang is lacking in some areas: the way in which type information is stored prevents circularly dependent types from being output with correct dependencies. Similarly, it cannot decode deeply nested templates. A truly robust implementation would traverse (or rebuild) clang’s representation of the type dependancy graph rather than operating directly from the C++ AST. I plan to upgrade the tool to use libtooling in the future, but the current state is adequate, as the limitations may be worked around. A further upgrade may involve outputting a generic data format that is parsed by the run-time library rather than outputting code that is compiled into the build.

Further, this system does not reflect any OOP concepts such as methods, constructors/destructors, and inheritance, and is hence not really suitable for use with general modern C++. It was made to suit exile’s C-style types with templates, and works accordingly. Such a system for modern C++ is hopefully in the works, and I’d be exited to play around with it.

Written on August 14, 2018