Proposal: RTTI


Feature awaiting further design consideration.

This is a proposal to add run-time type information to Loci.


This supports checking whether the actual type (which is known only at run-time, unless A is a class type) of a reference or pointer conforms to a given target type (B in this example).

void f(A& object) {
        if (is_a<B&>(object)) {

The performance for each case (depending on types of A and B) is:

  • If A is a class type, and B is a class type, then the ‘is_a’ statement is resolved at compile-time (hence no run-time overhead).
  • If A is a class type, and B is an interface type, then the compiler can again resolve it at compile-time.
  • If A is an interface type, but B is a class type, the implementation for this is just a single comparison, so this is a very efficient way to check the type of an interface reference/pointer.
  • If A is an interface type, and B is an interface type, then this call involves a number of comparisons proportional to the number of methods in the larger interface, unless it can be determined at compile-time that there is a valid static cast from A to B.


This operates just like C++:

int f(A& object) {
        B& castObject = dynamic_cast<B&>(object);
        return castObject.method();

This is effectively just the equivalent of calling is_a to assert the types are compatible, and then either treating the reference to A as a reference to B.


This allows access to information about a type from a reference or pointer to an instance of it:

void f(T& a, T& b) {
        rtti::type_info typeInfo = typeid(a);

        // type_info structures can be compared,
        // but is_a is likely to be more efficient.
        if (typeInfo == typeid(b)) {

        // Get the type's name.
        string typeName =;

        // Get the type's virtual table.
        void * vtable = typeInfo.vtable;

        // Get the type's size.
        size_t typeSize = typeInfo.size;

        for (const auto i: range<size_t>(0, typeInfo.methods.size())) {
                rtti::method_info methodInfo = typeInfo.methods[i];

                // Get method name.
                string methodName =;

                // etc.