Question

I’m considering a type erasure setup that uses typeid to resolve the type like so…

struct BaseThing
{
    virtual ~BaseThing() = 0 {}
};

template<typename T>
struct Thing : public BaseThing
{
    T x;
};

struct A{};
struct B{};

int main() 
{
    BaseThing* pThing = new Thing<B>();
    const std::type_info& x = typeid(*pThing);

    if( x == typeid(Thing<B>))
    {
        std::cout << "pThing is a Thing<B>!\n";
        Thing<B>* pB = static_cast<Thing<B>*>(pThing);
    }
    else if( x == typeid(Thing<A>))
    {
        std::cout << "pThing is a Thing<A>!\n";
        Thing<A>* pA = static_cast<Thing<A>*>(pThing);
    }
}

I’ve never seen anyone else do this. The alternative would be for BaseThing to have a pure virtual GetID() which would be used to deduce the type instead of using typeid. In this situation, with only 1 level of inheritance, what’s the cost of typeid vs the cost of a virtual function call? I know typeid uses the vtable somehow, but how exactly does it work?

This would be desirable instead of GetID() because it takes quite a bit of hackery to try to make sure the IDs are unique and deterministic.

Answer 1

Typically, you don’t just want to know the type, but also do something with the object as that type. In that case, dynamic_cast is more useful:

int main() 
{
    BaseThing* pThing = new Thing<B>();

    if(Thing<B>* pThingB = dynamic_cast<Thing<B>*>(pThing)) {
    {
        // Do something with pThingB
    }
    else if(Thing<A>* pThingA = dynamic_cast<Thing<A>*>(pThing)) {
    {
        // Do something with pThingA
    }
}

I think this is why you rarely see typeid used in practice.

Update:

Since this question concerns performance. I ran some benchmarks on g++ 4.5.1. With this code:

struct Base {
  virtual ~Base() { }
  virtual int id() const = 0;
};

template <class T> struct Id;

template<> struct Id<int> { static const int value = 1; };
template<> struct Id<float> { static const int value = 2; };
template<> struct Id<char> { static const int value = 3; };
template<> struct Id<unsigned long> { static const int value = 4; };

template <class T>
struct Derived : Base {
  virtual int id() const { return Id<T>::value; }
};

static const int count = 100000000;

static int test1(Base *bp)
{
  int total = 0;
  for (int iter=0; iter!=count; ++iter) {
    if (Derived<int>* dp = dynamic_cast<Derived<int>*>(bp)) {
      total += 5;
    }
    else if (Derived<float> *dp = dynamic_cast<Derived<float>*>(bp)) {
      total += 7;
    }
    else if (Derived<char> *dp = dynamic_cast<Derived<char>*>(bp)) {
      total += 2;
    }
    else if (
      Derived<unsigned long> *dp = dynamic_cast<Derived<unsigned long>*>(bp)
    ) {
      total += 9;
    }
  }
  return total;
}

static int test2(Base *bp)
{
  int total = 0;
  for (int iter=0; iter!=count; ++iter) {
    const std::type_info& type = typeid(*bp);

    if (type==typeid(Derived<int>)) {
      total += 5;
    }
    else if (type==typeid(Derived<float>)) {
      total += 7;
    }
    else if (type==typeid(Derived<char>)) {
      total += 2;
    }
    else if (type==typeid(Derived<unsigned long>)) {
      total += 9;
    }
  }
  return total;
}

static int test3(Base *bp)
{
  int total = 0;
  for (int iter=0; iter!=count; ++iter) {
    int id = bp->id();
    switch (id) {
      case 1: total += 5; break;
      case 2: total += 7; break;
      case 3: total += 2; break;
      case 4: total += 9; break;
    }
  }
  return total;
}

Without optimization, I got these runtimes:

test1: 2.277s
test2: 0.629s
test3: 0.469s

With optimization -O2, I got these runtimes:

test1: 0.118s
test2: 0.220s
test3: 0.290s

So it appears that dynamic_cast is the fastest method when using optimization with this compiler.