Question

I needed run-time polymorphism, so I used dynamic_cast.
But now I had two problems — dynamic_cast was extremely slow! (Scroll down for benchmark.)

Long story short, I ended up solving the problem this way, using static_cast:

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

    template<class T>
    T *as()
    { return this->type_id() == T::ID ? static_cast<T *>(this) : 0; }

    template<class T>
    T const *as() const
    { return this->type_id() == T::ID ? static_cast<T const *>(this) : 0; }
};

struct Derived : public Base
{
    enum { ID = __COUNTER__ };  // warning: can cause ABI incompatibility
    int type_id() const { return ID; }
};

int main()
{
    Base const &value = Derived();
    Derived const *p = value.as<Derived>();  // "static" dynamic_cast
}

But I’m surely not the first person to run into this problem, so I thought it might be worth asking:

Instead of coming up with a home-baked solution like this, is there a well-known pattern/library I can use for solving this problem in the future?

---

Sample benchmark

To get a feel for what I’m talking about, try the code below — dynamic_cast was approximately 15 times slower than a mere virtual call on my machine (110 ms. against 1620 ms. with the code below):

#include <cstdio>
#include <ctime>

struct Base { virtual unsigned vcalc(unsigned i) const { return i * i + 1; } };
struct Derived1 : public Base 
{ unsigned vcalc(unsigned i) const { return i * i + 2; } };
struct Derived2 : public Derived1
{ unsigned vcalc(unsigned i) const { return i * i + 3; } };

int main()
{
    Base const &foo = Derived2();
    size_t const COUNT = 50000000;
    {
        clock_t start = clock();
        unsigned n = 0;
        for (size_t i = 0; i < COUNT; i++)
            n = foo.vcalc(n);
        printf("virtual call: %d ms (result: %u)\n",
            (int)((clock() - start) * 1000 / CLOCKS_PER_SEC), n);
        fflush(stdout);
    }
    {
        clock_t start = clock();
        unsigned n = 0;
        for (size_t i = 0; i < COUNT; i++)
            n = dynamic_cast<Derived1 const &>(foo).vcalc(n);
        printf("virtual call after dynamic_cast: %d ms (result: %u)\n",
            (int)((clock() - start) * 1000 / CLOCKS_PER_SEC), n);
        fflush(stdout);
    }
    return 0;
}

When I simply remove the word virtual and change dynamic_cast to static_cast, I get a 79 ms running time — so a virtual call is only slower than a static call by ~25%!

Answer 1

Most uses of dynamic_cast can be replaced with double dispatch (aka. visitor pattern). That would amount to two virtual calls, which by your benchmark is still 7.5 times faster than dynamic_cast.