RTTI in Multithreaded Applications

When using RTTI in multi-threaded applications, it's important to consider thread safety and performance implications. Here are some best practices:

Thread-safe Type Comparisons

RTTI operations like typeid() and type comparisons are inherently thread-safe. You can safely use them in multiple threads without additional synchronization.

1#include <iostream>
2#include <thread>
3#include <vector>
4
5class Base {
6 public:
7  virtual ~Base() {}
8};
9
10class Derived : public Base {};
11
12void checkType(Base* obj) {
13  if (typeid(*obj) == typeid(Derived)) {  
14    std::cout << "Object is Derived\n";
15  } else {
16    std::cout << "Object is Base\n";
17  }
18}
19
20int main() {
21  std::vector<std::thread> threads;
22  Base baseObj;
23  Derived derivedObj;
24
25  for (int i = 0; i < 3; ++i) {
26    threads.emplace_back(checkType, &baseObj);
27    threads.emplace_back(checkType, &derivedObj);
28  }
29
30  for (auto& t : threads) {
31    t.join();
32  }
33}

1Object is Derived
2Object is Base
3Object is Base
4Object is Derived
5Object is Base
6Object is Derived

Avoid RTTI in Performance-Critical Sections

While thread-safe, RTTI operations can introduce slight performance overhead. In highly concurrent, performance-critical sections, consider alternative type-checking mechanisms.

1#include <chrono>
2#include <iostream>
3#include <thread>
4
5class Monster {
6 public:
7  virtual ~Monster() {}
8  virtual int getType() const { return 0; }  
9};
10
11class Dragon : public Monster {
12 public:
13  int getType() const override { return 1; }  
14};
15
16void performActionRTTI(const Monster& m) {
17  if (typeid(m) == typeid(Dragon)) {  
18    // Dragon-specific action
19  }
20}
21
22void performActionCustom(const Monster& m) {
23  if (m.getType() == 1) {  
24    // Dragon-specific action
25  }
26}
27
28int main() {
29  using namespace std::chrono;
30  Dragon dragon;
31  const int iterations = 1000000;
32
33  auto start = high_resolution_clock::now();
34  for (int i = 0; i < iterations; ++i) {
35    performActionRTTI(dragon);
36  }
37  auto end = high_resolution_clock::now();
38  std::cout << "RTTI: "
39    << duration_cast<microseconds>(
40      end - start).count() << " microseconds\n";
41
42  start = high_resolution_clock::now();
43  for (int i = 0; i < iterations; ++i) {
44    performActionCustom(dragon);
45  }
46  end = high_resolution_clock::now();
47  std::cout << "Custom: "
48    << duration_cast<microseconds>(
49      end - start).count() << " microseconds\n";
50}

1RTTI: 12366 microseconds
2Custom: 2860 microseconds

Use RTTI Judiciously in Lock-Free Programming

While RTTI operations are thread-safe, they may introduce hidden synchronization points. In lock-free algorithms, this could potentially impact performance or correctness.

Consider Thread-Local Caching

If you're frequently performing type checks on the same objects, consider caching the results in thread-local storage to reduce RTTI overhead.

1#include <thread>
2#include <typeinfo>
3#include <unordered_map>
4
5thread_local std::unordered_map<
6  const void*, const std::type_info*> typeCache;
7
8template <typename Base, typename Derived>
9bool isSameType(const Base* obj) {
10  auto it = typeCache.find(
11    static_cast<const void*>(obj));
12  if (it == typeCache.end()) {
13    const std::type_info& type = typeid(*obj);
14    typeCache[static_cast<const void*>(obj)] = &type;
15    return type == typeid(Derived);
16  }
17  return *it->second == typeid(Derived);
18}

In summary, be cautious with dynamic_cast in multithreaded code.

While dynamic_cast is thread-safe, it can be slower than typeid. Use it sparingly in performance-critical multithreaded code.

By following these practices, you can effectively use RTTI in multithreaded applications while minimizing potential performance impacts and ensuring thread safety.