RTTI in Multithreaded Applications
What are the best practices for using RTTI in multi-threaded 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.
#include <iostream>
#include <thread>
#include <vector>
class Base {
public:
virtual ~Base() {}
};
class Derived : public Base {};
void checkType(Base* obj) {
if (typeid(*obj) == typeid(Derived)) {
std::cout << "Object is Derived\n";
} else {
std::cout << "Object is Base\n";
}
}
int main() {
std::vector<std::thread> threads;
Base baseObj;
Derived derivedObj;
for (int i = 0; i < 3; ++i) {
threads.emplace_back(checkType, &baseObj);
threads.emplace_back(checkType, &derivedObj);
}
for (auto& t : threads) {
t.join();
}
}Object is Derived
Object is Base
Object is Base
Object is Derived
Object is Base
Object is DerivedAvoid 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.
#include <chrono>
#include <iostream>
#include <thread>
class Monster {
public:
virtual ~Monster() {}
virtual int getType() const { return 0; }
};
class Dragon : public Monster {
public:
int getType() const override { return 1; }
};
void performActionRTTI(const Monster& m) {
if (typeid(m) == typeid(Dragon)) {
// Dragon-specific action
}
}
void performActionCustom(const Monster& m) {
if (m.getType() == 1) {
// Dragon-specific action
}
}
int main() {
using namespace std::chrono;
Dragon dragon;
const int iterations = 1000000;
auto start = high_resolution_clock::now();
for (int i = 0; i < iterations; ++i) {
performActionRTTI(dragon);
}
auto end = high_resolution_clock::now();
std::cout << "RTTI: "
<< duration_cast<microseconds>(
end - start).count() << " microseconds\n";
start = high_resolution_clock::now();
for (int i = 0; i < iterations; ++i) {
performActionCustom(dragon);
}
end = high_resolution_clock::now();
std::cout << "Custom: "
<< duration_cast<microseconds>(
end - start).count() << " microseconds\n";
}RTTI: 12366 microseconds
Custom: 2860 microsecondsUse 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.
#include <thread>
#include <typeinfo>
#include <unordered_map>
thread_local std::unordered_map<
const void*, const std::type_info*> typeCache;
template <typename Base, typename Derived>
bool isSameType(const Base* obj) {
auto it = typeCache.find(
static_cast<const void*>(obj));
if (it == typeCache.end()) {
const std::type_info& type = typeid(*obj);
typeCache[static_cast<const void*>(obj)] = &type;
return type == typeid(Derived);
}
return *it->second == typeid(Derived);
}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.
Run Time Type Information (RTTI) and typeid()
Learn to identify and react to object types at runtime using RTTI, dynamic casting and the typeid() operator