Performance Impact of RTTI

Using RTTI in a large-scale application can have several performance implications. It's important to understand these to make informed decisions about when and how to use RTTI.

Memory Overhead

RTTI requires additional memory to store type information for each class. This includes:

• The vtable pointer in each object
• The vtable itself, which includes pointers to virtual functions and type information
• The type information structure

For a large application with many classes, this can add up to a significant amount of memory.

Runtime Cost

Using RTTI operations like typeid() and dynamic_cast() incurs a runtime cost:

• typeid() typically involves following the vtable pointer and comparing type info objects
• dynamic_cast() can be more expensive, as it may need to traverse the entire class hierarchy

Here's a simple benchmark to illustrate:

1#include <chrono>
2#include <iostream>
3#include <memory>
4#include <vector>
5
6class Base {
7 public:
8  virtual ~Base() = default;
9};
10
11class Derived : public Base {};
12
13int main() {
14  using namespace std::chrono;
15  const int iterations = 1000000;
16
17  // Use smart pointers for automatic memory management
18  std::vector<std::unique_ptr<Base>> objects;
19  objects.reserve(iterations);
20
21  for (int i = 0; i < iterations; ++i) {
22    objects.push_back(std::make_unique<Derived>());
23  }
24
25  auto start = high_resolution_clock::now();
26
27  for (int i = 0; i < iterations; ++i) {
28    if (typeid(*objects[i]) == typeid(Derived)) { 
29      // Do nothing
30    }
31  }
32
33  auto end = high_resolution_clock::now();
34  auto duration =
35      duration_cast<milliseconds>(end - start);
36
37  std::cout << "Time taken: " << duration.count()
38    << " milliseconds\n";
39}

1Time taken: 43 milliseconds

Code Size

RTTI can increase the size of your compiled code. The compiler needs to generate and include type information for all classes that might be used in RTTI operations.

Impact on Optimization

In some cases, the presence of RTTI can limit certain compiler optimizations. For example, if the compiler can't prove that a dynamic_cast() will always succeed, it may not be able to optimize it away.

Mitigation Strategies

To mitigate these performance impacts:

1. Use RTTI judiciously. Only enable it for classes that truly need it.
2. Consider alternatives like virtual functions or custom type identification systems for performance-critical code.
3. Profile your application to identify any RTTI-related bottlenecks.
4. Some compilers allow you to disable RTTI for parts of your code that don't need it.

Here's an example of a custom type ID system that could be faster than RTTI:

1#include <iostream>
2
3class Base {
4 public:
5  enum class Type { Base, Derived };
6  virtual Type GetType() const {
7    return Type::Base;
8  }
9  virtual ~Base() = default;
10};
11
12class Derived : public Base {
13 public:
14  Type GetType() const override {
15    return Type::Derived;
16  }
17};
18
19int main() {
20  Base* obj = new Derived();
21  // This is typically faster than using typeid
22  if (obj->GetType() == Base::Type::Derived) {
23    std::cout << "Derived";
24  }
25  delete obj;
26}

1Derived

In conclusion, while RTTI is a powerful feature, it's important to be aware of its performance implications in large-scale applications. Use it when its benefits outweigh the performance costs, and consider alternatives when performance is critical.