Performance Impact of Weak Pointers

Weak pointers do have a slight performance overhead compared to raw pointers due to the additional bookkeeping they perform. However, in most cases, this overhead is negligible and outweighed by the benefits they provide in terms of safety and expressive power.

When you create a weak pointer, it internally maintains a reference to a control block shared with the corresponding shared pointers. This control block keeps track of the reference counts and manages the object's lifetime. Operations like creating, copying, and destroying weak pointers involve atomic operations on the reference counts, which can have a small performance cost.

However, accessing the pointed-to object through a weak pointer (by creating a shared pointer using lock()) has minimal overhead compared to directly using a raw pointer. The main cost is the atomic operation to check and increment the reference count when locking the weak pointer.

Consider the following example:

1#include <memory>
2#include <iostream>
3
4class MyClass {
5public:
6  void DoSomething() {
7    std::cout << "Doing something";
8  }
9};
10
11void ProcessObject(std::weak_ptr<
12  MyClass> WeakPtr) {
13  if (auto SharedPtr{WeakPtr.lock()}) {
14    SharedPtr->DoSomething();
15  }
16}
17
18int main() {
19  auto Object{std::make_shared<MyClass>()};
20  ProcessObject(Object);
21}

1Doing something

In this case, the overhead of using a weak pointer in ProcessObject is minimal. The lock() operation checks the reference count atomically, and if the object still exists, it returns a shared_ptr that can be used to access the object efficiently.

In most scenarios, the performance impact of using weak pointers is negligible compared to the benefits they provide in terms of safety, expressiveness, and avoiding resource leaks. However, if you have extremely performance-critical code and have verified that weak pointers are a bottleneck, you may consider using raw pointers with careful manual lifetime management in those specific cases.