We are looking into the idiom:

if (auto spt = wp.lock()) {
    spt->hello();
}

🔍 What’s Happening?

• wp.lock() returns a shared_ptr<T> if the object wp points to is still alive.
• If the object has expired (i.e., the shared_ptr owning it is destroyed), it returns an empty shared_ptr (nullptr inside).

✅ Why if (auto spt = wp.lock()) works:

C++ allows you to use a shared_ptr in a boolean context — it implicitly checks:

if (spt) // is equivalent to: if (spt.get() != nullptr)

So, this is shorthand for:

auto spt = wp.lock();
if (spt != nullptr) {
    spt->hello();
}

But C++ lets you write it concisely:

if (auto spt = wp.lock()) {
    spt->hello();
}

🧠 Summary: