9.3 The Noexcept Specifier and Vector Move Optimizations

In C++, marking a function as noexcept is a contract with the compiler. By promising that a function will never throw an exception, you allow the compiler to perform optimizations that would otherwise be impossible.

Let's look at noexcept syntax, terminate rules, and vector move optimizations.

You declare a function as non throwing by appending noexcept to its signature. If a function marked noexcept does throw an exception at runtime, C++ does not attempt to unwind the stack. It immediately calls std::terminate to abort the application. Because stack unwinding is bypassed, the compiler can generate smaller, faster code frames.

The most critical application of noexcept is for move constructors. When a std::vector runs out of capacity, it must reallocate its heap buffer. To do this, it allocates a new buffer and moves elements from the old buffer to the new one.

If a move constructor throws an exception halfway through this reallocation, the vector cannot roll back the moves easily, which would violate the strong exception safety guarantee. To prevent this, the C++ standard library checks if your class has a noexcept move constructor:

* If marked noexcept: The vector moves the elements to the new buffer in constant time.

* If NOT marked noexcept: The vector falls back to slow copies, copying every element and then deleting the old ones, to preserve exception safety.

#include <vector>
#include <iostream>

class Item {
public:
    Item() = default;
    // Move constructor must be marked noexcept to avoid vector copy fallback!
    Item(Item&& other) noexcept {
        // Move resources...
    }
    Item& operator=(Item&& other) noexcept {
        // Move resources...
        return *this;
    }
};

* The Delivery Driver Metaphor: Imagine a shipping manager who only accepts delicate glassware from a delivery driver if the driver guarantees they will not drop them. If the driver cannot promise safety (noexcept), the manager orders a slow, heavily padded conveyor belt instead (slow copy fallback) to ensure nothing breaks.