cpp core · advanced

Exception-Safety Guarantees

Exception safety classifies what happens to program state when an operation throws. No guarantee: state may be corrupted and resources leaked. Basic guarantee: no leaks and all invariants hold, but the value may be unspecified. Strong guarantee: commit-or-rollback — if it throws, state is exactly as before (a transaction). Nothrow: the operation is noexcept and cannot throw. The copy-and-swap idiom achieves the strong guarantee by doing all throwing work on a temporary copy, then committing with a noexcept swap; RAII delivers the basic guarantee automatically.

The code

// The four exception-safety guarantees, weakest -> strongest:
// 1. no guarantee : a throw may corrupt/leak  (avoid)
// 2. basic        : no leak, object stays valid (value unspecified)
// 3. strong       : commit-or-rollback - state unchanged if it throws
// 4. nothrow      : marked noexcept, cannot throw

// Strong guarantee via copy-and-swap:
T& operator=(T other) { // copy may throw - but only the temp
    swap(*this, other); // noexcept swap commits atomically
    return *this;       // old state freed by temp's dtor
}

What this lesson walks through

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