C++ · 7 · Advanced / Job-Ready31 / 36 · 86%

Exception Safety Guarantees

Four levels of promise your function can make when something throws.

Examples: basic, strong, nothrow, noexcept

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The Four Levels

State which one every public function provides.

Complexity

No guarantee	avoid	may corrupt or leak
Basic	valid state	invariants hold, values unspecified
Strong	commit or rollback	as if never called
Nothrow	noexcept	never throws

Achieving Strong Safety

Do all the throwing work first, then commit with non-throwing operations.

Example

example

void Container::add(const Item& item) {
    auto copy = data_;        // may throw — nothing committed yet
    copy.push_back(item);     // may throw — still nothing committed
    data_.swap(copy);         // noexcept: the commit point
}

noexcept

A promise, enforced by std::terminate if you break it.

Example

example

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void cleanup() noexcept;                     // must not throw
Buffer(Buffer&&) noexcept;                   // enables vector move
static_assert(std::is_nothrow_move_constructible_v<Buffer>);

Rules That Prevent Disaster

Two hard rules and one habit.

Common pitfalls

✕Never let an exception escape a destructor — during unwinding it calls std::terminate.
✕Catch by const reference (catch (const std::exception& e)), never by value.
✕Use RAII everywhere so unwinding cleans up automatically.
✕Don't use exceptions for ordinary control flow — return std::optional or std::expected.