C++ RAII: Resource Acquisition Is Initialization Finally/ScopeExit
Example
For cases when we don't want to write special classes to handle some resource, we may write a generic class:
template<typename Function>
class Finally final
{
public:
explicit Finally(Function f) : f(std::move(f)) {}
~Finally() { f(); } // (1) See below
Finally(const Finally&) = delete;
Finally(Finally&&) = default;
Finally& operator =(const Finally&) = delete;
Finally& operator =(Finally&&) = delete;
private:
Function f;
};
// Execute the function f when the returned object goes out of scope.
template<typename Function>
auto onExit(Function &&f) { return Finally<std::decay_t<Function>>{std::forward<Function>(f)}; }
And its example usage
void foo(std::vector<int>& v, int i)
{
// ...
v[i] += 42;
auto autoRollBackChange = onExit([&](){ v[i] -= 42; });
// ... code as recursive call `foo(v, i + 1)`
}
Note (1): Some discussion about destructor definition has to be considered to handle exception:
~Finally() noexcept { f(); }:std::terminateis called in case of exception~Finally() noexcept(noexcept(f())) { f(); }: terminate() is called only in case of exception during stack unwinding.~Finally() noexcept { try { f(); } catch (...) { /* ignore exception (might log it) */} }Nostd::terminatecalled, but we cannot handle error (even for non stack unwinding).
