ScopeFail(c17)

Version >= C++ 17

感謝 int std::uncaught_exceptions()，我們可以實現僅在失敗時執行的操作（在範圍內丟擲異常）。以前 bool std::uncaught_exception() 只允許檢測是否有任何堆疊展開正在執行。

#include <exception>
#include <iostream>

template <typename F>
class ScopeFail
{
private:
    F f;
    int uncaughtExceptionCount = std::uncaught_exceptions();
public:
    explicit ScopeFail(const F& f) : f(f) {}
    ScopeFail(const ScopeFail&) = delete;
    ScopeFail& operator =(const ScopeFail&) = delete;

    // f() should not throw, else std::terminate is called.
    ~ScopeFail() {
        if (uncaughtExceptionCount != std::uncaught_exceptions()) {
            f();
        }
    }
};

struct Foo {
    ~Foo() {
        try {
            ScopeFail logFailure{[](){std::cout << "Fail 1\n";}};
            // Scope succeeds,
            // even if Foo is destroyed during stack unwinding
            // (so when 0 < std::uncaught_exceptions())
            // (or previously std::uncaught_exception() == true)
        } catch (...) {
        }
        try {
            ScopeFail logFailure{[](){std::cout << "Failure 2\n";}};

            throw std::runtime_error("Failed"); // returned value
                                                // of std::uncaught_exceptions increases
        } catch (...) { // returned value of std::uncaught_exceptions decreases
        }
    }

};

int main()
{
    try {
        Foo foo;

        throw std::runtime_error("Failed"); // std::uncaught_exceptions() == 1
    } catch (...) { // std::uncaught_exceptions() == 0
    }
}

輸出：

Failure 2