我有一个用例，我认为它最终应该是c++ 11语言中完全可以接受的一部分，因为我认为从流的角度来看，它更容易阅读。我的用例是线程的消费者/生产者链，其中在运行结束时发送一个哨兵nullptr以关闭所有线程。

如果c++支持它，你会希望你的代码看起来像这样:

    extern Queue downstream, upstream;

    int Example()
    {
        try
        {
           while(!ExitRequested())
           {
             X* x = upstream.pop();
             if (!x) break;
             x->doSomething();
             downstream.push(x);
           } 
        }
        finally { 
            downstream.push(nullptr);
        }
    }

我认为把finally声明放在循环的开始更符合逻辑，因为它发生在循环退出之后……但这只是一厢情愿的想法，因为我们无法在c++中实现它。注意，下游队列连接到另一个线程，所以你不能在下游的析构函数中放入哨兵推(nullptr)，因为此时它不能被销毁……它需要保持活动状态，直到另一个线程接收到nullptr。

下面是如何使用带有lambda的RAII类来做同样的事情:

    class Finally
    {
    public:

        Finally(std::function<void(void)> callback) : callback_(callback)
        {
        }
        ~Finally()
        {
            callback_();
        }
        std::function<void(void)> callback_;
    };

下面是你如何使用它:

    extern Queue downstream, upstream;

    int Example()
    {
        Finally atEnd([](){ 
           downstream.push(nullptr);
        });
        while(!ExitRequested())
        {
           X* x = upstream.pop();
           if (!x) break;
           x->doSomething();
           downstream.push(x);
        }
    }

为什么即使是托管语言也会提供final块，尽管垃圾收集器会自动释放资源?

实际上，基于垃圾收集器的语言需要更多的“finally”。垃圾收集器不会及时销毁您的对象，因此不能依赖它正确地清理与内存无关的问题。

就动态分配数据而言，许多人认为应该使用智能指针。

然而……

RAII将异常安全的责任从对象的用户转移到设计人员

可悲的是，这是它自己的失败。旧的C编程习惯很难改掉。当您使用用C或非常C风格编写的库时，将不会使用RAII。除了重写整个API前端，这就是你必须要处理的。那么，“终于”这个词的缺失真的很伤人。

另一个“finally”块模拟使用c++ 11 lambda函数

template <typename TCode, typename TFinallyCode>
inline void with_finally(const TCode &code, const TFinallyCode &finally_code)
{
    try
    {
        code();
    }
    catch (...)
    {
        try
        {
            finally_code();
        }
        catch (...) // Maybe stupid check that finally_code mustn't throw.
        {
            std::terminate();
        }
        throw;
    }
    finally_code();
}

让我们希望编译器会优化上面的代码。

现在我们可以这样写代码:

with_finally(
    [&]()
    {
        try
        {
            // Doing some stuff that may throw an exception
        }
        catch (const exception1 &)
        {
            // Handling first class of exceptions
        }
        catch (const exception2 &)
        {
            // Handling another class of exceptions
        }
        // Some classes of exceptions can be still unhandled
    },
    [&]() // finally
    {
        // This code will be executed in all three cases:
        //   1) exception was not thrown at all
        //   2) exception was handled by one of the "catch" blocks above
        //   3) exception was not handled by any of the "catch" block above
    }
);

如果你愿意，你可以把这个习语包装成“try - finally”宏:

// Please never throw exception below. It is needed to avoid a compilation error
// in the case when we use "begin_try ... finally" without any "catch" block.
class never_thrown_exception {};

#define begin_try    with_finally([&](){ try
#define finally      catch(never_thrown_exception){throw;} },[&]()
#define end_try      ) // sorry for "pascalish" style :(

现在"finally"块在c++ 11中可用:

begin_try
{
    // A code that may throw
}
catch (const some_exception &)
{
    // Handling some exceptions
}
finally
{
    // A code that is always executed
}
end_try; // Sorry again for this ugly thing

就我个人而言，我不喜欢“宏”版本的“finally”习语，宁愿使用纯粹的“with_finally”函数，即使在这种情况下语法更笨重。

您可以在这里测试上面的代码:http://coliru.stacked-crooked.com/a/1d88f64cb27b3813

PS

如果你的代码中需要一个finally块，那么作用域守卫或ON_FINALLY/ON_EXCEPTION宏可能会更好地满足你的需求。

下面是ON_FINALLY/ON_EXCEPTION的用法示例:

void function(std::vector<const char*> &vector)
{
    int *arr1 = (int*)malloc(800*sizeof(int));
    if (!arr1) { throw "cannot malloc arr1"; }
    ON_FINALLY({ free(arr1); });

    int *arr2 = (int*)malloc(900*sizeof(int));
    if (!arr2) { throw "cannot malloc arr2"; }
    ON_FINALLY({ free(arr2); });

    vector.push_back("good");
    ON_EXCEPTION({ vector.pop_back(); });

    ...

我想出了一个finally宏，可以像¹Java中的finally关键字一样使用;它使用std::exception_ptr及其友项，lambda函数和std::promise，因此它要求c++ 11或以上;它还使用了clang也支持的复合语句表达式GCC扩展。

警告:这个答案的早期版本使用了这个概念的不同实现，有更多的限制。

首先，让我们定义一个helper类。

#include <future>

template <typename Fun>
class FinallyHelper {
    template <typename T> struct TypeWrapper {};
    using Return = typename std::result_of<Fun()>::type;

public:    
    FinallyHelper(Fun body) {
        try {
            execute(TypeWrapper<Return>(), body);
        }
        catch(...) {
            m_promise.set_exception(std::current_exception());
        }
    }

    Return get() {
        return m_promise.get_future().get();
    }

private:
    template <typename T>
    void execute(T, Fun body) {
        m_promise.set_value(body());
    }

    void execute(TypeWrapper<void>, Fun body) {
        body();
    }

    std::promise<Return> m_promise;
};

template <typename Fun>
FinallyHelper<Fun> make_finally_helper(Fun body) {
    return FinallyHelper<Fun>(body);
}

然后是实际的宏观。

#define try_with_finally for(auto __finally_helper = make_finally_helper([&] { try 
#define finally });                         \
        true;                               \
        ({return __finally_helper.get();})) \
/***/

它可以这样使用:

void test() {
    try_with_finally {
        raise_exception();
    }    

    catch(const my_exception1&) {
        /*...*/
    }

    catch(const my_exception2&) {
        /*...*/
    }

    finally {
        clean_it_all_up();
    }    
}

使用std::promise使其非常容易实现，但它可能也引入了相当多不必要的开销，这些开销可以通过只从std::promise中重新实现所需的功能来避免。

注意:有一些东西不像java版本的finally那样工作。我能想到的是:

it's not possible to break from an outer loop with the break statement from within the try and catch()'s blocks, since they live within a lambda function; there must be at least one catch() block after the try: it's a C++ requirement; if the function has a return value other than void but there's no return within the try and catch()'s blocks, compilation will fail because the finally macro will expand to code that will want to return a void. This could be, err, avoided by having a finally_noreturn macro of sorts.

总而言之，我不知道我自己是否会使用这些东西，但玩它很有趣。：）

不一定，但你可以在一定程度上模仿他们，例如:

int * array = new int[10000000];
try {
  // Some code that can throw exceptions
  // ...
  throw std::exception();
  // ...
} catch (...) {
  // The finally-block (if an exception is thrown)
  delete[] array;
  // re-throw the exception.
  throw; 
}
// The finally-block (if no exception was thrown)
delete[] array;

注意，final -block本身可能在原始异常被重新抛出之前抛出一个异常，从而丢弃原始异常。这与Java final -block中的行为完全相同。同样，你不能在try&catch块中使用return。

不，c++不支持'finally'块。原因是c++支持RAII:“资源获取是初始化”——对于一个真正有用的概念来说，这是一个糟糕的名字。

其思想是，对象的析构函数负责释放资源。当对象具有自动存储持续时间时，当创建对象的块退出时，对象的析构函数将被调用——即使该块在出现异常时退出。以下是Bjarne Stroustrup对这个话题的解释。

RAII的一个常见用途是锁定互斥量:

// A class with implements RAII
class lock
{
    mutex &m_;

public:
    lock(mutex &m)
      : m_(m)
    {
        m.acquire();
    }
    ~lock()
    {
        m_.release();
    }
};

// A class which uses 'mutex' and 'lock' objects
class foo
{
    mutex mutex_; // mutex for locking 'foo' object
public:
    void bar()
    {
        lock scopeLock(mutex_); // lock object.

        foobar(); // an operation which may throw an exception

        // scopeLock will be destructed even if an exception
        // occurs, which will release the mutex and allow
        // other functions to lock the object and run.
    }
};

RAII also simplifies using objects as members of other classes. When the owning class' is destructed, the resource managed by the RAII class gets released because the destructor for the RAII-managed class gets called as a result. This means that when you use RAII for all members in a class that manage resources, you can get away with using a very simple, maybe even the default, destructor for the owner class since it doesn't need to manually manage its member resource lifetimes. (Thanks to Mike B for pointing this out.)

For those familliar with C# or VB.NET, you may recognize that RAII is similar to .NET deterministic destruction using IDisposable and 'using' statements. Indeed, the two methods are very similar. The main difference is that RAII will deterministically release any type of resource -- including memory. When implementing IDisposable in .NET (even the .NET language C++/CLI), resources will be deterministically released except for memory. In .NET, memory is not deterministically released; memory is only released during garbage collection cycles.

†有些人认为“破坏是资源放弃”是RAII习语更准确的名称。