编辑

如果你不中断/继续/返回等等，你可以添加一个捕获到任何未知的异常，并把always代码放在它后面。这也是当您不需要重新抛出异常的时候。

try{
   // something that might throw exception
} catch( ... ){
   // what to do with uknown exception
}

//final code to be called always,
//don't forget that it might throw some exception too
doSomeCleanUp(); 

那么问题是什么呢?

通常，在其他编程语言中，finally通常无论如何都运行(通常是指不管任何返回、中断、继续等等)，除了某种系统exit()——这在每种编程语言中有很大不同——例如，PHP和Java只是在那一刻退出，但Python无论如何都执行finally，然后退出。

但是我上面描述的代码并不是这样工作的 =>下面的代码只输出一些错误!：

#include <stdio.h>
#include <iostream>
#include <string>

std::string test() {
    try{
       // something that might throw exception
       throw "exceptiooon!";

       return "fine";
    } catch( ... ){
       return "something wrong!";
    }
    
    return "finally";
}

int main(void) {
    
    std::cout << test();
    
    
    return 0;
}

在c++中，由于RAII, final是不需要的。

RAII将异常安全的责任从对象的用户转移到对象的设计者(和实现者)。我认为这是正确的地方，因为你只需要让异常安全正确一次(在设计/实现中)。通过使用finally，您需要在每次使用对象时都获得正确的异常安全性。

在我看来，代码看起来更整洁了(见下文)。

例子:

一个数据库对象。为了确保使用了DB连接，必须打开并关闭它。通过使用RAII，这可以在构造函数/析构函数中完成。

C+像火车

void someFunc()
{
    DB    db("DBDesciptionString");
    // Use the db object.

} // db goes out of scope and destructor closes the connection.
  // This happens even in the presence of exceptions.

RAII的使用使得正确使用DB对象变得非常容易。DB对象将通过使用析构函数正确地关闭自身，无论我们如何尝试和滥用它。

Java终于来了

void someFunc()
{
    DB      db = new DB("DBDesciptionString");
    try
    {
        // Use the db object.
    }
    finally
    {
        // Can not rely on finaliser.
        // So we must explicitly close the connection.
        try
        {
            db.close();
        }
        catch(Throwable e)
        {
           /* Ignore */
           // Make sure not to throw exception if one is already propagating.
        }
    }
}

当最终使用对象时，对象的正确使用委托给对象的用户。例如，正确地显式地关闭DB连接是对象用户的责任。现在，您可能会认为这可以在终结器中完成，但资源可能有有限的可用性或其他约束，因此您通常希望控制对象的释放，而不是依赖于垃圾收集器的非确定性行为。

这也是一个简单的例子。 当需要释放多个资源时，代码可能会变得复杂。

更详细的分析可以在这里找到:http://accu.org/index.php/journals/236

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。

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);
        }
    }

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(); });

    ...

I also think that RIIA is not a fully useful replacement for exception handling and having a finally. BTW, I also think RIIA is a bad name all around. I call these types of classes 'janitors' and use them a LOT. 95% of the time they are neither initializing nor acquiring resources, they are applying some change on a scoped basis, or taking something already set up and making sure it's destroyed. This being the official pattern name obsessed internet I get abused for even suggesting my name might be better.

我只是认为不合理的做法是，要求某些特殊列表的每一个复杂设置都必须编写一个类来包含它，以避免在清理过程中出现问题时需要捕获多个异常类型时的复杂性。这将导致大量的临时类，否则这些类是不必要的。

是的，对于专为管理特定资源而设计的类，或者专为处理一组类似资源而设计的泛型类，这是没问题的。但是，即使所有涉及的东西都有这样的包装器，清理的协调也可能不仅仅是简单的反序析构函数调用。

我认为c++有一个final。我的意思是，天哪，在过去的几十年里，它被粘上了这么多零碎的东西，似乎奇怪的是，人们突然对一些东西变得保守起来，比如最终，它可能非常有用，可能不像其他一些已经添加的东西那么复杂(尽管这只是我的猜测)。