为了避免所有我可以在谷歌上搜索到的标准答案，我将提供一个你们都可以随意攻击的例子。
c#和Java(以及其他很多语言)有很多类型，有些“溢出”行为我一点也不喜欢(例如type。MaxValue +类型。SmallestValue ==类型。MinValue，例如int。MaxValue + 1 = int.MinValue)。
但是，鉴于我的邪恶本性，我将通过将此行为扩展为重写DateTime类型来对这种伤害进行侮辱。(我知道DateTime在. net中是密封的，但为了这个例子，我使用了一种与c#完全相似的伪语言，除了DateTime没有密封之外)。
被覆盖的Add方法:
/// <summary>
/// Increments this date with a timespan, but loops when
/// the maximum value for datetime is exceeded.
/// </summary>
/// <param name="ts">The timespan to (try to) add</param>
/// <returns>The Date, incremented with the given timespan.
/// If DateTime.MaxValue is exceeded, the sum wil 'overflow' and
/// continue from DateTime.MinValue.
/// </returns>
public DateTime override Add(TimeSpan ts)
{
try
{
return base.Add(ts);
}
catch (ArgumentOutOfRangeException nb)
{
// calculate how much the MaxValue is exceeded
// regular program flow
TimeSpan saldo = ts - (base.MaxValue - this);
return DateTime.MinValue.Add(saldo)
}
catch(Exception anyOther)
{
// 'real' exception handling.
}
}
当然，如果可以很容易地解决这个问题，但事实仍然是，我不明白为什么不能使用异常(从逻辑上讲，我可以看到，当性能是一个问题时，在某些情况下应该避免异常)。
我认为在许多情况下，它们比if结构更清晰，并且不会破坏方法所做的任何契约。
恕我直言，“永远不要在常规程序流程中使用它们”的反应似乎并不是每个人都有，因为这种反应的力量可以证明。
还是我说错了?
我读过其他的帖子，处理各种特殊情况，但我的观点是，如果你们都是:
清晰的
尊重你的方法
拍我。
有一些通用的机制，语言可以允许一个方法退出而不返回值，并unwind到下一个“catch”块:
Have the method examine the stack frame to determine the call site, and use the metadata for the call site to find either information about a try block within the calling method, or the location where the calling method stored the address of its caller; in the latter situation, examine metadata for the caller's caller to determine in the same fashion as the immediate caller, repeating until one finds a try block or the stack is empty. This approach adds very little overhead to the no-exception case (it does preclude some optimizations) but is expensive when an exception occurs.
Have the method return a "hidden" flag which distinguishes a normal return from an exception, and have the caller check that flag and branch to an "exception" routine if it's set. This routine adds 1-2 instructions to the no-exception case, but relatively little overhead when an exception occurs.
Have the caller place exception-handling information or code at a fixed address relative to the stacked return address. For example, with the ARM, instead of using the instruction "BL subroutine", one could use the sequence:
adr lr,next_instr
b subroutine
b handle_exception
next_instr:
要正常退出，子例程只需执行bx lr或pop {pc};在异常退出的情况下，子例程将在执行返回之前从LR中减去4，或者使用sub LR，#4,pc(取决于ARM的变化，执行模式等)。如果调用者没有被设计为适应它，这种方法将会非常严重地故障。
A language or framework which uses checked exceptions might benefit from having those handled with a mechanism like #2 or #3 above, while unchecked exceptions are handled using #1. Although the implementation of checked exceptions in Java is rather nuisancesome, they would not be a bad concept if there were a means by which a call site could say, essentially, "This method is declared as throwing XX, but I don't expect it ever to do so; if it does, rethrow as an "unchecked" exception. In a framework where checked exceptions were handled in such fashion, they could be an effective means of flow control for things like parsing methods which in some contexts may have a high likelihood of failure, but where failure should return fundamentally different information than success. I'm unaware of any frameworks that use such a pattern, however. Instead, the more common pattern is to use the first approach above (minimal cost for the no-exception case, but high cost when exceptions are thrown) for all exceptions.
正如其他人已经多次提到的，最小惊讶原则将禁止您仅为控制流的目的而过度使用异常。另一方面，没有任何规则是100%正确的，总有一些情况下，异常是“合适的工具”——就像goto本身，顺便说一下，它在Java等语言中以break和continue的形式发布，这通常是跳出大量嵌套循环的完美方式，而这种循环并不总是可以避免的。
下面的博文解释了一个相当复杂但也相当有趣的非本地ControlFlowException的用例:
http://blog.jooq.org/2013/04/28/rare-uses-of-a-controlflowexception
它解释了在jOOQ (Java的SQL抽象库)内部，当满足某些“罕见”条件时，如何偶尔使用这种异常来提前中止SQL呈现过程。
这种条件的例子有:
Too many bind values are encountered. Some databases do not support arbitrary numbers of bind values in their SQL statements (SQLite: 999, Ingres 10.1.0: 1024, Sybase ASE 15.5: 2000, SQL Server 2008: 2100). In those cases, jOOQ aborts the SQL rendering phase and re-renders the SQL statement with inlined bind values. Example:
// Pseudo-code attaching a "handler" that will
// abort query rendering once the maximum number
// of bind values was exceeded:
context.attachBindValueCounter();
String sql;
try {
// In most cases, this will succeed:
sql = query.render();
}
catch (ReRenderWithInlinedVariables e) {
sql = query.renderWithInlinedBindValues();
}
If we explicitly extracted the bind values from the query AST to count them every time, we'd waste valuable CPU cycles for those 99.9% of the queries that don't suffer from this problem.
Some logic is available only indirectly via an API that we want to execute only "partially". The UpdatableRecord.store() method generates an INSERT or UPDATE statement, depending on the Record's internal flags. From the "outside", we don't know what kind of logic is contained in store() (e.g. optimistic locking, event listener handling, etc.) so we don't want to repeat that logic when we store several records in a batch statement, where we'd like to have store() only generate the SQL statement, not actually execute it. Example:
// Pseudo-code attaching a "handler" that will
// prevent query execution and throw exceptions
// instead:
context.attachQueryCollector();
// Collect the SQL for every store operation
for (int i = 0; i < records.length; i++) {
try {
records[i].store();
}
// The attached handler will result in this
// exception being thrown rather than actually
// storing records to the database
catch (QueryCollectorException e) {
// The exception is thrown after the rendered
// SQL statement is available
queries.add(e.query());
}
}
If we had externalised the store() logic into "re-usable" API that can be customised to optionally not execute the SQL, we'd be looking into creating a rather hard to maintain, hardly re-usable API.
结论
从本质上讲，我们对这些非本地goto的使用就像[Mason Wheeler][5]在他的回答中所说的那样:
“我刚刚遇到了一种情况，此时我无法正确处理它，因为我没有足够的上下文来处理它，但调用我的例程(或调用堆栈的更上层)应该知道如何处理它。”
controlflowexception的两种用法与它们的替代方法相比都很容易实现，允许我们重用广泛的逻辑，而无需从相关的内部重构它。
但是对于未来的维护者来说，这种感觉还是有点令人惊讶。代码感觉相当微妙，虽然在这种情况下这是正确的选择，但我们总是不喜欢在本地控制流中使用异常，因为在本地控制流中很容易避免使用普通的if - else分支。
我不认为使用异常来进行流控制有什么错。异常有点类似于延续，在静态类型语言中，异常比延续更强大，所以，如果你需要延续，但你的语言没有它们，你可以使用异常来实现它们。
好吧，实际上，如果你需要延续，而你的语言没有，你选择了错误的语言，你应该使用另一种语言。但有时你别无选择:客户端web编程就是最好的例子——没有办法绕过JavaScript。
An example: Microsoft Volta is a project to allow writing web applications in straight-forward .NET, and let the framework take care of figuring out which bits need to run where. One consequence of this is that Volta needs to be able to compile CIL to JavaScript, so that you can run code on the client. However, there is a problem: .NET has multithreading, JavaScript doesn't. So, Volta implements continuations in JavaScript using JavaScript Exceptions, then implements .NET Threads using those continuations. That way, Volta applications that use threads can be compiled to run in an unmodified browser – no Silverlight needed.
