构造函数何时抛出异常是正确的?(或者在Objective C的情况下:什么情况下init ` er才应该返回nil?)

在我看来,如果对象不完整,构造函数应该失败——因此拒绝创建对象。也就是说,构造函数应该与它的调用者有一个合同,以提供一个函数和工作对象,在哪些方法可以被有意义地调用?这合理吗?


当前回答

Throwing an exception during construction is a great way to make your code way more complex. Things that would seem simple suddenly become hard. For example, let's say you have a stack. How do you pop the stack and return the top value? Well, if the objects in the stack can throw in their constructors (constructing the temporary to return to the caller), you can't guarantee that you won't lose data (decrement stack pointer, construct return value using copy constructor of value in stack, which throws, and now have a stack that just lost an item)! This is why std::stack::pop does not return a value, and you have to call std::stack::top.

这个问题在这里有很好的描述,检查第10项,编写异常安全的代码。

其他回答

Throwing an exception during construction is a great way to make your code way more complex. Things that would seem simple suddenly become hard. For example, let's say you have a stack. How do you pop the stack and return the top value? Well, if the objects in the stack can throw in their constructors (constructing the temporary to return to the caller), you can't guarantee that you won't lose data (decrement stack pointer, construct return value using copy constructor of value in stack, which throws, and now have a stack that just lost an item)! This is why std::stack::pop does not return a value, and you have to call std::stack::top.

这个问题在这里有很好的描述,检查第10项,编写异常安全的代码。

构造函数抛出异常是合理的,只要它正确地清理了自己。如果您遵循RAII范式(资源获取即初始化),那么构造函数通常会做有意义的工作;如果构造函数不能完全初始化,那么编写良好的构造函数将自行清理。

严格地从Java的角度来说,任何时候初始化使用非法值的构造函数时,它都应该抛出异常。这样它就不会被构造成一个糟糕的状态。

构造函数的任务是使对象进入可用状态。关于这个问题,基本上有两种观点。

一组人赞成两阶段建设。构造函数只是将对象带入一个休眠状态,在这种状态下它拒绝做任何工作。还有一个额外的函数来进行实际的初始化。

我一直不明白这种方法背后的原因。我坚决支持单阶段构造,即对象在构造后完全初始化并可用。

如果单阶段构造函数未能完全初始化对象,则应该抛出。如果对象不能初始化,则必须不允许它存在,因此构造函数必须抛出。

是的,如果构造函数未能构建其内部部分之一,则它可以(通过选择)有责任抛出(并以某种语言声明)一个显式异常,这在构造函数文档中有适当的说明。

This is not the only option: It could finish the constructor and build an object, but with a method 'isCoherent()' returning false, in order to be able to signal an incoherent state (that may be preferable in certain case, in order to avoid a brutal interruption of the execution workflow due to an exception) Warning: as said by EricSchaefer in his comment, that can bring some complexity to the unit testing (a throw can increase the cyclomatic complexity of the function due to the condition that triggers it)

如果它因为调用者而失败(比如调用者提供了一个空参数,而被调用的构造函数需要一个非空参数),构造函数无论如何都会抛出一个未检查的运行时异常。