C ++抛出类成员 [英] C++ throwing class members

问题描述

我有以下C ++代码

template <class E>
class ExceptionWrapper {
public:
    explicit ExceptionWrapper(const E& e): e(e) {}

    void throwException() {
        throw e;
    }

private:
    E e;
};

...
try {
    ExceptionWrapper<E> w(...);
    w.throwException();
} catch (const E& e) {
    ...
}
...

问题：此代码有效吗？我可以说，返回对类成员的引用几乎总是无效的（我相信大家都同意这个声明）。但是，我的同事声称， throw 不是这种情况。

Question: is this code valid? I could argue that returning a reference to the class member is almost always not valid (and I am sure everybody agrees with this statement). However, my colleague claims that this is not the case with throw.

在改变 catch（const E& e）到 catch（E e）后，一个讨厌的bug似乎消失了， - 此代码不是有效。

P.S. after changing catch (const E& e) to catch (E e) a nasty bug seemingly disappeared which strengthens my position - that this code is not valid.

推荐答案

通过引用捕获e是无效的，因为e是w的成员，w在捕获范围内不存在。

my claim is that catching e by reference is not valid since e is a member of w and w is not alive in the catch scope.

是不正确的。 throw e; 抛出成员的副本，该副本在catch的范围内有效。

Your claim is incorrect. throw e; throws a copy of the member, and that copy is valid in catch's scope.

3（n3797草稿）：

§ 15.1 / 3 (n3797 draft):

抛出异常副本 - 初始化（
8.5
，
12.8
）一个临时对象，称为
异常对象
。
临时值是一个左值，用于初始化在匹配的
处理程序
（
15.3
）中命名的变量。如果
类型的异常对象是一个不完整的类型或指向一个不完整类型的指针，而不是
（可能是cv-qualified）
void
程序是不成形的。评估
throw-expression
使用操作数throws
异常;异常对象的类型是通过从操作数的
静态类型中删除任何顶级的
cv-qualifiers
并从
T $的数组中调整类型来确定的b $ b或函数返回
T
为指向
T
或指向函数返回
T
。

Throwing an exception copy-initializes ( 8.5 , 12.8 ) a temporary object, called the exception object . The temporary is an lvalue and is used to initialize the variable named in the matching handler ( 15.3 ). If the type of the exception object would be an incomplete type or a pointer to an incomplete type other than (possibly cv-qualified) void the program is ill-formed. Evaluating a throw-expression with an operand throws an exception; the type of the exception object is determined by removing any top-level cv-qualifiers from the static type of the operand and adjusting the type from "array of T " or "function returning T " to "pointer to T " or "pointer to function returning T ," respectively.

通过const引用捕获是捕获异常的首选方法。它允许捕获 std :: exception 的导数，而不会对异常对象进行切片。

Catching by const reference is the preferred way to catch exceptions. It allows catching derivatives of std::exception without slicing the exception object.

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