为什么匿名临时异常可以绑定到捕获站点的引用？

Question

考虑

#include <iostream>

struct Foo{};

int main(){
    try {
        throw Foo();
    } catch (Foo& e){
        std::cout << "Caught";
    }
}

输出是Caught，但为什么呢？我本以为 catch 应该是 const Foo&。我忘记了什么？

Answer 1

这是因为抛出的对象被认为是左值。 [except.handle]/14 状态：

The variable declared by the exception-declaration, of type cv T or cv T&, is initialized from the exception object, of type E, as follows:

if T is a base class of E, the variable is copy-initialized ([dcl.init]) from an lvalue of type T designating the corresponding base class subobject of the exception object;

otherwise, the variable is copy-initialized ([dcl.init]) from an lvalue of type E designating the exception object.

The lifetime of the variable ends when the handler exits, after the destruction of any objects with automatic storage duration initialized within the handler.

Answer 2

I would have thought that the catch should have been const Foo&.

不需要。

我怀疑您期望将左值引用绑定到 non-const。

会出现问题

[except.handle]

The variable declared by the exception-declaration, of type cv T or cv T&, is initialized from the exception object, of type E, as follows:

if T is a base class of E ...

otherwise, the variable is copy-initialized ([dcl.init]) from an lvalue of type E designating the exception object.

The lifetime of the variable ends when the handler exits, after the destruction of any objects with automatic storage duration initialized within the handler.

注意突出显示的“左值”。将 non-const 左值引用绑定到左值没有问题。

旁注：

A handler is a match for an exception object of type E if

The handler is of type cv T or cv T& and E and T are the same type (ignoring the top-level cv-qualifiers), or

顶级 cv 限定符被忽略，不需要匹配抛出的对象和处理程序类型。

... When the handler declares a reference to an object, any changes to the referenced object are changes to the exception object and will have effect should that object be rethrown.

捕获引用允许修改异常对象。

为什么匿名临时异常可以绑定到捕获站点的引用？

Why can an anonymous temporary exception be bound to a reference at a catch site?

c++

language-lawyer