问题描述
一次，我认为这样的代码会失败：
Once upon a time, I assumed that code like this would fail:
const MyClass& obj = MyClass(); obj.DoSomething();
，因为MyClass对象将在其full-expression结尾处被销毁，使obj成为悬挂参考。然而，我（在这里）学到这不是真的;标准实际上有一个特殊的规定，允许const引用保持临时存活，直到所述引用自己被销毁。但是，强调的是，只有 const 引用有这个权力。今天我在VS2012中运行下面的代码作为一个实验。
because the MyClass object would be destroyed at the end of its full-expression, leaving obj as a dangling reference. However, I learned (here) that this isn't true; the standard actually has a special provision that allows const references to keep temporaries alive until said references are destroyed themselves. But, it was emphasized, only const references have this power. Today I ran the code below in VS2012 as an experiment.
struct Foo
{
Foo() { std::cout << "ctor" << std::endl; }
~Foo() { std::cout << "dtor" << std::endl; }
};
void f()
{
Foo& f = Foo();
std::cout << "Hello world" << std::endl;
}
调用 f c $ c>是：
The output when calling f() was:
ctor Hello world dtor
所以我看了一下C ++ 11草案标准，只找到了这个（§12.2 / 4）：
So I had a look at the C++11 draft standard, and only found this (§ 12.2/4):
上面显然没有单词 const 。所以;有这个行为被改变为C ++ 11，是我错了关于 const 开始，或VS2012有一个错误，我只是没有找到相关部分标准？
The word const is conspicuously absent from the above. So; has this behavior been changed for C++11, was I wrong about the const thing to begin with, or does VS2012 have a bug and I just haven't found the relevant part of the standard?
推荐答案
行为没有改变，你只需要将你的警告级别更改为 / W4 。 VisualStudio实现了生存期扩展规则，即使对于非 - const lvalue引用作为编译器扩展。在这种情况下，将右值绑定到非 - const 引用的行为与将它绑定到 const 参考。
The behavior hasn't changed, you just need to turn your warning level up to /W4. VisualStudio implements the lifetime extension rule even for non-const lvalue references as a compiler extension. In this context, binding an rvalue to the non-const reference behaves the same as if you were binding it to a const reference.
使用 / W4 ，您会看到：
warning C4239: nonstandard extension used : 'initializing' : conversion from 'Foo' to 'Foo &' 1> A non-const reference may only be bound to an lvalue
不允许将右值绑定到非 const lvalue reference可以在
The text disallowing binding of an rvalue to a non-const lvalue reference can be found in §8.5.3/5
double& rd2 = 2.0; // error: not an lvalue and reference not const int i = 2; double& rd3 = i; // error: type mismatch and reference not const
- end example]
—end example ]
引用语句的后半部分允许将临时数据绑定到右值引用，如。
The second half of the quoted statement is what allows binding of a temporary to an rvalue reference, as shown in litb's answer.
string &&s = string("hello");
结合§12.2/ 5 意味着临时的生命周期现在将匹配它绑定到的（右值）引用的生存期。
This, combined with the lifetime extension rule in §12.2/5, means the lifetime of the temporary will now match the lifetime of the (rvalue) reference it is bound to.
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