c++ - 什么决定临时对象的生命周期何时扩展到 const 引用或右值引用？

Question

给定:

struct hurg { ... };

hurg get_hurg() { return hurg(); }
hurg&& get_mhurg() { return hurg(); }

我的理解and experimenting表明以下是不是未定义的行为(编辑:感谢答案，事实证明我错了，get_mhurg() 示例是未定义的行为):

{
    const hurg& a = get_hurg(); 
    hurg&& b = get_hurg();
    const hurg& c = get_mhurg(); 
    hurg&& d = get_mhurg();
    // do stuff with a, b, c, d
}
// a, b, c, d are now destructed

即 get_hurg() 和 get_mhurg() 返回的临时 hurg 对象的生命周期被延长到结束范围。

但是，在(来自 here 的函数)的情况下:

template <typename T>
auto id(T&& x) -> decltype(auto) { return decltype(x)(x); }    

像这样使用它:

{
    const hurg& x = id(hurg()); 
    // the hurg() 'x' refers to is already destructed

    hurg&& y = id(hurg());
    // the hurg() 'y' refers to is already destructed

    // undefined behavior: use 'x' and 'y'
}

在这种情况下，hurg 的生命周期不会延长。

什么决定了临时对象的生命周期一般何时延长？而且，特别是，什么时候将函数的结果绑定(bind)到 const 左值引用或右值引用是安全的？

更具体地说，id 案例中到底发生了什么？

Answer 1

来自 [class.temporary]:

There are two contexts in which temporaries are destroyed at a different point than the end of the fullexpression. The first context is when a default constructor is called to initialize an element of an array [...]

The second context is when a reference is bound to a temporary. The temporary to which the reference is bound or the temporary that is the complete object of a sub-object to which the reference is bound persists for the lifetime of the reference except:
(5.1) — A temporary object bound to a reference parameter in a function call (5.2.2) persists until the completion of the full-expression containing the call.
(5.2) — The lifetime of a temporary bound to the returned value in a function return statement (6.6.3) is not extended; the temporary is destroyed at the end of the full-expression in the return statement.
(5.3) — A temporary bound to a reference in a new-initializer (5.3.4) persists until the completion of the full-expression containing the new-initializer.

所以有两件事。首先，get_mhurg 是未定义的行为。您返回的临时对象的生命周期不会延长。其次，传递给 id 的临时值一直持续到包含函数调用的完整表达式结束，但没有进一步。与 get_mhurg 一样，临时文件不是完全扩展的。所以这也是未定义的行为。