c++ - "bind directly"在引用初始化中是什么意思？

Question

N4527 8.5.3 [dcl.init.ref]

5 A reference to type “cv1 T1” is initialized by an expression of type “cv2 T2” as follows:

(5.1) — [...]

• (5.1.1) — [...]

• (5.1.2) — [...]

(5.2) — Otherwise, the reference shall be an lvalue reference to a non-volatile const type (i.e., cv1 shall be const), or the reference shall be an rvalue reference.

• (5.2.1) — If the initializer expression

• • (5.2.1.1) — is an xvalue (but not a bit-field), class prvalue, array prvalue or function lvalue and “cv1 T1” is reference-compatible with “cv2 T2”, or
• • (5.2.1.2) — has a class type (i.e., T2 is a class type), where T1 is not reference-related to T2, and can be converted to an xvalue, class prvalue, or function lvalue of type “cv3 T3”, where “cv1 T1” is reference-compatible with “cv3 T3” (see 13.3.1.6),

then the reference is bound to the value of the initializer expression in the first case and to the result of the conversion in the second case (or, in either case, to an appropriate base class subobject).

• (5.2.2) — Otherwise:

• • (5.2.2.1) — If T1 or T2 is a class type and T1 is not reference-related to T2, user-defined conversions are considered using the rules for copy-initialization of an object of type “cv1 T1” by userdefined conversion (8.5, 13.3.1.4, 13.3.1.5); the program is ill-formed if the corresponding non-reference copy-initialization would be ill-formed. The result of the call to the conversion function, as described for the non-reference copy-initialization, is then used to direct-initialize the reference. For this direct-initialization, user-defined conversions are not considered.
• • (5.2.2.2) — Otherwise, a temporary of type “cv1 T1” is created and copy-initialized (8.5) from the initializer expression. The reference is then bound to the temporary.
• • If T1 is reference-related to T2:
• • (5.2.2.3) — cv1 shall be the same cv-qualification as, or greater cv-qualification than, cv2 ; and
• • (5.2.2.4) — if the reference is an rvalue reference, the initializer expression shall not be an lvalue.

In all cases except the last (i.e., creating and initializing a temporary from the initializer expression), the reference is said to bind directly to the initializer expression.

“最后一个案例”是什么意思？ 5.2.2(包括 5.2.2.1 和 5.2.2.2)还是 5.2.2.2(只有一个)？

也就是说，5.2.2.1是不是直接绑定(bind)了？

//case 5.2.1.2
struct X{};

struct Y{Y(X);};
const Y& y = X();   // bind directly

struct Z{operator X();};
const X& x = Z();   // bind directly

//case 5.2.2.1
struct A{operator int();};
const int& a = A(); // bind directly or not?

struct B{B(int);};
const B& b = 1;     // bind directly or not?

Answer 1

“最后一个案例”指的是 5.2.2.2:

— Otherwise, a temporary of type “cv1 T1” is created and copy-initialized (8.5) from the initializer expression. The reference is then bound to the temporary.

这种情况取决于先前的条件为假，即 5.2.2.1:

— If T1 or T2 is a class type and T1 is not reference-related to T2...

示例的最后两个片段不是这种情况，因为第一个片段 A是类类型并且int与 A 相关 .在第二个例子中，B是类类型并且B与 int 无关.由于这些都是错误的，因此 5.2.2.2 不适用(它们确实直接绑定(bind))。