重载转换函数模板
Overloading conversion function templates
考虑以下几点：
struct X {
template <class T> operator T(); // #1
template <class T> operator T&(); // #2
};
int a = X{}; // error: ambiguous
int& b = X{}; // calls #2
int const& c = X{}; // calls #2
b的情况很简单，#2是唯一可行的候选人。什么规则表明 #2 优先于 #1 用于初始化 int const&，但两者对于 int 的初始化有歧义？
在决定如何初始化给定初始化器的引用时，首先尝试直接绑定。 [dcl.init.ref]/(5.1.2):
If the reference is an lvalue reference and the initializer expression […] 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 lvalue of type “cv3 T3”, where “cv1 T1” is reference-compatible with “cv3 T3” (this conversion is selected by enumerating the applicable conversion functions (13.3.1.6) and choosing the best one through overload resolution (13.3)), then the reference is bound […] to the lvalue result
of the conversion […].
管理此过程的候选选择的措辞（13.3.1.6，如上所述）不包括第一个转换函数：
The conversion functions of S and its base classes are considered. Those non-explicit conversion functions
that are not hidden within S and yield type “lvalue reference to cv2 T2”(when initializing an lvalue reference or an rvalue reference to function) […], where “cv1 T” is reference-compatible (8.6.3)
with “cv2 T2”, are candidate functions. For direct-initialization, […].
显然，这种排除是引用初始化语义所特有的，所以第一种情况仍然是模棱两可的。
考虑以下几点：
struct X {
template <class T> operator T(); // #1
template <class T> operator T&(); // #2
};
int a = X{}; // error: ambiguous
int& b = X{}; // calls #2
int const& c = X{}; // calls #2
b的情况很简单，#2是唯一可行的候选人。什么规则表明 #2 优先于 #1 用于初始化 int const&，但两者对于 int 的初始化有歧义？
在决定如何初始化给定初始化器的引用时，首先尝试直接绑定。 [dcl.init.ref]/(5.1.2):
If the reference is an lvalue reference and the initializer expression […] has a class type (i.e.,
T2is a class type), whereT1is not reference-related toT2, and can be converted to an lvalue of type “cv3T3”, where “cv1T1” is reference-compatible with “cv3T3” (this conversion is selected by enumerating the applicable conversion functions (13.3.1.6) and choosing the best one through overload resolution (13.3)), then the reference is bound […] to the lvalue result of the conversion […].
管理此过程的候选选择的措辞（13.3.1.6，如上所述）不包括第一个转换函数：
The conversion functions of
Sand its base classes are considered. Those non-explicit conversion functions that are not hidden withinSand yield type “lvalue reference to cv2T2”(when initializing an lvalue reference or an rvalue reference to function) […], where “cv1T” is reference-compatible (8.6.3) with “cv2T2”, are candidate functions. For direct-initialization, […].
显然，这种排除是引用初始化语义所特有的，所以第一种情况仍然是模棱两可的。