我在研究 this SO question 的答案时出现了这个问题.考虑以下代码:

struct A{
    operator char() const{ return 'a'; }
    operator int() const{ return 10; }
};

struct B {
    void operator<< (int) { }
};

int main()
{
    A a;
    B b;
    b << a;
}

a 的转换至int可以通过a.operator char()后跟一个完整的促销，或a.operator int()然后是身份转换(即根本没有转换)。标准规定(§13.3.3.1 [over.best.ics]/p10，脚注省略，我的粗体；所有引用均来自 N3936):

If several different sequences of conversions exist that each convert the argument to the parameter type, the implicit conversion sequence associated with the parameter is defined to be the unique conversion sequence designated the ambiguous conversion sequence. For the purpose of ranking implicit conversion sequences as described in 13.3.3.2, the ambiguous conversion sequence is treated as a user-defined sequence that is indistinguishable from any other user-defined conversion sequence. If a function that uses the ambiguous conversion sequence is selected as the best viable function, the call will be ill-formed because the conversion of one of the arguments in the call is ambiguous.

这里，B::operator<<(int)是唯一可行的候选者 - 因此是最佳可行的候选者，即使参数的转换序列是不明确的转换序列。那么，根据加粗的句子，调用应该是格式错误的，因为“调用中的参数之一的转换是模棱两可的”。

然而，我测试的编译器(g++、clang 和 MSVC)实际上没有报告错误，这是有道理的，因为在通过重载决议选择要调用的函数后，函数的“参数 (8.3.5) 应被初始化 ( 8.5, 12.8, 12.1) 及其对应的 参数”(§5.2.2 [expr.call]/p4)。这个初始化是copy-initialization(§8.5 [dcl.init]/p15)，并且根据§8.5 [dcl.init ]/p17，导致新一轮的重载决议来确定要使用的转换函数:

The semantics of initializers are as follows. The destination type is the type of the object or reference being initialized and the source type is the type of the initializer expression. If the initializer is not a single (possibly parenthesized) expression, the source type is not defined.

• [...]
• If the destination type is a (possibly cv-qualified) class type: [...]
• Otherwise, if the source type is a (possibly cv-qualified) class type, conversion functions are considered. The applicable conversion functions are enumerated (13.3.1.5), and the best one is chosen through overload resolution (13.3). The user-defined conversion so selected is called to convert the initializer expression into the object being initialized. If the conversion cannot be done or is ambiguous, the initialization is ill-formed.
• [...]

而在这一轮的重载决议中，在 §13.3.3 [over.match.best]/p1 中有一个决胜局:

a viable function F1 is defined to be a better function than another viable function F2 if for all arguments i, ICSi(F1) is not a worse conversion sequence than ICSi(F2), and then

• for some argument j, ICSj(F1) is a better conversion sequence than ICSj(F2), or, if not that,
• the context is an initialization by user-defined conversion (see 8.5, 13.3.1.5, and 13.3.1.6) and the standard conversion sequence from the return type of F1 to the destination type (i.e., the type of the entity being initialized) is a better conversion sequence than the standard conversion sequence from the return type of F2 to the destination type.

(Example and remainder of the list omitted)

由于标准转换序列来自 int至int (精确匹配等级)优于 char 的标准转换序列至int (晋升排名)，第一胜第二，应该没有歧义——operator int()定义的转换将用于初始化，这与 §13.3.3.1 [over.best.ics]/p10 中的句子相矛盾，该句子说函数调用由于歧义而格式错误。

上面的分析有什么问题吗，还是那句话是标准的bug？

最佳答案

在为用户定义的转换序列决定最佳的用户定义转换时，我们有一个重载候选集。

§13.3.3/p1 说:

Define ICSi(F) as follows:

• [...]

• let ICSi(F) denote the implicit conversion sequence that converts the i-th argument in the list to the type of the i-th parameter of viable function F. 13.3.3.1 defines the implicit conversion sequences and 13.3.3.2 defines what it means for one implicit conversion sequence to be a better conversion sequence or worse conversion sequence than another.

Given these definitions, a viable function F1 is defined to be a better function than another viable function F2 if for all arguments i, ICSi(F1) is not a worse conversion sequence than ICSi(F2), and then

— [...]

— the context is an initialization by user-defined conversion (see 8.5, 13.3.1.5, and 13.3.1.6) and the standard conversion sequence from the return type of F1 to the destination type (i.e., the type of the entity being initialized) is a better conversion sequence than the standard conversion sequence from the return type of F2 to the destination type.

这适用于

§13.3.3.1.2/p2

2 The second standard conversion sequence converts the result of the user-defined conversion to the target type for the sequence. Since an implicit conversion sequence is an initialization, the special rules for initialization by user-defined conversion apply when selecting the best user-defined conversion for a user-defined conversion sequence (see 13.3.3 and 13.3.3.1).

因此涉及operator int的转换序列被选为最佳匹配。

最后我将 §13.3.3.1/p10 改写为

If several different sequences of conversions exist that each convert the argument to the parameter type and it was not possible to determine a best candidate, the implicit conversion sequence associated with the parameter is defined to be the unique conversion sequence designated the ambiguous conversion sequence. For the purpose of ranking implicit conversion sequences as described in 13.3.3.2, the ambiguous conversion sequence is treated as a user-defined sequence that is indistinguishable from any other user-defined conversion sequence134. If a function that uses the ambiguous conversion sequence is selected as the best viable function, the call will be ill-formed because the conversion of one of the arguments in the call is ambiguous.

关于c++ - 在重载决议中，选择使用模糊转换序列的函数是否必然导致调用格式错误？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/25395056/