c++ - 不允许对自定义类型进行多次隐式转换？

Question

class C {
public:
    C() { }
};

class B {
public:
    B(C c) { }
    B() { }
};

class A {
public:
    A(bool b) { }
    A(B b) { }
};

int main() {
    A a1 = true; // bool -> A        is allowed
    A a2 = B();  // B -> A           is allowed

    A a3 = 7;    // int -> bool -> A is allowed
    A a4 = C();  // C -> B -> A      isn't allowed
}

为什么我可以用 bool 使用两步隐式转换，但不能用 C 使用它？ 描述多步隐式转换的一般规则是什么？

Answer 1

没有多步用户自定义隐式转换。

int -> bool -> A

是允许的，因为 int->bool 转换不是用户定义的。

12.3 转换 [class.conv]

1 Type conversions of class objects can be specified by constructors and by conversion functions. These conversions are called user-defined conversions and are used for implicit type conversions (clause 4), for initialization (8.5), and for explicit type conversions (5.4, 5.2.9).

2 User-defined conversions are applied only where they are unambiguous (10.2, 12.3.2). Conversions obey the access control rules (clause 11). Access control is applied after ambiguity resolution (3.4).

3 [ Note: See 13.3 for a discussion of the use of conversions in function calls as well as examples below. —end note ]

4 At most one user-defined conversion (constructor or conversion function) is implicitly applied to a single value.