c++ - 失败和错误的类型转换

Question

您能否解释一下 ill-formed cast 和 failed cast 之间的区别。例如:

class A { virtual void f(); };
class B { virtual void g(); };
class D : public virtual A, private B { };

void g() {
    D d;
    B* bp = (B*)&d;    // cast needed to break protection
    A* ap = &d;        // public derivation, no cast needed
    D& dr = dynamic_cast<D&>(*bp);    // fails
    ap = dynamic_cast<A*>(bp);        // fails
    bp = dynamic_cast<B*>(ap);        // fails
    ap = dynamic_cast<A*>(&d);        // succeeds
    bp = dynamic_cast<B*>(&d);        // ill-formed (not a run-time check)
}

Answer 1

不管名称如何，当您使用 dynamic_cast 时要进行向上转换(派生->基础)，转换是在编译时完成的，其行为方式与 static_cast 相同或隐式转换 - 如果基数不明确或不可访问，则程序格式错误，这意味着编译器必须产生诊断。 §5.2.7 [expr.dynamic.cast]/p5:

[For the expression dynamic_cast<T>(v):]

If T is "pointer to cv1 B" and v has type "pointer to cv2 D" such that B is a base class of D, the result is a pointer to the unique B subobject of the D object pointed to by v. Similarly, if T is "reference to cv1 B" and v has type cv2 D such that B is a base class of D, the result is the unique B subobject of the D object referred to by v. ⁶⁷ The result is an lvalue if T is an lvalue reference, or an xvalue if T is an rvalue reference. In both the pointer and reference cases, the program is ill-formed if cv2 has greater cv-qualification than cv1 or if B is an inaccessible or ambiguous base class of D.

⁶⁷ The most derived object (1.8) pointed or referred to by v can contain other B objects as base classes, but these are ignored.

在其他情况下(向下或向侧面 throw )，检查在运行时执行。如果失败，则转换结果是指针转换的空指针，以及 std::bad_cast。引用转换的异常。