c++ - 可以编写 catch() 以从一种对象类型转换为另一种对象类型吗？

Question

给定这段代码，其中 A 在第 3 方库中，B 是我编写的代码:

class A {...};
class B { public: B( const A &a ); ... };

try
{
    A a(...);
    throw a;
}
catch ( const B &b )
{
    // ...do stuff with B, like log the details to a file...
}

如果一个对象 A 被抛出，那么我的 catch B 就会被跳过。我希望因为 B 有一个接受 A 对象的构造函数，所以这可能会起作用。

我可以做些什么来完成这项工作，还是我必须修改所有 catch 子句以处理同时捕获 A 和 B?

Answer 1

不，那行不通。将抛出的对象与处理程序匹配时，不考虑用户定义的转换。根据 C++11 标准的第 15.3/3 段:

A handler is a match for an exception object of type E if

• The handler is of type cv T or cv T& and E and T are the same type (ignoring the top-level cv-qualifiers), or

• the handler is of type cv T or cv T& and T is an unambiguous public base class of E, or

• the handler is of type cv T or const T& where T is a pointer type and E is a pointer type that can be converted to T by either or both of

  • a standard pointer conversion (4.10) not involving conversions to pointers to private or protected or ambiguous classes

  • a qualification conversion

• the handler is of type cv T or const T& where T is a pointer or pointer to member type and E is std::nullptr_t.