class Base
{
public:
class Foo { ... };
...
};
那么另一个类派生自基地：
class Derived : public Base
{
// no mention or redefinition of nested class "Foo" anywhere in "Derived"
};
这是否意味着我们现在有一个独特的Derived :: Foo,或者Derived :: Foo与Base :: Foo完全相同？
以下是这种情况的转折：如果有人抛出Derived :: Foo的实例呢？在这种情况下会被抓住吗？
catch ( const Base::Foo &ex )
{
// would this also catch an instance of Derived::Foo?
}
解决方法
typedef int newType1 ;
只要Derived没有重新声明newType1,那么我们期望Base :: newType1和Derived :: newType1是相同的类型,而嵌套类没有什么不同.如果我们参考标准草案9.2节班级成员第1段说(强调我的)：
[…]Members of a class are data members,member functions (9.3),nested types,and
enumerators. Data members and member functions are static or non-static; see 9.4. Nested types are
classes (9.1,9.7) and enumerations (7.2) defined in the class,and arbitrary types declared as members by use of a typedef declaration (7.1.3).
这证实了直觉嵌套类只是类型(也是成员),为了完整性,上面引用的部分9.7是嵌套类部分,从第10节派生类第1段我们看到：
[…]Unless redeclared in the derived class,members of a base class are also considered to be members of the derived class.[…]
由于它们是相同的类型,捕获将正常工作.
