Recently, two keywords were added to C++ that relate specifically to templates: typename and export.
Both play specialized roles in C++ programming. Each is briefly examined.
The typename keyword has two uses.
First, as mentioned earlier, it can be substituted for the keyword class in a template declaration. For example, the swapargs() template function could be specified like this:
template <typename X> void swapargs(X &a, X &b)
//这里typename代替了class在模版中的声明
{
X temp;
temp = a;
a = b;
b = temp;
}
template <typename X> void swapargs(X &a, X &b)
//这里typename代替了class在模版中的声明
{
X temp;
temp = a;
a = b;
b = temp;
}Here, typename specifies the generic type X. There is no difference between using class and using typename in this context.
The second use of typename is to inform the compiler that a name used in a template declaration is a type name rather than an object name. For example,
template<class _Kty,
class _Ty,
class _Pr = less<_Kty>,
class _Alloc = allocator<pair<const _Kty, _Ty> > >
class map
: public _Tree<_Tmap_traits<_Kty, _Ty, _Pr, _Alloc, false> >
{ // ordered red-black tree of {key, mapped} values, unique keys
public:
typedef map<_Kty, _Ty, _Pr, _Alloc> _Myt;
typedef _Tree<_Tmap_traits<_Kty, _Ty, _Pr, _Alloc, false> > _Mybase;
typedef _Kty key_type;
typedef _Ty mapped_type;
typedef _Ty referent_type; // retained
typedef _Pr key_compare;
typedef typename _Mybase::value_compare value_compare;
typedef typename _Mybase::allocator_type allocator_type;
typedef typename _Mybase::size_type size_type;
typedef typename _Mybase::difference_type difference_type;
typedef typename _Mybase::pointer pointer;
这里，可以看到，typedef typename _Mybase::value_compare
value_compare来定义一个变量类型。那么这个value_compare的类型来源于_Mybase::value_compare，而这里typename就是指明，这个_Mybase::value_compare是一个类型，而不是一个成员变量。
The export keyword can precede a template declaration. It allows other files to use a template declared in a different file by specifying only its declaration rather than duplicating its entire definition.