关于构造函数初始化

template <class elemType>
class Example2 {
public:
     explicit Example2( elemType val = 0 )
     : _val( val ){}
      
     bool min( elemType value ) { return _val < value; }
     void value( elemType new_val ) { _val = new_val; }
     void print( ostream &os ) { os << _val; }
private:
     elemType _val;
};
 
template<class elemType>
ostream& operator<< ( ostream &os, const Example2<elemType> &ex )
{ ex.print( os ); return os; }

 Example2< string > exs( "Walden" ); 这一句为什么能初始化exs的值 为Walden