阅读下列说明和C++-代码，将应填入 （n） 处的字句写在答题纸的对应栏内。
【说明】
某发票（lnvoice）由抬头（Head）部分、正文部分和脚注（Foot）部分构成。现采用装饰（ Decorator）模式实现打印发票的功能，得到如图5-1所示的类图。

【C++代码】
#include

using
namespace std;
class Invoice{
public:
（1） {
cout<<"This is the content of the
invoice!"< }
};
class
Decorator : public Invoice {
Invoice *ticket;
public:
Decorator(lnvoice *t) { ticket = t; }
void printInvoice（ ）{
if(ticket != NULL)
（2）;
}
};
class
HeadDecorator : public Decorator{
public:
HeadDecorator(lnvoice*t): Decorator(t) { }
void printInvoice（ ） {
cout<< "This is the
header of the invoice! "<< endl;
（3） ;
}
};
class FootDecorator : public Decorator{
public:
FootDecorator(Invoice *t): Decorator(t) { }
void printlnvoice（ ）{
（4） ;
cout<< "This is the
footnote of the invoice!"<< endl;
}
};
int main(void)
{
Invoice t;
FootDecorator f(&t);
HeadDecorator h(&f);
h.printInvoice（ ）;

cout<<”------------------------”< FootDecorator a(NULL) ;

HeadDecorator b( （5） );
b.printInvoice（ ）;
return 0;
}
程序的输出结果为：
This is the header of the invoice!
This is the content of the invoice!
This is the footnote of the invoice!
----------------------------
This is the header of the invoice!
This is the footnote of the invoice!