如何在结构C++中使用枚举

我试图在结构中使用enum，但我得到了以下错误：

union.cpp:27:21: error: ‘DOLLAR’ was not declared in this scope
 book.currency = DOLLAR;
                 ^

这是我的代码：

 struct shelf{
      char title[50];
      char author[50];
      union {
          float dollars;
          int yens;
      };
 
      enum {
          DOLLAR = 1, YEN
      } currency;
  } book;
 
  int main () {
      strcpy(book.title,"book 1");
      strcpy(book.author, "author 1");
 
      book.dollars = 100;
 
      book.currency = DOLLAR;
 
      cout << book.currency;
      return 0;
  }