Question

我尝试用Ada做的是创建一个Book对象，然后打印出那本书。问题是，语言不是很可写。当我编译，构建主文件，然后执行该文件时，它给了我这个错误：

raised CONSTRAINT_ERROR : book1.adb:28 length check failed

这是我在正文档中的内容：

package body Book1 is

function getAuthor (B: in Book) return AuthorName is

begin
  return B.getAuthor;
end getAuthor;

function getTitle (B: in Book) return TitleName is

begin
   return B.getTitle;
end getTitle;

function getNumberOfPages (B: in Book) return Pages is

begin
  return B.getNumberOfPages;
end getNumberOfPages;

function createBook (Author: in String; Title: in String; NumberOfPages: in Float)
                    return Book is
  B: Book;

begin
  B.getAuthor := Author;
  B.getTitle := Title;
  B.getNumberOfPages := NumberOfPages;
  return B;
end createBook;

end Book1;

这是我在spec文件中的内容：

package Book1 is

type Book is tagged private;

subtype AuthorName is String (1 .. 300);
subtype TitleName is String (1 .. 750);
subtype Pages is Float range 0.0 .. 5000000.0;

function getAuthor(B: in Book) return AuthorName;

function getTitle(B: in Book) return TitleName;

function getNumberOfPages(B: in Book) return Pages;

function createBook(Author: in String; Title: in String; NumberOfPages: in Float)
                   return Book;

private
  type Book is tagged
     record
     getAuthor: AuthorName;
     getTitle: TitleName;
     getNumberOfPages: Pages;
   end record;

end Book1;

这是我在测试文件中的内容：

with Book1, Ada.Text_IO, Ada.Float_Text_IO;
use Book1, Ada.Text_IO, Ada.Float_Text_IO;

procedure testBook is

theRealBook: Book := createBook(Author => "Jordan White",
                                      Title => "The Last Christmas",
                                      NumberOfPages => 32.0);

procedure Print_Book (B: in Book'Class) is

begin
  Put (getAuthor (B => B));
  Put (getTitle (B => B));
  Put (getNumberOfPages (B => B));
  New_Line;
end Print_Book;

begin
Print_Book (theRealBook);

end testBook;

我到底在做什么才能收到此错误，我该如何解决？

Answer 1

你的陈述：

theRealBook: Book := createBook(Author => "Jordan White",
                                      Title => "The Last Christmas",
                                      NumberOfPages => 32.0);

作者和标题的字符串隐含长度（字符串文字）。这意味着当您在此处分配作者和标题时：

  B.getAuthor := Author;
  B.getTitle := Title;

你正在假设＆＃34; Jordan white＆＃34;是一个长度为300的字符串。（标题相同的问题）我怀疑这是作业，所以不好让你知道如何找到长度，字符串的边界，以及如何分配数组切片。

错误修复：引发CONSTRAINT_ERROR：book1.adb：28长度检查失败

1 个答案: