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zorleone's questions

Martin Hope
zorleone
Asked: 2024-12-01 21:08:00 +0800 CST
  • 8

当尝试获取无界字符串的第一个或最后一个索引时，如下程序所示

with Ada.Strings; use Ada.Strings; -- for `Backward`
with Ada.Strings.Unbounded;
with Ada.Text_IO; use Ada.Text_IO;

procedure Foo is
    package U_Str renames Ada.Strings.Unbounded;

    S : U_Str.Unbounded_String := U_Str.To_Unbounded_String ("example unbounded string");
    I : Natural := U_Str.Index (Source => S,
                                Pattern => "s",
                                Going => Backward,
                                From => S'Last);
begin
    Put_Line (I'Image);
end Foo;

GNAT 抱怨U_Str.Unbounded_String自己是私有类型：

$ gnatmake foo.adb
x86_64-linux-gnu-gcc-10 -c foo.adb
foo.adb:12:41: prefix for "Last" attribute may not be private type
gnatmake: "foo.adb" compilation error

我不明白这一点，同样的程序用常规方法String可以正常工作：

with Ada.Strings; use Ada.Strings; -- for `Backward`
with Ada.Strings.Fixed;
with Ada.Text_IO; use Ada.Text_IO;

procedure Foo is
    package F_Str renames Ada.Strings.Fixed;

    S : String := "example fixed string";
    I : Natural := F_Str.Index (Source => S,
                                Pattern => "s",
                                Going => Backward,
                                From => S'Last);
begin
    Put_Line (I'Image);
end Foo;

$ gnatmake foo.adb 
x86_64-linux-gnu-gcc-10 -c foo.adb
x86_64-linux-gnu-gnatbind-10 -x foo.ali
x86_64-linux-gnu-gnatlink-10 foo.ali
$ ./foo 
 15

无界字符串没有'First和'Last属性吗？Ada 参考手册只说明

下列属性是针对前缀 A 定义的，该前缀 A 属于数组类型（在任何隐式取消引用之后），或表示受约束的数组子类型

但它没有提到任何有关私有类型的信息。这是 GNAT 错误还是我遗漏了什么？

string