可以在FreePascal中的变体记录的案例中复制标识符吗？
这是我的问题：我想创建一个记录类型，其中在变体记录的情况下，一些（但不是全部）将具有某个字段。根据{{3}}，这是完全合法的。然而，当我尝试编译以下代码时：
program example;
{$mode objfpc}{$H+}
uses sysutils;
type
maritalStates = (single, married, widowed, divorced);
TPerson = record
name: record
first, middle, last: string;
end;
sex: (male, female);
dob: TDateTime;
case maritalStatus: maritalStates of
single: ( );
married, widowed: (marriageDate: TDateTime);
divorced: (marriageDate, divorceDate: TDateTime;
isFirstDivorce: boolean)
end;
var
ExPerson: TPerson;
begin
ExPerson.name.first := 'John';
ExPerson.name.middle := 'Bob';
ExPerson.name.last := 'Smith';
ExPerson.sex := male;
ExPerson.dob := StrToDate('05/05/1990');
ExPerson.maritalStatus := married;
ExPerson.marriageDate := StrToDate('04/01/2015');
end.
编译失败，出现以下错误：
$ fpc ex.pas
Free Pascal Compiler version 3.0.0 [2016/02/14] for x86_64
Copyright (c) 1993-2015 by Florian Klaempfl and others
Target OS: Win64 for x64
Compiling ex.pas
ex.pas(19,18) Error: Duplicate identifier "marriageDate"
ex.pas(21,3) Error: Duplicate identifier "marriageDate"
ex.pas(35,4) Fatal: There were 2 errors compiling module, stopping
Fatal: Compilation aborted
Error: C:\lazarus\fpc\3.0.0\bin\x86_64-win64\ppcx64.exe returned an error exitcode
维基是完全错了，还是我错过了什么？有没有办法达到我想要的效果？
3 个答案:
答案 0 :(得分：3)
非常有趣的问题。我确信这是可能的。如果您将代码修改为：
..
married, widowed, divorced: (marriageDate: TDateTime);
divorced: (divorceDate: TDateTime; isFirstDivorce: boolean)
..
它有效，但它不是你想要的结果。由于婚姻日期和离婚日期相互叠加（如评论中所述！）
此图片取自“Pascal用户手册（第4版）”，您可以看到变体部分具有相同的存储位置。
根据Pascal users manual (4th edition)和“Turbo Pascal ISBN 3-89011-060-6”一书，所引用的wiki上的记录声明无效！
- 所有字段名称必须不同 - 即使它们出现在不同的变体中。
- 如果变量为空（即没有字段），则表单为： C：（）
- 字段列表只能包含一个变体部分，并且必须遵循记录的固定部分。
- 变体本身可能包含变体部分;因此，变体部分可以嵌套。
- 记录类型中引入的枚举类型常量标识符的范围扩展到封闭块。
第1点是相关的！书“Turbo Pascal”建议的解决方案是为多次出现的字段名称使用唯一的前缀。
在你的情况下你可以看起来像：
TPerson = record
name: record
first, middle, last: string;
end;
sex: (male, female);
dob: TDateTime;
case maritalStatus: maritalStates of
single: ( );
married, widowed: (marMarriageDate: TDateTime);
divorced: (divMarriageDate, divorceDate: TDateTime;
isFirstDivorce: boolean)
end;
另一个解决方案是将已婚，被驱逐......定义为记录类型。
..
married : (m: TMarried);
divorced : (d: TDivorced);
..
答案 1 :(得分：1)
这似乎有效
program example;
{$mode objfpc}{$H+}
uses sysutils;
type
TMarried = record
marriageDate : TDateTime
end;
TDivorced = record
marriageDate : TDateTime;
divorceDate : TDateTime;
isFirstDivorce: boolean
end;
TWidowed = TMarried;
maritalStates = (single, married, widowed, divorced);
TPerson = record
name: record
first, middle, last: string;
end;
sex: (male, female);
dob: TDateTime;
case maritalStatus: maritalStates of
single : ();
married : (m: TMarried);
widowed : (w: TWidowed);
divorced : (d: TDivorced);
end;
var ExPerson: TPerson;
begin
with ExPerson do
begin
name.first := 'John';
name.middle := 'Bob';
name.last := 'Smith';
sex := male;
dob := StrToDate('05/05/1990');
maritalStatus := married;
m.marriageDate := StrToDate('04/01/2015');
end;
end.
编辑：您也可以内联定义记录，但我认为上述内容更清晰。这是另一种方式：
program example;
{$mode objfpc}{$H+}
uses sysutils;
type
maritalStates = (single, married, widowed, divorced);
TPerson = record
name: record
first, middle, last: string;
end;
sex: (male, female);
dob: TDateTime;
case maritalStatus: maritalStates of
single : ();
married : (m: record marriageDate: TDateTime end);
widowed : (w: record marriageDate: TDateTime end);
divorced : (d: record
marriageDate : TDateTime;
divorceDate : TDateTime;
isFirstDivorce: boolean
end)
end;
var ExPerson: TPerson;
begin
with ExPerson do
begin
name.first := 'John';
name.middle := 'Bob';
name.last := 'Smith';
sex := male;
dob := StrToDate('05/05/1990');
maritalStatus := married;
m.marriageDate := StrToDate('04/01/2015');
end;
end.
答案 2 :(得分：0)
Baltasar建议编译，但不能做你想要的。 marriageDate和divorceDate会重叠，写入其中一个也会修改另一个，因为它们只是在同一个地址。
但在这种情况下，根本没有充分的理由可以使用变体记录。
为什么不简单：
type
maritalStates = (single, married, widowed, divorced);
TPerson = record
name: record
first,
middle,
last: string;
end;
sex: (male, female);
dob: TDateTime;
maritalStatus: maritalStates; // single, married, widowed, divorced
marriageDate: TDateTime; // married, widowed, divorced
divorceDate : TDateTime; // divorced
isFirstDivorce: boolean; // divorced
end;
使用和布局正是您所需要的。如果某个字段不适用（例如marriageDate为single，divorceDate为married），则您根本不使用该字段。
这与变体记录相同。在那里，您还只设置了适用的字段。请注意，编译器或运行时不会阻止您写入变体记录的错误字段，即在变体记录中，如果状态为single，您仍然可以写入或读取{{1}即使这没有任何意义。
如果要区分几种不同的设置，只需在注释中执行此操作，并忘记变体记录，此处不需要它。现在你可以做到：
divorceDate更新
只是为了表明完全没有必要制作这种记录变体，
我将发布我的Project62.dpr，它显示了相应字段和相同记录大小的完全相同的偏移量：
var
P: TPerson;
begin
P.name.first := 'Bob';
P.name.middle := 'The';
P.name.last := 'Builder';
P.sex := male;
P.dob := StrToDate('05/05/1980');
P.maritalStatus := divorced;
P.marriageDate := StrToDate('04/01/2013');
P.divorceDate := StrToDate('04/02/2016');
P.isFirstDivorce := True;
// etc...
输出（在Windows上）：
program Project62;
{$APPTYPE CONSOLE}
{$R *.res}
uses
System.SysUtils;
type
maritalStates = (single, married, widowed, divorced);
tsex = (male, female);
// No variant part
PPerson = ^TPerson;
TPerson = record
name: record
first,
middle,
last: string;
end;
sex: tsex;
dob: TDateTime;
maritalStatus: maritalStates; // single, married, widowed, divorced
marriageDate: TDateTime; // married, widowed, divorced
divorceDate : TDateTime; // divorced
isFirstDivorceDate: boolean; // divorced
end;
// Variant part like tonypdmtr's record
PPerson2 = ^TPerson2;
TPerson2 = record
name: record
first,
middle,
last: string;
end;
sex: tsex;
dob: TDateTime;
case maritalStatus: maritalStates of
single: ();
widowed: (w: record marriageDate: TDateTime; end); // overlaps with m.marriageDate and d.marriageDate
married: (m: record marriageDate: TDateTime; end); // overlaps with w.marriageDate and d.marriageDate
divorced: (d: record
marriageDate: TDateTime; // overlaps with w.marriageDate and m.marriageDate
divorceDate: TDateTime; // same offset as in my non-variant version
isFirstDivorceDate: Boolean // same offset as in my non-variant version
end);
end;
begin
try
Writeln('TPerson: size = ', Sizeof(TPerson));
Writeln('TPerson.maritalStatus: offset = ', NativeUInt(@PPerson(nil)^.maritalStatus));
Writeln('TPerson.marriageDate: offset = ', NativeUInt(@PPerson(nil)^.marriageDate));
Writeln('TPerson.divorceDate: offset = ', NativeUInt(@PPerson(nil)^.divorceDate));
Writeln('TPerson.isFirstDivorceDate: offset = ', NativeUInt(@PPerson(nil)^.isFirstDivorceDate));
Writeln;
Writeln('TPerson2: size = ', Sizeof(TPerson2));
Writeln('TPerson2.maritalStatus: offset = ', NativeUInt(@PPerson2(nil)^.maritalStatus));
Writeln('TPerson2.w.marriageDate: offset = ', NativeUInt(@PPerson2(nil)^.w.marriageDate));
Writeln('TPerson2.m.marriageDate: offset = ', NativeUInt(@PPerson2(nil)^.m.marriageDate));
Writeln('TPerson2.d.marriageDate: offset = ', NativeUInt(@PPerson2(nil)^.d.marriageDate));
Writeln('TPerson2.d.divorceDate: offset = ', NativeUInt(@PPerson2(nil)^.d.divorceDate));
Writeln('TPerson2.d.isFirstDivorceDate: offset = ', NativeUInt(@PPerson2(nil)^.d.isFirstDivorceDate));
except
on E: Exception do
Writeln(E.ClassName, ': ', E.Message);
end;
Readln;
end.
32位布局可以放在一个简单的图中：
TPerson: size = 56
TPerson.maritalStatus: offset = 24
TPerson.marriageDate: offset = 32
TPerson.divorceDate: offset = 40
TPerson.isFirstDivorceDate: offset = 48
TPerson2: size = 56
TPerson2.maritalStatus: offset = 24
TPerson2.w.marriageDate: offset = 32
TPerson2.m.marriageDate: offset = 32
TPerson2.d.marriageDate: offset = 32
TPerson2.d.divorceDate: offset = 40
TPerson2.d.isFirstDivorceDate: offset = 48
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