如何在Delphi中最佳实现三值逻辑？
我正在考虑
type
TExtBoolean = (ebTrue, ebFalse, ebUnknown);
使用
function ExtOr(A: TExtBoolean; B: TExtBoolean): TExtBoolean;
begin
if (A = ebTrue) or (B = ebTrue) then
Result := ebTrue
else if (A = ebFalse) and (B = ebFalse) then
Result := ebFalse
else
Result := ebUnknown;
end;
等等，这种方法似乎并不太优雅。是否存在更好的方法？
编辑：我所说的优雅是指易于使用。实现越优雅，越好。对于我来说，CPU效率并不是很重要。
您可以使用运算符重载来实现增强记录。它看起来像这样：
type
TTriBool = record
public
type
TTriBoolEnum = (tbFalse, tbTrue, tbUnknown);
public
Value: TTriBoolEnum;
public
class operator Implicit(const Value: Boolean): TTriBool;
class operator Implicit(const Value: TTriBoolEnum): TTriBool;
class operator Implicit(const Value: TTriBool): TTriBoolEnum;
class operator Equal(const lhs, rhs: TTriBool): Boolean;
class operator LogicalOr(const lhs, rhs: TTriBool): TTriBool;
function ToString: string;
end;
class operator TTriBool.Implicit(const Value: Boolean): TTriBool;
begin
if Value then
Result.Value := tbTrue
else
Result.Value := tbFalse;
end;
class operator TTriBool.Implicit(const Value: TTriBoolEnum): TTriBool;
begin
Result.Value := Value;
end;
class operator TTriBool.Implicit(const Value: TTriBool): TTriBoolEnum;
begin
Result := Value.Value;
end;
class operator TTriBool.Equal(const lhs, rhs: TTriBool): Boolean;
begin
Result := lhs.Value=rhs.Value;
end;
class operator TTriBool.LogicalOr(const lhs, rhs: TTriBool): TTriBool;
begin
if (lhs.Value=tbTrue) or (rhs.Value=tbTrue) then
Result := tbTrue
else if (lhs.Value=tbFalse) and (rhs.Value=tbFalse) then
Result := tbFalse
else
Result := tbUnknown;
end;
function TTriBool.ToString: string;
begin
case Value of
tbFalse:
Result := 'False';
tbTrue:
Result := 'True';
tbUnknown:
Result := 'Unknown';
end;
end;
一些使用示例：
var
x: Double;
tb1, tb2: TTriBool;
tb1 := True;
tb2 := x>3.0;
Writeln((tb1 or tb2).ToString);
tb1 := False;
tb2.Value := tbUnknown;
Writeln((tb1 or tb2).ToString);
输出结果为：
True Unknown
tbUnspecified？ - LU RDAS. 在这里你所说的优雅是指实现的优雅还是使用的优雅还是CPI效率还是可维护性？优雅是一个非常模糊的词...
我认为让它更易于使用的明显方法是将类型转换为可像ExtBoolean1 or (ExtBoolean2 and True)一样使用。
然而，所需的功能可能在Delphi 2006之前（本身就相当有缺陷）或短时间内完成，因此请使用您的DUnit并进行大量测试。
要列出要使用的功能及其描述：
概述其中一些想法：
type
TExtBoolean = record
Value: (ebUnknown, ebTrue, ebFalse);
function IsNull: boolean; inline;
function Defined: boolean; inline;
class operator Implicit ( from: boolean ): TExtBoolean; inline;
class operator Implicit ( from: TExtBoolean ): boolean;
class operator LogicalAnd( Value1, Value2: TExtBoolean ): TExtBoolean;
class operator LogicalAnd( Value1: TExtBoolean; Value2: boolean): TExtBoolean; inline;
class operator LogicalAnd( Value1: boolean; Value2: TExtBoolean ): TExtBoolean;
....
end;
const Unknown: TExtBoolean = (Value: ebUnknown);
...
var v1: TExtBoolean;
v1 := False;
v1 := True;
v1 := Unknown;
...
class operator TExtBoolean.Implicit ( from: boolean ): TExtBoolean;
begin
if from
then Result.Value := ebTrue
else Result.Value := ebFalse
end;
class operator TExtBoolean.Implicit ( from: TExtBoolean ): Boolean;
begin
case from.Value of
ebTrue: Result := True;
ebFalse: Result := False;
else raise EConvertError.Create('....');
end;
function TExtBoolean.Defined: boolean;
begin
Result := (Self.Value = ebTrue) or (Self.Value = ebFalse);
end;
// this implementation detects values other than ebTrue/ebFalse/ebUnkonwn
// that might appear in reality due to non-initialized memory garbage
// since hardware type of Value is byte and may be equal to 3, 4, ...255
function TExtBoolean.IsNull: boolean;
begin
Result := not Self.Defined
end;
class operator TExtBoolean.And( Value1, Value2: TExtBoolean ): TExtBoolean;
begin
if Value1.IsNull or Value2.IsNull
then Result.Value := eb.Undefined
else Result := boolean(Value1) and boolean(Value2);
// Or, sacrificing readability and safety for the sake of speed
// and removing duplicate IsNull checks
// else Result := (Value1.Value = ebTrue) and (Value2.Value = ebTrue);
end;
class operator TExtBoolean.LogicalAnd( Value1, TExtBoolean; Value2: boolean): TExtBoolean;
begin
Result := Value2 and Value1;
end;
class operator TExtBoolean.LogicalAnd( Value1: boolean; Value2: TExtBoolean ): TExtBoolean;
begin
if Value2.IsNull
then Result := Value2
else Result := Value1 and (Value2.Value = ebTrue);
// or if to accept a duplicate redundant check for readability sake
// and to avert potential later erros (refactoring, you may accidentally remove the check above)
// else Result := Value1 and boolean (Value2);
end;
PS. 上面的未指定检查是故意悲观的，倾向于犯错。 这是防止非初始化变量和可能的未来更改添加更多状态而采取的措施。 虽然这似乎过度保护，但至少Delphi XE2与我达成了一致：在类似情况下，请参见警告：
program Project20; {$APPTYPE CONSOLE}
uses System.SysUtils;
type enum = (e1, e2, e3);
var e: enum;
function name( e: enum ): char;
begin
case e of
e1: Result := 'A';
e2: Result := 'B';
e3: Result := 'C';
end;
end;
// [DCC Warning] Project20.dpr: W1035 Return value of function 'name' might be undefined
begin
for e := e1 to e3
do Writeln(name(e));
ReadLn;
end.