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Variant type conversions

All integer, real, string, character, and Boolean types are assignment-compatible with Variant. Expressions can be explicitly cast as variants, and the VarAsType and VarCast standard routines can be used to change the internal representation of a variant. The following code demonstrates the use of variants and some of the automatic conversions performed when variants are mixed with other types.

var

V1, V2, V3, V4, V5: Variant;

I: Integer;

D: Double;

S: string;

begin

V1 := 1; { integer value }

V2 := 1234.5678; { real value }

V3 := 'Hello world!'; { string value }

V4 := '1000'; { string value }

V5 := V1 + V2 + V4; { real value 2235.5678}

I := V1; { I = 1 (integer value) }

D := V2; { D = 1234.5678 (real value) }

S := V3; { S = 'Hello world!' (string value) }

I := V4; { I = 1000 (integer value) }

S := V5; { S = '2235.5678' (string value) }

end;

The compiler performs type conversions according to the following rules.

Source\Target	integer	real	string	character	Boolean
integer	converts integer formats	converts to real	converts to string reoresentation	same as string (left)	return False if 0, True otherwise
Real	rounds to nearest integer	converts real formats	converts to string reoresentation using regional settings	same as string (left)	return False if 0, True otherwise
String	converts to integer, truncating if necessary; raises exception if string is not numeric	converts to real using regional settings; raises exception if string is not numeric	converts string/character formats	same as string (left)	return False if string is False (non-case-sensitive) or a numeric string that evaluates to 0, True if string is True or a nonzero numeric string; raises exception otherwise
character	same as string (above)	same as string (above)	same as string (above)	same as string-to-string	same as string (above)
Boolean	False = 0, True = (255 if Byte)	False = 0, True =-1	False =”0”, True = “-1”	same as string (left)	False = False, True = True
Unassigned	returns 0	returns 0	returns empty string	same as string (left)	return False
Null	raises exception	raises exception	raises exception	same as string (left)	raises exception

Out-of-range assignments often result in the target variable getting the highest value in its range. Invalid assignments or casts raise the EVariantError exception.

Special conversion rules apply to the TDateTime real type declared in the System unit. When a TDateTime is converted to any other type, it treated as a normal Double. When an integer, real, or Boolean is converted to a TDateTime, it is first converted to a Double, then read as a date-time value. When a string is converted to a TDateTime, it is interpreted as a date-time value using the regional settings. When an Unassigned value is converted to TDateTime, it is treated like the real or integer value 0. Converting a Null value to TDateTime raises an exception.

On Windows, if a variant references a COM interface, any attempt to convert it reads the object’s default property and converts that value to the requested type. If the object has no default property, an exception is raised.

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Variant types

译文

变体类型转换

所有整数、实数、串、字符和布尔类型对变体Variant都是赋值兼容的。表达式能被明确作为变体转换，VarAsType和VarCast标准例程可以用于改变变体的内部表示。下面的代码演示了变体与其他类型混合时的用法以及自动转换。

var

V1, V2, V3, V4, V5: Variant;

I: Integer;

D: Double;

S: string;

begin

V1 := 1; { 整数值 }

V2 := 1234.5678; { 实数值 }

V3 := 'Hello world!'; { 串值 }

V4 := '1000'; { 串值 }

V5 := V1 + V2 + V4; { 实数值 2235.5678}

I := V1; { I = 1 (整数值) }

D := V2; { D = 1234.5678 (实数值) }

S := V3; { S = 'Hello world!' (串值) }

I := V4; { I = 1000 (整数值) }

S := V5; { S = '2235.5678' (串值) }

end;

编译器根据下列规则对变体进行类型转换。

源\目标	整数	实数	串	字符	布尔型
整数	转换整数格式	转换为实数	转换到串表示	同串（左）	如果为0则返回False 否则返回True
实数	四舍五入到最近的整数	转换实数格式	用本地设置转换到串表示	同串（左）	如果为0则返回False 否则返回True
串	转换到整数，如果必需则截断；如果串不是数字则引发异常	用本地设置转换到实数；如果串不是数字则引发异常	转换串/字符格式	同串（左）	如果串是False（大小写不敏感）或串是一个等于0的数字串则返回False，如果串是True或串是一个非0的数字串则返回True，其他则引发异常
字符	同串（上）	同串（上）	同串（上）	同串到串	同串（上）
布尔型	False = 0, True = (255 if Byte)	False = 0, True =-1	False =”0” True = “-1”	同串（左）	False = False, True = True
Unassigned	返回0	返回0	返回空串	同串（左）	返回False
Null	引发异常	引发异常	引发异常	同串（左）	引发异常

越界赋值通常导致目标变量得到其范围内的最大值。非法的赋值或转换引发EVariantError异常。

在System单元中对TDateTime类型声明了特殊的转换规则。当一个TDateTime类型转换到任意其他类型时，被视为一般的Double类型。当整数、实数或布尔型转换到TDateTime类型时，首先转换为Double型，然后作为日期-时间值读取。当一个Unassigned值被转换到TDateTime类型时，它将被视为实数或整数值0。转换一个Null值到TDateTime类型时，将引发异常。

在Windows系统中，如果一个变体引用是一个COM接口，那么任何转换尝试将读取对象的缺省属性的值并将其转换为请求的类型。如果对象没有缺省属性，那么将引发异常。

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