BCD and currency types in Delphi

With these years of Delphi, unexpectedly to currency and Tbcdfield smattering, the following gave a very good explanation, it is worth reading.

1. BCD type

BCD is binary-coded Decimal? In Delphi, the BCD field type can precisely hold floating-point data types.

The data type of the BCD code supported by Delphi is named TBCD, which is defined as follows:

TBCD = Packed record

Precision:byte; {1..64}

Signspecialplaces:byte; {sign:1, special:1, places:6}

fraction:packed Array [0..31] of Byte; {BCD nibbles, 00..99 per Byte, high Nibble 1st}

End

The support for BCD is in the Fmtbcd cell, so to use the BCD function, you need to reference this cell.

The BCD functions of Delphi are:

Bcdadd	Calculates the and of two BCD codes
Bcdcompare	Compare the size of two BCD
Bcddivide	Dividing BCD data
Bcdmultiply	BCD Data multiplication
Bcdprecision	Returns the number of data for the BCD. A value of 123, such as the BCD return value of 3,BCD value 9382, is 4.
Bcdscale	Returns the number of decimal digits of the BCD code
Bcdsubtract	Two BCD code subtraction
Bcdtocurr	Convert the BCD code to the current format data type
Bcdtodouble	Convert BCD code to a double-formatted data type
Bcdtointeger	BCD code converted to the data type in integer format
Bcdtostr	Convert BCD code to string
Bcdtostrf	BCD code converted to a string with formatting control
Currtobcd	Current data type converted to BCD code
Doubletobcd	Double data type conversion to BCD code
Formatbcd	Format the BCD code as a string
Integertobcd	Integer integer type converted to BCD code
Isbcdnegative	Determine if the BCD is negative
Normalizebcd	Converts the value of one BCD to the value of a different BCD code based on the given precision and number of decimal digits
Nullbcd	Determine if the BCD is null
Strtobcd	Convert string to BCD code
Trystrtobcd	Convert string to BCD code, conversion failure returns the given default value

2. Currency type

Identical to the money type in SQL Server, the currency type in Delphi:

1) occupies 8 bytes.

2) Always 4 decimal places.

3) Range: -2^63 ~ 2^63-1 ( -922,337,203,685,477.5808 ~ 922,337,203,685,477.5807). the storage format is equivalent to always multiplying by 10000 and then saving in integer format.

3. BCD field type (tbcdfield)

The decimal and numeric data types are now available in many databases, they can precisely hold floating-point types, and decimal and numeric types can be mapped to BCD field types.

In the tdatabase control of BDE, there is a ENABLEBCD option:

The tadoquery of ADO also has the ENABLEBCD option.

The ENABLEBCD option is used to describe how to handle numeric types (decimal and numeric) fields:

1) When ENABLEBCD is true, the numeric Type field is mapped to the Tbcdfield class.

2) When ENABLEBCD is false, the numeric type field is mapped to the Tfloatfield class.

Tbcdfield is defined in the Db.pas file:

TBCDField = class(TNumericField)

  private

  FCurrency: Boolean;

  FCheckRange: Boolean;

  FMinValue: Currency;

  FMaxValue: Currency;

  FPrecision: Integer;

  procedure SetCurrency(Value: Boolean);

  procedure SetMaxValue(Value: Currency);

  procedure SetMinValue(Value: Currency);

  procedure SetPrecision(Value: Integer);

  procedure UpdateCheckRange;

  protected

  class procedure CheckTypeSize(Value: Integer); override;

  procedure CopyData(Source, Dest: Pointer); override;

  function GetAsBCD: TBcd; override;

  function GetAsCurrency: Currency; override;

  function GetAsFloat: Double; override;

  function GetAsInteger: Longint; override;

  function GetAsString: string; override;

  function GetAsVariant: Variant; override;

  function GetDataSize: Integer; override;

  function GetDefaultWidth: Integer; override;

  procedure GetText(var Text: string; DisplayText: Boolean); override;

  function GetValue(var Value: Currency): Boolean;

  procedure SetAsBCD(const Value: TBcd); override;

  procedure SetAsCurrency(Value: Currency); override;

  procedure SetAsFloat(Value: Double); override;

  procedure SetAsInteger(Value: Longint); override;

  procedure SetAsString(const Value: string); override;

  procedure SetVarValue(const Value: Variant); override;

  public

  constructor Create(AOwner: TComponent); override;

  property Value: Currency read GetAsCurrency write SetAsCurrency;

  published

  { Lowercase to avoid name clash with C++ Currency type }

  property currency: Boolean read FCurrency write SetCurrency default False;

  property MaxValue: Currency read FMaxValue write SetMaxValue;

  property MinValue: Currency read FMinValue write SetMinValue;

  property Precision: Integer read FPrecision write SetPrecision default 0;

  property Size default 4;

end;

Because Tbcdfield uses the currency type to hold the data (note: Not saved with TBCD), and currency is fixed with only 4 decimal places, the accuracy exceeds 4 bits using Tfloatfield (that is, ENABLEBCD bit false).

4. Reference Documents

1. Direct support for BCD code in Delphi.

Http://www.cnblogs.com/ywangzi/archive/2012/11/14/2769823.html

BCD and currency types in Delphi