delphi - 如何获取给定变体的实际类型的内存大小？

Question

考虑以下代码:

procedure Test;
var
  IntGenuine: UInt64;
  IntVariant: Variant;

begin
  IntGenuine := 0;
  IntVariant := UInt64(0); // The type of the variant is UInt64 now

  WriteLn('Size of IntGenuine = ', SizeOf(IntGenuine)); // Output: 8
  WriteLn('Size of IntVariant = ', SizeOf(IntVariant)); // Output: 24
end;

我知道语句 SizeOf(IntVariant) 等同于 SizeOf(Variant)。它获取类型 Variant 的大小，而不是变体实际类型的大小(在本例中为 UInt64)。

如何获取给定变体的实际类型的内存大小？

Answer 1

您可以创建一个类似这样的函数:

Program Project1;

{$APPTYPE CONSOLE}
uses
  Variants, SysUtils;

function GetVarTypeSize(AVarType : TVarType; var isArray : boolean) : integer;
begin

  isArray := AVarType <> (AVarType and VarTypeMask);    

  case AVarType and VarTypeMask of
    varSmallInt: result := SizeOf(SmallInt);
    varInteger:  result := SizeOf(Integer);
    varSingle:   result := SizeOf(Single);
    varDouble:   result := SizeOf(Double);
    varCurrency: result := SizeOf(Currency);
    varDate:     result := SizeOf(TDateTime);
    varOleStr:   result := SizeOf(PWideChar);
    varDispatch: result := SizeOf(Pointer);
    varError:    result := SizeOf(HRESULT);
    varBoolean:  result := SizeOf(WordBool);
    varUnknown:  result := SizeOf(Pointer);
    varShortInt: result := SizeOf(ShortInt);
    varByte:     result := SizeOf(Byte);
    varWord:     result := SizeOf(Word);
    varLongWord: result := SizeOf(LongWord);
    varInt64:    result := SizeOf(Int64);
    varUInt64:   result := SizeOf(UInt64);
    varString:   result := SizeOf(Pointer);
    varAny:      result := SizeOf(Pointer);
    varArray:    result := SizeOf(PVarArray);
    varByRef:    result := SizeOf(Pointer);
    varUString:  result := SizeOf(Pointer);
    varRecord:   result := SizeOf(TVarRecord);
  else
    result := -1;  //unknown
  end;
end;

var
  v : Variant;
  b : boolean;
begin
  v := 3.141592654;   // double
  Write(GetVarTypeSize(VarType(v), b));
  if b then WriteLn(' : Is array') else WriteLn;

  v := 3;             // byte
  Write(GetVarTypeSize(VarType(v), b));
  if b then WriteLn(' : Is array') else WriteLn;

  v := integer(3);    // integer
  Write(GetVarTypeSize(VarType(v), b));
  if b then WriteLn(' : Is array') else WriteLn;

  v := Now;           // DateTime
  Write(GetVarTypeSize(VarType(v), b));
  if b then WriteLn(' : Is array') else WriteLn;

  v := VarArrayCreate([0,9], varDouble);   //array ! careful
  Write(GetVarTypeSize(VarType(v), b));
  if b then WriteLn(' : Is array') else WriteLn;

  ReadLn;
end.

这里VarTypeMask将屏蔽定义变体数组的位。如果基本变体是数组类型，则屏蔽它会告诉您数组元素的类型。

您可以阅读更多in the documentation .