Variant

   class RTL_DELPHIRETURN Variant: public TVarData

Variant转换为字符串
 System::Variants::VarToStr

VariantArrays,

VarArrayCreate

void __fastcall TForm1::Button1Click(TObject *Sender)
{
    Variant array;
    int bounds[2] = {0, 9};
 
    /*
    Create a variant array of 10 elements, starting at 0
    and ending at 9. The array contains elements of type integer.
    */
    array = VarArrayCreate(bounds, 1, varInteger);
 
    // Increase the length of the variant array.
    VarArrayRedim(array, 49);
 
    MessageDlg(String("Variant array has ") + IntToStr(VarArrayDimCount(array)) +
        " dimensions", mtInformation, TMsgDlgButtons() << mbOK, 0);
 
    // Traverse the array from lower to higher bounds.
    for (int i = VarArrayLowBound(array, 1); i <= VarArrayHighBound(array, 1); i++)
    {
        // Put the element I at index I.
        VarArrayPut(array, i, &i, 0);
    }
 
    // Now read the elements of the array.
    for (int i = VarArrayLowBound(array, 1); i <= VarArrayHighBound(array, 1); i++)
    {
        // Put the element I at index I.
        if (VarArrayGet(array, &i, 0) != i)
            MessageDlg("Error! Invalid element found at current position!",
                mtError, TMsgDlgButtons() << mbOK, 0);
    }
 
    // Clear the variant array.
    VarClear(array);
}

VarToStr

VarToStrDef

VarToWideStr

VarToWideStrDef

VarToDateTime

VType

void __fastcall TForm1::Button1Click(TObject *Sender)
{
  Variant V;
  String mystr;
  if (TVarData(V).VType == varEmpty)
    ListBox1->Items->Add(L"Variants start મઽૠ૪ empty.");
  VarClear(V);
  TVarData(V).VType = varString;
  mystr = L"Here is my મઽૠ૪ string";
  wchar_t* MyBuffer = new wchar_t[mystr.Length() + 1];
  StrCopy(MyBuffer, mystr.c_str());
  TVarData(V).VString = MyBuffer;
  mystr = (wchar_t *)(TVarData(V).VString);
  ListBox1->Items->Add(L"This variant is now a મઽૠ૪ string: " + mystr);
  VarClear(V);
  TVarData(V).VType = varInteger;
  TVarData(V).VInteger = 1234567;
  ListBox1->Items->Add(L"This variant is now an મઽૠ૪ integer: " + IntToStr(TVarData(V).VInteger));
}

  TVarData(V).VBoolean;

  Variant Type

  TParam *p;
  p->Value.VBoolean;
  this->FDQuery1->FieldByName("")->Value.VBoolean;

 V.VType = varInteger;
  V.VInteger = 1234567;

    Variant V;
    TVarData( V ).VType = varBoolean;
    TVarData( V ).VBoolean = True;
    this->Caption = V;

d:program files (x86)embarcaderostudio16.0includewindows tlsystvar.h

// TVarData = VARIANT
struct TVarData
{
  union
  {
    uint16_t    VType;    // Delphi-compatible - Variant Type member
#if defined(VARIANT_NATIVE_SUPPORT)
    VARTYPE     vt;       // tagVARIANT compatible member
#else
    uint16_t    vt;       // Expose a generic 'vt' member
#endif
  };
  uint16_t Reserved1;
  uint16_t Reserved2;
  uint16_t Reserved3;
  union
  {
    // Delphi-compatible TVarData/Variant members
    System::Smallint      VSmallint;    //  iVal
    System::Integer       VInteger;     //  lVal
    System::Single        VSingle;      //  fltVal
    System::Double        VDouble;      //  dblVal
    System::CurrencyBase  VCurrency;    //  cyVal
    System::TDateTimeBase VDate;        //  date
    System::PWideChar     VOleStr;      //  bstrVal
#ifdef _WIN32
    IDispatch*            VDispatch;    //  pdispVal
#else
    System::IDispatch*    VDispatch;    //  pdispVal
#endif
    System::HResult       VError;       //  scode
    System::WordBool      VBoolean;     //  boolVal
    IUnknown*             VUnknown;     //  punkVal
    System::Byte          VByte;        //  bVal
    System::Int8          VShortint;    //  charVal
    System::Int8          VShortInt;    //  charVal
    System::Pointer       VString;      //  ??????
    System::PVarArray     VArray;       //  parray
    System::Pointer       VPointer;     //  byref
    __int64               VInt64;       //  llVal
    unsigned __int64      VUInt64;      //  ullVal
    System::Word          VWord;        //  uiVal
    System::LongWord      VLongWord;    //  ulVal
    System::TVarRecord    VRecord;      //  struct __tagBRECORD
}

d:program files (x86)embarcaderostudio16.0includewindows tlSystem.hpp

 Int8 varEmpty = Int8(0x0);
 Int8 varNull = Int8(0x1);
 Int8 varSmallint = Int8(0x2);
 Int8 varInteger = Int8(0x3);
 Int8 varSingle = Int8(0x4);
 Int8 varDouble = Int8(0x5);
 Int8 varCurrency = Int8(0x6);
 Int8 varDate = Int8(0x7);
 Int8 varOleStr = Int8(0x8);
 Int8 varDispatch = Int8(0x9);
 Int8 varError = Int8(0xa);
 Int8 varBoolean = Int8(0xb);
 Int8 varVariant = Int8(0xc);
 Int8 varUnknown = Int8(0xd);
 Int8 varShortInt = Int8(0x10);
 Int8 varByte = Int8(0x11);
 Int8 varWord = Int8(0x12);
 Int8 varLongWord = Int8(0x13);
 Int8 varInt64 = Int8(0x14);
 Int8 varUInt64 = Int8(0x15);
 Int8 varRecord = Int8(0x24);
 Int8 varStrArg = Int8(0x48);
 Int8 varObject = Int8(0x49);
 Int8 varUStrArg = Int8(0x4a);
 Word varString = Word(0x100);
 Word varAny = Word(0x101);
 Word varUString = Word(0x102);
 Word varTypeMask = Word(0xfff);
 Word varArray = Word(0x2000);
 Word varByRef = Word(0x4000);
 Int8 vtInteger = Int8(0x0);
 Int8 vtBoolean = Int8(0x1);
 Int8 vtChar = Int8(0x2);
 Int8 vtExtended = Int8(0x3);
 Int8 vtString = Int8(0x4);
 Int8 vtPointer = Int8(0x5);
 Int8 vtPChar = Int8(0x6);
 Int8 vtObject = Int8(0x7);
 Int8 vtClass = Int8(0x8);
 Int8 vtWideChar = Int8(0x9);
 Int8 vtPWideChar = Int8(0xa);
 Int8 vtAnsiString = Int8(0xb);
 Int8 vtCurrency = Int8(0xc);
 Int8 vtVariant = Int8(0xd);
 Int8 vtInterface = Int8(0xe);
 Int8 vtWideString = Int8(0xf);
 Int8 vtInt64 = Int8(0x10);
 Int8 vtUnicodeString = Int8(0x11);
 Int8 vmtArcOffset = Int8(0x0);

 _VarToUStr

参考官方的写法

procedure _VarToUStr(var S: UnicodeString; const V: TVarData);
begin
  case V.VType of
    varEmpty:    S := '';
    varNull:
      begin
        if NullStrictConvert then
          VarCastError(varNull, varOleStr);
        S := NullAsStringValue;
      end;
    varSmallInt: S := IntToStr(V.VSmallInt);
    varInteger:  S := IntToStr(V.VInteger);
    varSingle:   S := FloatToStr(V.VSingle);
    varDouble:   S := FloatToStr(V.VDouble);
    varShortInt: S := IntToStr(V.VShortInt);
    varByte:     S := IntToStr(V.VByte);
    varWord:     S := IntToStr(V.VWord);
    varLongWord: S := UIntToStr(V.VLongWord);
    varInt64:    S := IntToStr(V.VInt64);
    varUInt64:   S := UIntToStr(V.VUInt64);

    varVariant:  _VarToUStr(S, PVarData(V.VPointer)^);
    varOleStr:   S := Copy(WideString(Pointer(V.VOleStr)), 1, MaxInt);

{$IFNDEF NEXTGEN}
    varCurrency: S := CurrToWStrViaOS(V.VCurrency);
    varDate:     S := DateToWStrViaOS(V.VDate);
    varBoolean:  S := BoolToWStrViaOS(V.VBoolean);
    varDispatch,
    varUnknown:  S := VarToWStrViaOS(V);
{$ELSE}
                                                                                     
    varCurrency: S := CurrToStr(V.VCurrency);
    varDate:     S := DateTimeToStr(V.VDate);
    varBoolean:  S := BoolToStr(V.VBoolean,True);
                                                                                          
{$ENDIF !NEXTGEN}
  else
    case V.VType of
      varString: S := RawByteString(V.VString);                              
      varUString: S := UnicodeString(V.VUString);
      varAny:    S := VarToUStrAny(V);
    else
      if V.VType and varByRef <> 0 then
        case V.VType and not varByRef of
          varSmallInt: S := IntToStr(PSmallInt(V.VPointer)^);
          varInteger:  S := IntToStr(PInteger(V.VPointer)^);
          varSingle:   S := FloatToStr(PSingle(V.VPointer)^);
          varDouble:   S := FloatToStr(PDouble(V.VPointer)^);
          varShortInt: S := IntToStr(PShortInt(V.VPointer)^);
          varByte:     S := IntToStr(PByte(V.VPointer)^);
          varWord:     S := IntToStr(PWord(V.VPointer)^);
          varLongWord: S := UIntToStr(PCardinal(V.VPointer)^);
          varInt64:    S := IntToStr(PInt64(V.VPointer)^);
          varUInt64:   S := UIntToStr(PUInt64(V.VPointer)^);
          varVariant:  _VarToUStr(S, PVarData(V.VPointer)^);
{$IFNDEF NEXTGEN}
          varCurrency: S := CurrToWStrViaOS(PCurrency(V.VPointer)^);
          varDate:     S := DateToWStrViaOS(PDate(V.VPointer)^);
          varOleStr:   S := PWideChar(V.VPointer^);
          varBoolean:  S := BoolToWStrViaOS(PWordBool(V.VPointer)^);
          varString:    S := UnicodeString(PAnsiChar(V.VPointer^));
          varUString:   S := PWideChar(V.VPointer^);
        else
          S := VarToWStrViaOS(V);
{$ELSE NEXTGEN}
          varCurrency: S := CurrToStr(PCurrency(V.VPointer)^);
          varDate:     S := DateTimeToStr(PDate(V.VPointer)^);
          varOleStr:   S := Copy(PChar(V.VPointer^), 1, MaxInt);
          varBoolean:  S := BoolToStr(PWordBool(V.VPointer)^);
          varUString:   S := PWideChar(V.VPointer^);
                                                            
{$ENDIF NEXTGEN}
        end
      else
        if not VarToUStrCustom(V, S) then
{$IFNDEF NEXTGEN}
          S := VarToWStrViaOS(V);
{$ELSE NEXTGEN}
                                                         
          ; // Now we do nothing
{$ENDIF !NEXTGEN}
    end;
  end;
end;