Delphi Basics : Variant command

DelphiBasics

Variant
Type

A variable type that can hold changing data types

System unit

type Variant;

Description

The Variant data type provides a flexible general purpose data type.

It can hold anything but structured data and pointers.

But use with care - there are penalties in performance, potentials for run time errors and poor code clarity when using Variants.

Use VarType in conjunction with VarTypeMask to determine the curent data types a Variant is set to.

Variants are useful in very specific circumstances, where data types and their content are determined at run time rather than at compile time.

Notes

Variant strings cannot be indexed.

Related commands

Null		A variable that has no value
PVariant		Pointer to a Variant value

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Example code : Illustrating a few Variant assignments and the resulting data types

var
  myVar : Variant;

begin
  // Assign various values to a Variant
  // and then show the resulting Variant type
  ShowMessage('Variant value = not yet set');
  ShowBasicVariantType(myVar);

  // Simple value
  myVar := 123;
  ShowMessage('Variant value = 123');
  ShowBasicVariantType(myVar);

  // Calculated value using a Variant and a constant
  myVar := myVar + 456;
  ShowMessage('Variant value = 123 + 456');
  ShowBasicVariantType(myVar);

  myVar := 'String '+IntToStr(myVar);
  ShowMessage('Variant value = String 579');
  ShowBasicVariantType(myVar);
end;

// Show the type of a variant
procedure TForm1.ShowBasicVariantType(varVar: Variant);
var
  typeString : string;
  basicType  : Integer;

begin
  // Get the Variant basic type :
  // this means excluding array or indirection modifiers
  basicType := VarType(varVar) and VarTypeMask;

  // Set a string to match the type
  case basicType of
    varEmpty     : typeString := 'varEmpty';
    varNull      : typeString := 'varNull';
    varSmallInt  : typeString := 'varSmallInt';
    varInteger   : typeString := 'varInteger';
    varSingle    : typeString := 'varSingle';
    varDouble    : typeString := 'varDouble';
    varCurrency  : typeString := 'varCurrency';
    varDate      : typeString := 'varDate';
    varOleStr    : typeString := 'varOleStr';
    varDispatch  : typeString := 'varDispatch';
    varError     : typeString := 'varError';
    varBoolean   : typeString := 'varBoolean';
    varVariant   : typeString := 'varVariant';
    varUnknown   : typeString := 'varUnknown';
    varByte      : typeString := 'varByte';
    varWord      : typeString := 'varWord';
    varLongWord  : typeString := 'varLongWord';
    varInt64     : typeString := 'varInt64';
    varStrArg    : typeString := 'varStrArg';
    varString    : typeString := 'varString';
    varAny       : typeString := 'varAny';
    varTypeMask  : typeString := 'varTypeMask';
  end;

  // Show the Variant type
  ShowMessage('Variant type  = '+typeString);
end;

Show full unit code

  Variant value = not yet set
  Variant type = varEmpty
  Variant value = 123
  Variant type = varByte
  Variant value = 123 + 456
  Variant type = varInt64
  Variant value = String 579
  Variant type = varString