RAD Studio VCL Reference
TCustomVariantType Class

TCustomVariantType is the base class for Delphi custom variant types.

TCustomVariantType = class(TObject, IInterface);
class TCustomVariantType : public TObject, public IInterface;

Use TCustomVariantType to define new data types for Delphi Variants. TCustomVariantType descendants do not represent the Variants themselves. Rather, they act as utility classes that the custom Variants call on to perform their operations. 

To create a System::Variant type,  

1.Map the storage of the System::Variant's data on to the TVarData record. 

2.Write a method that creates instances of your custom System::Variant. This fills in the System::Variant's data as defined in step 1. 

3.Declare a class that descends from TCustomVariantType. 

4.Implement all required behavior (including type conversion rules) in the new class. This involves overriding the appropriate virtual methods (typically BinaryOp, UnaryOp, CompareOp or Compare, Cast, CastTo, Clear, Copy, IsClear, LeftPromotion, and RightPromotion). 

5.In the initialization section of the unit that defines your TCustomVariantType descendant, create an instance of your class. When you instantiate your object, it automatically registers itself with the System::Variant-handling system so that the new System::Variant type is enabled. 

6.In the finalization section of the unit that defines your TCustomVariantType descendant, free the instance of your class. This automatically unregisters the System::Variant type.

Note: C++ can use Custom System::Variant types, but TCustomVariantType descendants must be implemented in Delphi.
For an example, see the FMTBcd unit, which implements a custom System::Variant type for binary-coded decimal.
Tip: To create a custom System::Variant that allows users to call methods or procedures and to get or set property values, use TInvokeableVariantType as a base class instead.


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