# Poco::Dynamic

# class VarHolder

Library: Foundation
Package: Dynamic
Header: Poco/Dynamic/VarHolder.h

## Description

Interface for a data holder used by the Var class. Provides methods to convert between data types. Only data types for which VarHolder specialization exists are supported.

Provided are specializations for all C++ built-in types with addition of std::string, Poco::UTF16String, DateTime, LocalDateTime, Timestamp, std::vector<Var> and DynamicStruct.

Additional types can be supported by adding specializations. When implementing specializations, the only condition is that they reside in Poco namespace and implement the pure virtual functions clone() and type().

Those conversions that are not implemented shall fail back to this base class implementation. All the convert() function overloads in this class throw BadCastException.

## Inheritance

Known Derived Classes: VarHolderImpl

## Member Summary

Member Functions: clone, cloneHolder, convert, convertSignedFloatToUnsigned, convertSignedToUnsigned, convertToSmaller, convertToSmallerUnsigned, convertUnsignedToSigned, isArray, isBoolean, isDate, isDateTime, isDeque, isInteger, isList, isNumeric, isOrdered, isSigned, isString, isStruct, isTime, isUUID, isVector, size, type

## Types

### ArrayValueType

typedef Var ArrayValueType;

## Constructors

### VarHolder

VarHolder();

Creates the VarHolder.

## Destructor

### ~VarHolder

virtual ~VarHolder();

Destroys the VarHolder.

## Member Functions

### clone

virtual VarHolder * clone(
Placeholder < VarHolder > * pHolder = 0
) const = 0;

Implementation must implement this function to deep-copy the VarHolder. If small object optimization is enabled (i.e. if POCO_NO_SOO is not defined), VarHolder will be instantiated in-place if it's size is smaller than POCO_SMALL_OBJECT_SIZE.

### convert

virtual void convert(
Int8 & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
Int16 & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
Int32 & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
Int64 & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
UInt8 & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
UInt16 & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
UInt32 & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
UInt64 & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
DateTime & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
LocalDateTime & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
Timestamp & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
UUID & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
long long & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to suport the conversion.

### convert

virtual void convert(
unsigned long long & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to suport the conversion.

### convert

virtual void convert(
bool & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
float & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
double & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
char & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
std::string & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### convert

virtual void convert(
Poco::UTF16String & val
) const;

Throws BadCastException. Must be overridden in a type specialization in order to support the conversion.

### isArray

virtual bool isArray() const;

Returns true.

### isBoolean

virtual bool isBoolean() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isDate

virtual bool isDate() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isDateTime

virtual bool isDateTime() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isDeque

virtual bool isDeque() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isInteger

virtual bool isInteger() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isList

virtual bool isList() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isNumeric

virtual bool isNumeric() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isOrdered

virtual bool isOrdered() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isSigned

virtual bool isSigned() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isString

virtual bool isString() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isStruct

virtual bool isStruct() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isTime

virtual bool isTime() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isUUID

virtual bool isUUID() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### isVector

virtual bool isVector() const;

Returns false. Must be properly overridden in a type specialization in order to support the diagnostic.

### size

virtual std::size_t size() const;

Returns 1 iff Var is not empty or this function overridden.

### type

virtual const std::type_info & type() const = 0;

Implementation must return the type information (typeid) for the stored content.

### cloneHolder

template < typename T > VarHolder * cloneHolder(
Placeholder < VarHolder > * pVarHolder,
const T & val
) const;

Instantiates value holder wrapper.

Called from clone() member function of the implementation.

When the smal object optimization is enabled (POCO_NO_SOO not defined), if size of the wrapper is larger than POCO_SMALL_OBJECT_SIZE, holder is instantiated on the heap, otherwise it is instantiated in-place (in the pre-allocated buffer inside the holder).

### convertSignedFloatToUnsigned

template < typename F, typename T > void convertSignedFloatToUnsigned(
const F & from,
T & to
) const;

This function is meant for converting floating point data types to unsigned integral data types. Negative values can not be converted and if one is encountered, RangeException is thrown. If upper limit is within the target data type limits, the conversion is performed.

### convertSignedToUnsigned

template < typename F, typename T > void convertSignedToUnsigned(
const F & from,
T & to
) const;

This function is meant for converting signed integral data types to unsigned data types. Negative values can not be converted and if one is encountered, RangeException is thrown. If upper limit is within the target data type limits, the conversion is performed.

### convertToSmaller

template < typename F, typename T > void convertToSmaller(
const F & from,
T & to
) const;

This function is meant to convert signed numeric values from larger to smaller type. It checks the upper and lower bound and if from value is within limits of type T (i.e. check calls do not throw), it is converted.

### convertToSmallerUnsigned

template < typename F, typename T > void convertToSmallerUnsigned(
const F & from,
T & to
) const;

This function is meant for converting unsigned integral data types, from larger to smaller type. Since lower limit is always 0 for unsigned types, only the upper limit is checked, thus saving some cycles compared to the signed version of the function. If the value to be converted is smaller than the maximum value for the target type, the conversion is performed.

### convertUnsignedToSigned

template < typename F, typename T > void convertUnsignedToSigned(
const F & from,
T & to
) const;

This function is meant for converting unsigned integral data types to signed integral data types. Negative values can not be converted and if one is encountered, RangeException is thrown. If upper limit is within the target data type limits, the conversion is performed.