Class Poco::Dynamic::Var
Author: Applied Informatics Software Engineering GmbH and Contributors

# Poco::Dynamic

# class Var

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

## Description

Var allows to store data of different types and to convert between these types transparently. Var puts forth the best effort to provide intuitive and reasonable conversion semantics and prevent unexpected data loss, particularly when performing narrowing or signedness conversions of numeric data types.

An attempt to convert or extract from a non-initialized ("empty") Var variable shall result in an exception being thrown.

Loss of signedness is not allowed for numeric values. This means that if an attempt is made to convert the internal value which is a negative signed integer to an unsigned integer type storage, a RangeException is thrown. Overflow is not allowed, so if the internal value is a larger number than the target numeric type size can accommodate, a RangeException is thrown.

Precision loss, such as in conversion from floating-point types to integers or from double to float on platforms where they differ in size (provided internal actual value fits in float min/max range), is allowed.

String truncation is allowed — it is possible to convert between string and character when string length is greater than 1. An empty string gets converted to the char '\0', a non-empty string is truncated to the first character.

Boolean conversion is performed as follows:

A string value "false" (not case sensitive), "0" or "" (empty string) can be converted to a boolean value false, any other string not being false by the above criteria evaluates to true (e.g: "hi" -> true). Integer 0 values are false, everything else is true. Floating point values equal to the minimal FP representation on a given platform are false, everything else is true.

Arithmetic operations with POD types as well as between Var's are supported, subject to following limitations:

- for std::string and const char* values, only '+' and '+=' operations are supported

- for integral and floating point numeric values, following operations are supported:

    '+', '+=', '-', '-=', '*', '*=' , '/' and '/='

- for integral values, following operations are supported:

    prefix and postfix increment (++) and decrement (--)

- for all other types, InvalidArgumentException is thrown upon attempt of an arithmetic operation

A Var can be created from and converted to a value of any type for which a specialization of VarHolderImpl is available. For supported types, see VarHolder documentation.

## Member Summary

Member Functions: at, begin, clear, convert, empty, end, extract, isArray, isBoolean, isDate, isDateTime, isDeque, isEmpty, isInteger, isList, isNumeric, isSigned, isString, isStruct, isTime, isVector, operator !, operator !=, operator &&, operator *, operator *=, operator +, operator ++, operator +=, operator -, operator --, operator -=, operator /, operator /=, operator <, operator <=, operator =, operator ==, operator >, operator >=, operator T, operator [], operator ||, parse, size, swap, toString, type

## Types

### ConstIterator

typedef const VarIterator ConstIterator;

### Iterator

typedef Poco::Dynamic::VarIterator Iterator;

### Ptr

typedef SharedPtr < Var > Ptr;

## Constructors

### Var

Var();

Creates an empty Var.

### Var

template < typename T > Var(
const T & val
);

Creates the Var from the given value.

### Var

Var(
const char * pVal
);

### Var

Var(
const Var & other
);

Copy constructor.

## Destructor

### ~Var

~Var();

Destroys the Var.

## Member Functions

### at

char & at(
std::size_t n
);

Returns character at position n. This function only works with Var containing a std::string.

### begin

ConstIterator begin() const;

Returns the const Var iterator.

### begin

Iterator begin();

Returns the Var iterator.

### clear

void clear();

Empties Var.

### convert

template < typename T > void convert(
T & val
) const;

Invoke this method to perform a safe conversion.

Example usage:

    Var any("42");
    int i;
    any.convert(i);

Throws a RangeException if the value does not fit into the result variable. Throws a NotImplementedException if conversion is not available for the given type. Throws InvalidAccessException if Var is empty.

### convert

template < typename T > T convert() const;

Invoke this method to perform a safe conversion.

Example usage:

    Var any("42");
    int i = any.convert<int>();

Throws a RangeException if the value does not fit into the result variable. Throws a NotImplementedException if conversion is not available for the given type. Throws InvalidAccessException if Var is empty.

### empty

void empty();

Deprecated. This function is deprecated and should no longer be used.

Empties Var. This function is deprecated and will be removed. Please use clear().

### end

ConstIterator end() const;

Returns the const Var iterator.

### end

Iterator end();

Returns the Var iterator.

### extract

template < typename T > const T & extract() const;

Returns a const reference to the actual value.

Must be instantiated with the exact type of the stored value, otherwise a BadCastException is thrown. Throws InvalidAccessException if Var is empty.

### isArray

bool isArray() const;

Returns true if Var is an array.

### isBoolean

bool isBoolean() const;

Returns true if stored value is boolean. Returns false for boolean strings (e.g. "true" is string, not number)

### isDate

bool isDate() const;

Returns true if stored value represents a date.

### isDateTime

bool isDateTime() const;

Returns true if stored value represents a date/time.

### isDeque

bool isDeque() const;

Returns true if Var represents a deque.

### isEmpty

bool isEmpty() const;

Returns true if empty.

### isInteger

bool isInteger() const;

Returns true if stored value is integer.

### isList

bool isList() const;

Returns true if Var represents a list.

### isNumeric

bool isNumeric() const;

Returns true if stored value is numeric. Returns false for numeric strings (e.g. "123" is string, not number)

### isSigned

bool isSigned() const;

Returns true if stored value is signed.

### isString

bool isString() const;

Returns true if stored value is std::string.

### isStruct

bool isStruct() const;

Returns true if Var represents a struct.

### isTime

bool isTime() const;

Returns true if stored value represents time or date/time.

### isVector

bool isVector() const;

Returns true if Var represents a vector.

### operator !

bool operator ! () const;

Logical NOT operator.

### operator !=

template < typename T > bool operator != (
const T & other
) const;

Inequality operator

### operator !=

bool operator != (
const Var & other
) const;

Inequality operator overload for Var

### operator !=

bool operator != (
const char * other
) const;

Inequality operator overload for const char*

### operator &&

template < typename T > bool operator && (
const T & other
) const;

Logical AND operator.

### operator &&

bool operator && (
const Var & other
) const;

Logical AND operator operator overload for Var.

### operator *

template < typename T > const Var operator * (
const T & other
) const;

Multiplication operator for multiplying Var with POD

### operator *

const Var operator * (
const Var & other
) const;

Multiplication operator overload for Var

### operator *=

template < typename T > Var & operator *= (
const T & other
);

Multiplication assignment operator

### operator *=

Var & operator *= (
const Var & other
);

Multiplication assignment operator overload for Var

### operator +

template < typename T > const Var operator + (
const T & other
) const;

Addition operator for adding POD to Var

### operator +

const Var operator + (
const Var & other
) const;

Addition operator specialization for Var

### operator +

const Var operator + (
const char * other
) const;

Addition operator specialization for adding const char* to Var

### operator ++

Var & operator ++ ();

Pre-increment operator

### operator ++

const Var operator ++ (
int
);

Post-increment operator

### operator +=

template < typename T > Var & operator += (
const T & other
);

Addition assignment operator for addition/assignment of POD to Var.

### operator +=

Var & operator += (
const Var & other
);

Addition assignment operator overload for Var

### operator +=

Var & operator += (
const char * other
);

Addition assignment operator overload for const char*

### operator -

template < typename T > const Var operator - (
const T & other
) const;

Subtraction operator for subtracting POD from Var

### operator -

const Var operator - (
const Var & other
) const;

Subtraction operator overload for Var

### operator --

Var & operator -- ();

Pre-decrement operator

### operator --

const Var operator -- (
int
);

Post-decrement operator

### operator -=

template < typename T > Var & operator -= (
const T & other
);

Subtraction assignment operator

### operator -=

Var & operator -= (
const Var & other
);

Subtraction assignment operator overload for Var

### operator /

template < typename T > const Var operator / (
const T & other
) const;

Division operator for dividing Var with POD

### operator /

const Var operator / (
const Var & other
) const;

Division operator overload for Var

### operator /=

template < typename T > Var & operator /= (
const T & other
);

Division assignment operator

### operator /=

Var & operator /= (
const Var & other
);

Division assignment operator specialization for Var

### operator <

template < typename T > bool operator < (
const T & other
) const;

Less than operator

### operator <

bool operator < (
const Var & other
) const;

Less than operator overload for Var

### operator <=

template < typename T > bool operator <= (
const T & other
) const;

Less than or equal operator

### operator <=

bool operator <= (
const Var & other
) const;

Less than or equal operator overload for Var

### operator =

template < typename T > Var & operator = (
const T & other
);

Assignment operator for assigning POD to Var

### operator =

Var & operator = (
const Var & other
);

Assignment operator specialization for Var

### operator ==

template < typename T > bool operator == (
const T & other
) const;

Equality operator

### operator ==

bool operator == (
const char * other
) const;

Equality operator overload for const char*

### operator ==

bool operator == (
const Var & other
) const;

Equality operator overload for Var

### operator >

template < typename T > bool operator > (
const T & other
) const;

Greater than operator

### operator >

bool operator > (
const Var & other
) const;

Greater than operator overload for Var

### operator >=

template < typename T > bool operator >= (
const T & other
) const;

Greater than or equal operator

### operator >=

bool operator >= (
const Var & other
) const;

Greater than or equal operator overload for Var

### operator T

template < typename T > operator T() const;

Safe conversion operator for implicit type conversions. If the requested type T is same as the type being held, the operation performed is direct extraction, otherwise it is the conversion of the value from type currently held to the one requested.

Throws a RangeException if the value does not fit into the result variable. Throws a NotImplementedException if conversion is not available for the given type. Throws InvalidAccessException if Var is empty.

### operator []

template < typename T > Var & operator[] (
const T & n
);

### operator []

template < typename T > const Var & operator[] (
const T & n
) const;

### operator []

Var & operator[] (
const std::string & name
);

Index operator by name, only use on Vars where isStruct returns true! In all other cases InvalidAccessException is thrown.

### operator []

const Var & operator[] (
const std::string & name
) const;

Index operator by name, only use on Vars where isStruct returns true! In all other cases InvalidAccessException is thrown.

### operator ||

template < typename T > bool operator || (
const T & other
) const;

Logical OR operator

### operator ||

bool operator || (
const Var & other
) const;

Logical OR operator operator overload for Var

### parse

static Var parse(
const std::string & val
);

Parses the string which must be in JSON format

### size

std::size_t size() const;

Returns the size of this Var. This function returns 0 when Var is empty, 1 for POD or the size (i.e. length) for held container.

### swap

void swap(
Var & other
);

Swaps the content of the this Var with the other Var.

### toString

std::string toString() const;

Returns the stored value as string.

### toString

static std::string toString(
const Var & var
);

Converts the Var to a string in JSON format. Note that toString(const Var&) will return a different result than Var::convert<std::string>() and Var::toString()!

### type

const std::type_info & type() const;

Returns the type information of the stored content.