# Poco

# template < typename C >

# class Nullable

Library: Foundation
Package: Core
Header: Poco/Nullable.h

## Description

Nullable is a simple wrapper class for value types that allows objects or native type variables to have "null" value.

The class is useful for passing parameters to functions when parameters are optional and no default values should be used or when a non-assigned state is needed, such as in e.g. fetching null values from database.

A Nullable can be default constructed. In this case, the Nullable will have a Null value and isNull() will return true. Calling value() (without default value) on a Null object will throw a NullValueException.

A Nullable can also be constructed from a value. It is possible to assign a value to a Nullable, and to reset a Nullable to contain a Null value by calling clear().

For use with Nullable, the value type should support default construction.

## Member Summary

Member Functions: assign, clear, isNull, operator !=, operator <, operator =, operator ==, operator >, operator C &, operator NullType &, operator const C &, swap, value

## Constructors

### Nullable

Nullable();

Creates an empty Nullable.

### Nullable

Nullable(
const NullType & param205
);

Creates an empty Nullable.

### Nullable

Nullable(
const C & value
);

Creates a Nullable with the given value.

### Nullable

Nullable(
const Nullable & other
);

Creates a Nullable by copying another one.

## Destructor

### ~Nullable

~Nullable();

Destroys the Nullable.

## Member Functions

### assign

Nullable & assign(
const C & value
);

Assigns a value to the Nullable.

### assign

Nullable & assign(
const Nullable & other
);

Assigns another Nullable.

### assign

Nullable & assign(
NullType
);

Sets value to null.

### clear

void clear();

Clears the Nullable.

### isNull

bool isNull() const;

Returns true if the Nullable is empty.

### operator !=

bool operator != (
const C & value
) const;

Compares Nullable with value for non equality

### operator !=

bool operator != (
const Nullable < C > & other
) const;

Compares two Nullables for non equality

### operator !=

bool operator != (
const NullType & param207
) const;

Compares with NullData for non equality

### operator <

bool operator < (
const Nullable < C > & other
) const;

Compares two Nullable objects. Return true if this object's value is smaler than the other object's value. Null value is smaller than a non-null value.

### operator =

Nullable & operator = (
const C & value
);

Assigns a value to the Nullable.

### operator =

Nullable & operator = (
const Nullable & other
);

Assigns another Nullable.

### operator =

Nullable & operator = (
NullType
);

Assigns another Nullable.

### operator ==

bool operator == (
const Nullable < C > & other
) const;

Compares two Nullables for equality

### operator ==

bool operator == (
const C & value
) const;

Compares Nullable with value for equality

### operator ==

bool operator == (
const NullType & param206
) const;

Compares Nullable with NullData for equality

### operator >

bool operator > (
const Nullable < C > & other
) const;

Compares two Nullable objects. Return true if this object's value is greater than the other object's value. A non-null value is greater than a null value.

### operator C &

operator C & ();

Get reference to the value

### operator NullType &

operator NullType & ();

Get reference to the value

### operator const C &

operator const C & () const;

Get const reference to the value

### swap

void swap(
Nullable & other
);

Swaps this Nullable with other.

### value

C & value();

Returns the Nullable's value.

Throws a NullValueException if the Nullable is empty.

### value

const C & value() const;

Returns the Nullable's value.

Throws a NullValueException if the Nullable is empty.

### value

const C & value(
const C & deflt
) const;

Returns the Nullable's value, or the given default value if the Nullable is empty.