import std.stdio;

import std.exception : enforce;

​

void main() {

    Nullable!(int*) v = null;

    writeln("Surprise! ", v.isNull() );

}

​

struct Nullable(T)

{

    private T _value;

    private bool _isNull = true;

 /**

Constructor initializing $(D this) with $(D value).

 */ 

    this(T value)

    {

        _value = value;

        _isNull = false;

    }

 /**

Returns $(D true) if and only if $(D this) is in the null state.

 */ 

    bool isNull()

    {

        return _isNull;

    }

 /**

Forces $(D this) to the null state.

 */ 

    void nullify()

    {

        // destroy

        //static if (is(typeof(_value.__dtor()))) _value.__dtor();

        _isNull = true;

    }

 /**

Assigns $(D value) to the internally-held state. If the assignment

succeeds, $(D this) becomes non-null.

 */ 

    void opAssign(T value)

    {

        _value = value;

        _isNull = false;

    }

 /**

Gets the value. Throws an exception if $(D this) is in the null

state. This function is also called for the implicit conversion to $(D

T).

 */

    ref T get()

    {

        enforce(!isNull);

        return _value;

    }

 /**

Implicitly converts to $(D T). Throws an exception if $(D this) is in

the null state.

 */

    alias get this;

}