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CNullable<T> : A convenient Nullable<T> equivalent class for C++/CLI

, 18 May 2005
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CNullable<T> can be used to represent a value type (or simple native type) such that the type can be nulled. You can also compare it to nullptr using the == and != operators both of which have been overloaded.

Overview

Starting with Whidbey, C# supports nullable types (value types that can also be null) and it does this using the System::Nullable<T> generic class defined in the mscorlib assembly. It also provides a short-cut form, T? var which corresponds to System.Nullable<T> var. Take a look at the following C# code snippet :-

int? q1 = null;

if (q1.HasValue)
    Console.WriteLine(q1.Value);
else
    Console.WriteLine("It's null");    

q1 = 99;
q1++;

int? q2 = q1;
if (q2 != null)
{
    q2 = null;
}

Now, take a look at the corresponding C++ code (C# code in comments) :-

//int? q1 = null;
Nullable<int> q1;

//if (q1.HasValue)
//    Console.WriteLine(q1.Value);
//else
//    Console.WriteLine("It's null");

if(q1.HasValue)
    Console::WriteLine(q1.Value);
else
    Console::WriteLine("It's null");

//q1 = 99;

q1 = Nullable<int>(99);

//q1++;

if(q1.HasValue)
    q1 = Nullable<int>(q1.Value + 1);

//int? q2 = q1;

Nullable<int> q2 = q1;

//if (q2 != null)
//{
//    q2 = null;
//}

if( q2.HasValue )
{
    q2 = Nullable<int>();
}

Not very convenient, is it? Now look at this C++ code :-

//int? q1 = null;
CNullable<int> q1;

//if (q1.HasValue)
//    Console.WriteLine(q1.Value);
//else
//    Console.WriteLine("It's null");

if(q1.HasValue)
    Console::WriteLine(q1.Value);
else
    Console::WriteLine("It's null");

//q1 = 99;

q1 = 99;

//q1++;

(*q1)++;

////int? q2 = q1;

CNullable<int> q2 = q1;

//if (q2 != null)
//{
//    q2 = null;
//}

if(q2 != nullptr)
{
    q2 = nullptr;
}

Well, that's a lot more closer to the C# code as far as ease-of-use is concerned, is it not? That's why I wrote my CNullable<> template ref class for C++/CLI.

Class Reference

template<typename T> ref class CodeProject::Extra::CNullable sealed : INullableValue

CNullable can be used to represent a value type (or simple native type) such that the type can be nulled. You can also compare it to nullptr using the == and != operators both of which have been overloaded.

Constructors

  • CNullable()

    The default constructor creates a CNullable object assigned to nullptr.

    CNullable<char> x1;
  • CNullable(void* p)

    This overload handles the case where a nullptr is passed to the constructor. If the pointer passed is not a nullptr, an InvalidOperationException gets thrown.

    CNullable<double> x2(nullptr);
  • CNullable(const T% t)

    Constructs a CNullable object using the passed in template-argument type.

    CNullable<int> x3(100);
  • CNullable(const CNullable% n)

    Copy constructor.

    CNullable<int> x4(x3);

Operators

  • operator T()

    Converts to type T.

    Note - If the object is currently nulled, an InvalidOperationException is thrown.

    int y = x4;
  • void operator =(void* p)

    Assignment operator that handles the nullptr case.

    Note - If p is a non-null pointer, an InvalidOperationException is thrown.

    x3 = nullptr;
  • const T operator =(const T% t)

    Assignment operator for type T.

    x4 = x3 = 13;
  • T% operator*()

    The underlying type is exposed by overloading the dereference operator.

    x5 = (*x4)++;
  • bool operator ==(void* p)

    Allows == comparison with nullptr.

    if( x5 == nullptr )
    {
    }
  • bool operator !=(void* p)

    Allows != comparison with nullptr.

    if( x5 != nullptr )
    {
    }
  • operator String^()

    Gives the string representation of the type. Internally calls the ToString method.

    Console::WriteLine( (String^)x5 );

Methods

  • virtual String^ ToString() override

    Returns an empty string if the object is currently nulled, else calls ToString on the underlying template-argument type member and returns that string.

    Console::WriteLine( x5.ToString() );
  • Nullable<T> CreateNullable()

    Creates a new Nullable<T> object (useful to interact with .NET code that explicitly expects a Nullable<T>). Normally you'd be able to pass a CNullable object directly since it implements INullableValue.

    Nullable<int> n1 = x5.CreateNullable();
  • static CNullable^ FromNullable(Nullable<T> n)

    Static method to create a new CNullable object from a Nullable<T> object.

    CNullable<int> x6 = CNullable<int>::FromNullable(n1);

Properties

  • property virtual bool HasValue::get

    Implementation of INullableValue::HasValue. Indicates whether the value is valid or whether it's a null-value.

    if(x6.HasValue)
  • property virtual Object^ Value::get

    Implementation of INullableValue::Value. If HasValue is true, it returns the value of the internal template argument type member, otherwise throws an InvalidOperationException exception.

    Console::WriteLine(x6.Value);

History

  • May 18th, 2005 : Article first published on The Code Project.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

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About the Author

Nish Nishant

United States United States
Nish Nishant is a Software Architect/Consultant based out of Columbus, Ohio. He has over 15 years of software industry experience in various roles including Lead Software Architect, Principal Software Engineer, and Product Manager. Nish is a recipient of the annual Microsoft Visual C++ MVP Award since 2002 (13 consecutive awards as of 2014).

Nish is an industry acknowledged expert in the Microsoft technology stack. He authored
C++/CLI in Action for Manning Publications in 2005, and had previously co-authored
Extending MFC Applications with the .NET Framework for Addison Wesley in 2003. In addition, he has over 140 published technology articles on CodeProject.com and another 250+ blog articles on his
WordPress blog. Nish is vastly experienced in team management, mentoring teams, and directing all stages of software development.

Contact Nish : You can reach Nish on his google email id voidnish.

Website and Blog

Comments and Discussions

 
QuestionCan you port it to C++? Pin
CP Visitor18-May-05 3:11
memberCP Visitor18-May-05 3:11 
GeneralAnd if you're not using .Net... Pin
Stuart Dootson18-May-05 3:10
memberStuart Dootson18-May-05 3:10 
GeneralRe: And if you're not using .Net... Pin
Nishant Sivakumar18-May-05 7:16
staffNishant Sivakumar18-May-05 7:16 
Generalfixed link Pin
peterchen14-Apr-11 10:33
memberpeterchen14-Apr-11 10:33 

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