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) :-
Nullable<int> q1;
if(q1.HasValue)
Console::WriteLine(q1.Value);
else
Console::WriteLine("It's null");
q1 = Nullable<int>(99);
if(q1.HasValue)
q1 = Nullable<int>(q1.Value + 1);
Nullable<int> q2 = q1;
if( q2.HasValue )
{
q2 = Nullable<int>();
}
Not very convenient, is it? Now look at this C++ code :-
CNullable<int> q1;
if(q1.HasValue)
Console::WriteLine(q1.Value);
else
Console::WriteLine("It's null");
q1 = 99;
(*q1)++;
CNullable<int> q2 = q1;
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.
Nish Nishant is a Principal Software Architect based out of Columbus, Ohio. He has over 17 years of software industry experience in various roles including Lead Software Architect, Principal Software Engineer, and Product Manager. Nish was a Microsoft Visual C++ MVP between 2002 and 2015.
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 : If you are interested in hiring Nish as a consultant, you can reach him via his google email id
voidnish.
Company Website :
www.ganymedesoftwaresolutions.com