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Using managed reference types in C++

, 15 Oct 2001
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A quick introduction to using .NET managed reference types in C++
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Introduction

One of the nice things about .NET are the common base classes. These classes are essentially the API of .NET, and are available from all languages equally. Once you know how to use a String in VB.NET you also know how to use it in C# and C++. Once you have struggled up the learning curve for one language you can then go on to use that knowledge in other .NET languages.

In .NET there are value types and reference types. Value types refer to simple data structures such as int's and enumerations, and are stored on the stack. Reference types are created on the .NET managed heap, which allows the garbage collector to track their lifetime and free instances when they are no longer required.

Think of a reference type as a pointer - though not in the traditional C/C++ sense of the word. The actual location of a variable on the managed heap will change as the garbage collector recovers unused memory and compacts the heap, so in a short time a traditional pointer to a spot on the heap will be invalid. A .NET reference, on the other hand, will always give you access to your values no matter where it has been moved on the heap. A variable of reference type will always either contain a reference to a value of that type, or null. Assigning the value of a reference variable to another variable copies the reference, not the value stored. Be warned!

Value types are stored on the stack and are accessed directly. Once the memory containing that value is freed, the value type instance is destroyed. Hence, references to value types are not allowed. If it were allowed it would be possible to have a reference point to an invalid memory location. A value type will always point to a variable of that type, and cannot be null. Assigning a value type to another variable results in a copy of the value being made.

Creating an instance of a reference type

Value types are easy since they are declared on the stack. It would be insane if you had to call new each time you wanted to create an int or double. Reference types are a little more complicated in that they cannot be created on the stack.

Reference types are created on the .NET managed heap, and so must be created using the overloaded new operator. The new operator for managed types not only creates the object on the managed heap, but also initialises the value of the variable to 0. The value passed back from new will not be a .NET reference, and not pointer in the traditional sense. In the following examples we will concentrate on the String class. You'll use it a lot, and it has some tricks up its sleave.

To create an instance of a reference type you simply declare a pointer of the variables type and create the object using new.

String* s = new String("This is a string");

Attempting to declare a managed object on the stack simply won't work:

String s = "This is a string";  // WILL NOT COMPILE

The String class' constructor contains many different overrides for many different occasions, but does not contain an override for String().

Other ways of declaring a String are as follows:

String* s = new String("This is an ANSI string");
String* s = "This is an ANSI string";
String* s = L"This is a UNICODE string";
String* s = S"This is a .NET string";

ANSI and UNICODE strings should be familiar to you. .NET strings (those prefixed by 'S') are new and offer better performance than standard C++ literal strings. As well as this, all instances of identical string literals actually point to the same string. If s1 and s2 are two String's, then the following code:

s1 = S"This is a .NET string";
s2 = S"This is a .NET string";
if (s1 == s2)
	printf("s1 == s2\n");
else
	printf("s1 != s2\n");

s1 = "This is a C++ literal string";
s2 = "This is a C++ literal string";
if (s1 == s2)
	printf("s1 == s2\n");
else
	printf("s1 != s2\n");

would produce

s1 == s2
s1 != s2

Note that C++ literal strings can be used where ever .NET strings are used, but .NET strings cannot be used where C++ strings are expected.

Note also the use of printf in the above snippet. Just because we are using .NET types and methods doesn't mean we lose our standard non-managed libraries. In managed C++ we get the best of both worlds.

Creating your own managed types

Creating your own managed types is achieved using the new __gc keyword.

__gc class MyClass
{
   public: 
       int ID;
};

You then use this class as you would any other managed class:

MyClass* mc = new MyClass;
mc->ID = 5;

Because mc is a managed type it will be automatically initialised to 0 (ie mc->ID will be set as 0).

Using managed types in non-managed functions

The final point is that when combining managed and unmanaged code you will invariably come across situations where you need to pass a managed pointer to a function expecting an unmanaged (fixed) pointer.

To allow this, a new keyword __pin has been introduced that essentially pins down the managed pointer so that the garbage collector will not move it.

MyClass __pin* pMC = mc;
printf("The pinned value of mc is %d\n", pMC->ID);

History

16 Oct 2001 - updated source files for VS.NET beta 2

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Chris Maunder
Founder CodeProject
Canada Canada
Chris is the Co-founder, Administrator, Architect, Chief Editor and Shameless Hack who wrote and runs The Code Project. He's been programming since 1988 while pretending to be, in various guises, an astrophysicist, mathematician, physicist, hydrologist, geomorphologist, defence intelligence researcher and then, when all that got a bit rough on the nerves, a web developer. He is a Microsoft Visual C++ MVP both globally and for Canada locally.
 
His programming experience includes C/C++, C#, SQL, MFC, ASP, ASP.NET, and far, far too much FORTRAN. He has worked on PocketPCs, AIX mainframes, Sun workstations, and a CRAY YMP C90 behemoth but finds notebooks take up less desk space.
 
He dodges, he weaves, and he never gets enough sleep. He is kind to small animals.
 
Chris was born and bred in Australia but splits his time between Toronto and Melbourne, depending on the weather. For relaxation he is into road cycling, snowboarding, rock climbing, and storm chasing.
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Comments and Discussions

 
GeneralVery nice...Does it come in pink PinmvpSacha Barber23-Jul-10 21:49 
GeneralCalling managed C++ functions in unmanaged code Pinmemberswarnimad26-Sep-06 1:18 
In our current project, to optimize efforts, one dll was written in .Net framework (with /clr settings). Now we need to access this dll from other projects which are not CLR compliant.
 
To do this, they have some mechanism in place –
 
DataExport dll is the .Net dll (/clr)
 
A new project DataExportingLinkActive has been created as a wrapper to DataExport dll. This project also has /clr setting in it. But it has a mix of managed and unmanaged code.
 
Now while compiling this project, it gives me following linking errors –
 
DataExportLink.obj : error LNK2019: unresolved external symbol "public: void __clrcall CDataExport::AddRowXMLChartMaster(class System::String ^,class System::String ^,class System::String ^,class System::String ^,class System::String ^,class System::String ^)" (?AddRowXMLChartMaster@CDataExport@@$$FQAMXP$AAVString@System@@00000@Z) referenced in function "public: void __thiscall CDataExportLink::AddRowXMLChartMaster(class ATL::CStringT > >,class ATL::CStringT > >,class ATL::CStringT > >,class ATL::CStringT > >,class ATL::CStringT > >,class ATL::CStringT > >)" (?AddRowXMLChartMaster@CDataExportLink@@$$FQAEXV?$CStringT@DV?$StrTraitMFC_DLL@DV?$ChTraitsCRT@D@ATL@@@@@ATL@@00000@Z)
 
The help for this error says that there is an attempt to call a __clrcall method in __thiscall method. Which also means that the caller is expecting __thiscall method but it is not getting it (__clrcall method is available instead).
 
I have been hunting to get this resolved. But no breakthrough yet.
One more clue: These projects were working fine in VS 2003. Now in VS 2005, they are giving these errors!
 
Please let me know if any one of you has encountered such a problem and have succeeded resolving the same.
 
Any kind of help would be appreciated..Smile | :)
 
Thanks in advance
 

 

 
Swarnima
GeneralC# interface - MC++ implementation Pinmemberharanath29-May-03 22:51 
GeneralHELP!!! Pinmemberomoshima24-Apr-03 7:05 
Questionwhere can I get vs.net beta2 now? PinmemberATL31-Jan-02 1:47 
GeneralWell written PinmemberBen Bryant4-Apr-01 7:21 
GeneralDisturbing.. PinmemberAnonymous3-Apr-01 5:04 
GeneralRe: Disturbing.. PinmemberAnonymous3-Apr-01 8:23 
GeneralRe: Disturbing.. PinmemberWilliam E. Kempf3-Apr-01 9:45 
GeneralRe: Disturbing.. PinmemberChris Maunder3-Apr-01 15:44 
GeneralATL7 is out? PinmemberGerald Schwab3-Apr-01 17:15 
GeneralRe: ATL7 is out? PinmemberChris Maunder4-Apr-01 13:20 
GeneralRe: Disturbing.. Pinmemberharanath27-May-03 4:35 
GeneralRe: Disturbing.. PinsussAnonymous27-Apr-05 16:20 
GeneralRe: Disturbing.. PinadminChris Maunder27-Apr-05 17:09 

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