Tracking Reference Operator (C++ Component Extensions)

 

The latest version of this topic can be found at Tracking Reference Operator (C++ Component Extensions).

A tracking reference (%) behaves like an ordinary C++ reference (&) except that when an object is assigned to a tracking reference, the object’s reference count is incremented.

All Platforms

A tracking reference has the following characteristics.

• Assignment of an object to a tracking reference causes the object’s reference count to be incremented.

• A native reference (&) is the result when you dereference a *. A tracking reference (%) is the result when you dereference a ^. As long as you have a % to an object, the object will stay alive in memory.

• The dot (.) member-access operator is used to access a member of the object.

• Tracking references are valid for value types and handles (for example String^).

• A tracking reference cannot be assigned a null or nullptr value. A tracking reference may be reassigned to another valid object as many times as required.

• A tracking reference cannot be used as a unary take-address operator.

Windows Runtime

A tracking reference behaves like a standard C++ reference, except that a % is reference-counted. The following snippet shows how to convert between % and ^ types:

Foo^ spFoo = ref new Foo();  
Foo% srFoo = *spFoo;  
Foo^ spFoo2 = %srFoo;  

The following example shows how to pass a ^ to a function that takes a %.

  
ref class Foo sealed {};  
  
    // internal or private  
    void UseFooHelper(Foo% f)  
    {  
        auto x = %f;  
    }  
  
    // public method on ABI boundary  
    void UseFoo(Foo^ f)  
    {  
        if (f != nullptr) { UseFooHelper(*f); }  
    }  

Common Language Runtime

In C++/CLI, you can use a tracking reference to a handle when you bind to an object of a CLR type on the garbage-collected heap.

In the CLR, the value of a tracking reference variable is updated automatically whenever the garbage collector moves the referenced object.

A tracking reference can be declared only on the stack. A tracking reference cannot be a member of a class.

It is not possible to have a native C++ reference to an object on the garbage-collected heap.

For more information about tracking references in C++/CLI, see:

• How to: Use Tracking References in C++/CLI

• How to: Use Tracking References and Value Types

• How to: Use Tracking References and Interior Pointers

• How to: Write Template Functions that Take Native, Value, or Reference Parameters

Examples

Example

The following sample for C++/CLI shows how to use a tracking reference with native and managed types.

// tracking_reference_1.cpp  
// compile with: /clr  
ref class MyClass {  
public:  
   int i;  
};  
  
value struct MyStruct {  
   int k;  
};  
  
int main() {  
   MyClass ^ x = ref new MyClass;  
   MyClass ^% y = x;   // tracking reference handle to reference object   
  
   int %ti = x->i;   // tracking reference to member of reference type  
  
   int j = 0;  
   int %tj = j;   // tracking reference to object on the stack  
  
   int * pi = new int[2];  
   int % ti2 = pi[0];   // tracking reference to object on native heap  
  
   int *% tpi = pi;   // tracking reference to native pointer  
  
   MyStruct ^ x2 = ref new MyStruct;  
   MyStruct ^% y2 = x2;   // tracking reference to value object  
  
   MyStruct z;  
   int %tk = z.k;   // tracking reference to member of value type  
  
   delete[] pi;  
}  
  

Example

The following sample for C++/CLI shows how to bind a tracking reference to an array.

// tracking_reference_2.cpp  
// compile with: /clr  
using namespace System;  
  
int main() {  
   array<int> ^ a = ref new array< Int32 >(5);  
   a[0] = 21;  
   Console::WriteLine(a[0]);  
   array<int> ^% arr = a;  
   arr[0] = 222;  
   Console::WriteLine(a[0]);  
}  

Output

21  
222