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How to: Use Tracking References and Value Types

The following sample shows simple boxing through a tracking reference to a value type:

// tracking_reference_valuetypes_1.cpp
// compile with: /clr
using namespace System;
int main() {
   int i = 10;
   int % j = i;
   Object ^ o = j;   // j is implicitly boxed and assigned to o
}

The following sample shows you can have both tracking references and native references to value types.

// tracking_reference_valuetypes_2.cpp
// compile with: /clr
using namespace System;
int main() {
   int i = 10;
   int & j = i;
   int % k = j;
   i++;   // 11
   j++;   // 12
   k++;   // 13
   Console::WriteLine(i);
   Console::WriteLine(j);
   Console::WriteLine(k);
}

Output

13
13
13

The following sample shows that tracking references can be used with value types and native types.

// tracking_reference_valuetypes_3.cpp
// compile with: /clr
value struct G {
   int i;
};

struct H {
   int i;
};

int main() {
   G g;
   G % v = g;
   v.i = 4;
   System::Console::WriteLine(v.i);
   System::Console::WriteLine(g.i);

   H h;
   H % w = h;
   w.i = 5;
   System::Console::WriteLine(w.i);
   System::Console::WriteLine(h.i);
}

Output

4
4
5
5

The following sample shows that you can bind a tracking reference to a value type on the garbage-collected heap.

// tracking_reference_valuetypes_4.cpp
// compile with: /clr
using namespace System;
value struct V {
   int i;
};

void Test(V^ hV) {   // hv boxes another copy of original V on GC heap
   Console::WriteLine("Boxed new copy V: {0}", hV->i);
}

int main() {
   V v;   // V on the stack
   v.i = 1;
   V ^hV1 = v;   // v is boxed and assigned to hV1
   v.i = 2;
   V % trV = *hV1;   // trV is bound to boxed v, the v on the gc heap.
   Console::WriteLine("Original V: {0}, Tracking reference to boxed V: {1}", v.i, trV.i);
   V ^hV2 = trV;   // hv2 boxes another copy of boxed v on the GC heap
   hV2->i = 3;
   Console::WriteLine("Tracking reference to boxed V: {0}", hV2->i);
   Test(trV);
   v.i = 4;
   V ^% trhV = hV1;  // creates tracking reference to boxed type handle
   Console::WriteLine("Original V: {0}, Reference to handle of originally boxed V: {1}", v.i, trhV->i);
}

Output

Original V: 2, Tracking reference to boxed V: 1
Tracking reference to boxed V: 3
Boxed new copy V: 1
Original V: 4, Reference to handle of originally boxed V: 1
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