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pin_ptr

Updated: October 2008

pin_ptr lets you declare a pinning pointer. A pinning pointer is an interior pointer that prevents the object pointed into from moving on the garbage-collected heap (the value of a pinning pointer is not changed by the common language runtime). This is necessary when passing the address of a managed class to an unmanaged function because the address will not change unexpectedly during resolution of the unmanaged function call.

[cli::]pin_ptr<cv_qualifier type> var = &initializer;

where:

cv_qualifier

const or volatile qualifiers. A pinning pointer is volatile by default. It is redundant but not an error to declare a pinning pointer volatile.

initializer

A member of a reference type, element of a managed array, or any other object that you can assign to a native pointer.

type

The type of initializer.

var

The name of the pin_ptr variable.

An object or sub-object of a managed class can be pinned, in which case the common language runtime will not move it during garbage collection. The principal use of this is to pass a pointer to managed data as an actual parameter of an unmanaged function call. During a collection cycle, the runtime will inspect the metadata created for the pinning pointer and will not move the item it points to.

Pinning a sub-object defined in a managed object has the effect of pinning the entire object.

Pinning an object also pins its value fields (that is, fields of primitive or value type). However, fields declared by tracking handle are not pinned.

If the pinning pointer is reassigned to point to a new value, the previous instance pointed to is no longer considered pinned.

When a pinning pointer goes out of scope, the object is no longer considered pinned, unless there are other pinning pointers pointing to or into the object. You do not have to explicitly unpin a pointer.

If no pinning pointers point to the object (all pinning pointers went out of scope, were reassigned to point to other objects, or were assigned nullptr), the object is guaranteed not to be pinned.

A pinning pointer can point to a reference handle, value type or boxed type handle, member of a managed type or to an element of a managed array. It cannot point to a reference type.

NoteNote:

Taking the address of a pin_ptr that points to a native object results in undefined behavior.

Pinning pointers can only be declared as non-static local variables on the stack.

Pinning pointers cannot be used as:

  • function parameters

  • the return type of a function

  • a member of a class

  • the target type of a cast.

A pin_ptr represents a superset of the functionality of a native pointer. Therefore, anything that can be assigned to a native pointer can also be assigned to a pin_ptr. An interior pointer is permitted to perform the same set of operations as native pointers, including comparison and pointer arithmetic.

pin_ptr is in the cli namespace. See cli Namespace for more information.

For information about interior pointers, see interior_ptr.

For information about pinning pointers, see

// pin_ptr_1.cpp
// compile with: /clr 
using namespace System;
#define SIZE 10

#pragma unmanaged
// native function that initializes an array
void native_function(int* p) {
   for(int i = 0 ; i < 10 ; i++)
    p[i] = i;
}
#pragma managed

public ref class A {
private:
   array<int>^ arr;   // CLR integer array

public:
   A() {
      arr = gcnew array<int>(SIZE);
   }

   void load() {
   pin_ptr<int> p = &arr[0];   // pin pointer to first element in arr
   int* np = p;   // pointer to the first element in arr
   native_function(np);   // pass pointer to native function
   }

   int sum() {
      int total = 0;
      for (int i = 0 ; i < SIZE ; i++)
         total += arr[i];
      return total;
   }
};

int main() {
   A^ a = gcnew A;
   a->load();   // initialize managed array using the native function
   Console::WriteLine(a->sum());
}
45

This sample shows that an interior pointer can be converted to a pinning pointer and that the return type of the & operator is an interior pointer when the operand is on the managed heap.

// pin_ptr_2.cpp
// compile with: /clr
using namespace System;

ref struct G {
   G() : i(1) {}
   int i;
};

ref struct H {
   H() : j(2) {}
   int j;
};

int main() {
   G ^ g = gcnew G;   // g is a whole reference object pointer
   H ^ h = gcnew H;

   interior_ptr<int> l = &(g->i);   // l is interior pointer

   pin_ptr<int> k = &(h->j);   // k is a pinning interior pointer

   k = l;   // ok
   Console::WriteLine(*k);
};
1

This sample shows that a pinning pointer can be cast to another type:

// pin_ptr_3.cpp
// compile with: /clr
using namespace System;

ref class ManagedType {
public:
   int i;
};

int main() {
   ManagedType ^mt = gcnew ManagedType;
   pin_ptr< int > pt = &mt->i;
   *pt = 8;
   Console::WriteLine(mt->i);

   char *pc = ( char* ) pt;
   *pc = 255;
   Console::WriteLine(mt->i);
}
8 255

Compiler option: /clr

Date

History

Reason

October 2008

Clarified pinning behavior of fields.

Customer feedback.

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