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pin_ptr (C++/CLI)

Declares a pinning pointer, which is used only with the common language runtime.

(There are no remarks for this language feature that apply to all runtimes.)

(This language feature is not supported in the Windows Runtime.)

A pinning pointer is an interior pointer that prevents the object pointed to from moving on the garbage-collected heap. That is, the value of a pinning pointer is not changed by the common language runtime. This is required when you pass the address of a managed class to an unmanaged function so that the address will not change unexpectedly during resolution of the unmanaged function call.

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

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

type

The type of initializer.

var

The name of the pin_ptr variable.

initializer

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

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.

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

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

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

An object is pinned only while a pin_ptr points to it. The object is no longer pinned when its pinning pointer goes out of scope, or is set to nullptr. After the pin_ptr goes out of scope, the object that was pinned can be moved in the heap by the garbage collector. Any native pointers that still point to the object will not be updated, and de-referencing one of them could raise an unrecoverable exception.

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 an element of a managed array. It cannot point to a reference type.

Taking the address of a pin_ptr that points to a native object causes 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.

pin_ptr is in the cli namespace. For more information, see Platform, default, and cli Namespaces (C++ Component Extensions).

For more information about interior pointers, see interior_ptr (C++/CLI).

For more information about pinning pointers, see How to: Pin Pointers and Arrays and How to: Declare Pinning Pointers and Value Types.

Compiler option: /clr

Example

The following example uses pin_ptr to constrain the position of the first element of an array.

// 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());
}

Output

45

Example

The following example shows that an interior pointer can be converted to a pinning pointer, and that the return type of the address-of 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);
};

Output

1

Example

The following example 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);
}

Output

8255

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