Pointer to managed array in C++/CLI

Question

Alright, I know how you normally would declare a pointer:

void SomeFunction(array<float> ^managedArray)
{
  pin_ptr<float> managedArrayPtr = &managedArray[0];
}

This works fine except when managedArray contains no elements. In that case, it throws an IndexOutOfRangeException.

In C# you can do this:

void SomeFunction(float[] managedArray)
{
  fixed (float* managedArrayPtr = managedArray)
  {
  }
}

Which does no memory access and works even if managedArray is empty. Do I really have to check for the number of elements everywhere I use pointers to managed arrays or does C++/CLI have a way to do it like C#? It should be using the 'lea' instruction in ASM which does no memory access.

Any help is greatly appreciated!

Hans Passant · Accepted Answer · 2010-08-19 18:16:52Z

3

There isn't much point in trying to read from the array when it is empty. Just check for that:

void SomeFunction(array<float> ^managedArray)
{
    if (managedArray->Length > 0) {
        pin_ptr<float> managedArrayPtr = managedArray;
        //etc...
    }
}

answered Aug 19, 2010 at 18:16

Hans Passant

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3 Comments

user425451 Over a year ago

You're right, but it's annoying to check that everywhere. I was hoping for a shorthand since something like this is used very often in pointer languages. Anyways I'll use this.

Hans Passant Over a year ago

Everywhere? You need to use this kind of code sparingly, pinning isn't healthy for the garbage collector.

user425451 Over a year ago

I'm wrapping a native library and I do indeed use it everywhere. Sure, it might not really be 'everywhere' generally, but while wrapping this library, yes everywhere.

Brian Gideon · Accepted Answer · 2010-08-19 16:56:13Z

1

Good question. Unfortunately I am not that familiar with C++/CLI. I do know that you can do the pinning manually using the GCHandle struct and will work on empty arrays. It is not as elegant as using pin_ptr though.

void SomeFunction(array<float> ^managedArray) 
{ 
 GCHandle handle = GCHandle::Alloc(managedArray, GCHandleType::Pinned);
 try 
 {
  float* ptr = (float*)(void*)handle.AddrOfPinnedObject();
 }
 finally
 {
  handle.Free();
 }
}

answered Aug 19, 2010 at 16:56

Brian Gideon

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2 Comments

user425451 Over a year ago

But then I might as well just check the length. I was hoping for a quick way, like an operator or something.

Brian Gideon Over a year ago

Yeah, checking the length is probably the way to go. I googled quite a bit I could not find an elegant solution either.

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