CODEWITHSUNDEEP
c#cpointersvoid
user12345613
user12345613

Reputation: 833

What is void** in C#?

I'm looking through the source of a C# program that uses a library written in C. I came across this line and was unsure what it was:

cvbimUNSAFE.GetImageVPA ((cvbim.IMG)cvImg.Image, 0, (void**)&lpImageBits, &pVPAT);

What is an object of type void **? I did some Google searches and could only find information about void*, which is a pointer to a sort of catch all top level type, if I understood correctly.

Upvotes: 9

Views: 9593

Answers (6)

V. S.
V. S.

Reputation: 1244

In C# void** is a pointer to void* pointer. Here is a simple example:

   public unsafe static void Main(string[] args)
   {
        int x = 1;
        void* v = &x;

        //dereferencing v, it prints 1
        Console.WriteLine(*(int*)v);

        void** vtp = &v;
        //dereferencing vtp, it prints 1
        Console.WriteLine(*(int*)*vtp);

        Console.ReadLine();
    }

Upvotes: 0

jason
jason

Reputation: 241641

It's a pointer to a pointer to something not specified. Basically, just think of it as a memory pointer to a raw memory pointer.

So, int** is a pointer to a pointer to an int, but void** is a pointer to a pointer, but it's not specified what that pointer is pointing at.

I did some google searches and could only find information about void*, which is a pointer to a sort of catch all top level type, if I understood correctly.

Not quite. void* is a pointer to something, it's just not specified what that something is and should just be thought of as a pointer to a raw hunk of memory that you have to apply some structure to. For example, malloc returns a void* because it's returning a pointer to a raw hunk of memory.

Upvotes: 18

James Johnson
James Johnson

Reputation: 46047

It's a void pointer. See this article for details:

http://msdn.microsoft.com/en-us/library/y31yhkeb%28VS.80%29.aspx

And you can take a look at this SO question for details on how to implement it in C#:

How to declare a void pointer in C#

On a side note, that method should be marked as unsafe if it's not.

Upvotes: 2

Simone
Simone

Reputation: 11797

In C, it represents a pointer to a void* object. In other word, when you dereference it you get a void*.

I guess this is used because lpImageBits will be modified inside the function you are invoking.

Upvotes: 1

Vinicius Kamakura
Vinicius Kamakura

Reputation: 7778

In this case, I am guessing this library will allocate the amount of memory necessary to hold the image so it needs a double indirection so it can change the address lpImageBits points to.

Upvotes: 1

Jez
Jez

Reputation: 29993

That's not C#. It looks like C++. In that case, a void** is a pointer to a void pointer.

Upvotes: -3

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