问题
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.
回答1:
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.
回答2:
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.
回答3:
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.
回答4:
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.
回答5:
That's not C#. It looks like C++. In that case, a void** is a pointer to a void pointer.
来源：https://stackoverflow.com/questions/7205541/what-is-void-in-c