I'm currently in the code generation phase of building a compiler for a Java-like language. I'm trying to understand how to implement dynamic dispatch for virtual methods.
I get how to build a virtual function table for every class and store a pointer to it in every object. What I don't get is- when generating code for a function call, how do you know what the offset is for that function in the table?
Thanks.
How do you know what anything is in your language? You write it down somewhere while parsing.
What I did in one of my toy languages was to keep a "vtable size" for each class, and when you add a new method to a class, you write down the vtable size as the offset of the method somewhere (i.e. you create a lookup table that maps method name to info about it, like its parameter types and its offset in the vtable), then add to the size to account for the newly-added method.
Of course, this assumes your language actually uses a vtable, like for example C++. If you use messaging in the style of Smalltalk or Objective-C, this table that you build actually gets saved to your compiled product and just used directly. Now a table look-up is slower than just accessing an offset directly, but also has the advantage that a caller does not need to know the type of an object to call a method on it, and you can easily add methods to objects without having to recompile the entire program.
It is mentioned in Scott Meyer's book that part of the overhead caused by using shared pointers is that they need virutal function to destroy the pointed object correctly. My question is why? Is this not supposed to be the responsibility of the class of that pointed object to have a virtual destructor?
Is this not supposed to be the reponsibility of the class of that pointed object to have a virtual destructor?
That would be one possible way to design a shared pointer, but std::shared_ptr allows you to do the following, even if Base does not have a virtual destructor:
std::shared_ptr<Base> p { new Derived{} };
It does this by capturing the correct deleter for the argument when the std::shared_ptr is constructed, then calls that when the reference count hits zero rather than just using delete (of course, you can pass your own custom deleter to use instead). This is commonly referred to as type erasure, and this technique is generally implemented using virtual function calls.
I hooked the CopyItems method of IFileOperation to monitor/intercept file copy in windows.
My problem is how can i retrieve the full file name from IShellItem(last param of CopyItems)
function New_CopyItems(p: Pointer; punkItems: IUnknown;psiDestinationFolder: IShellItem): HResult; stdcall;
The psiDestinationFolder have a method called GetDisplayName that return only the folder name of current file was begin copied!! But i want to get full file name and don't know what i should to do !? There is any other method to help me getting full name ?? Or i have to using another API ....?
Excuse me if my English is bad!
The CopyItems method copies potentially multiple items. So right off the bat you are mistaken looking for a single file name. This is a very complex API and you do need to read the documentation carefully and understand clearly how the function works.
The psiDestinationFolder parameter is an IShellItem that identifies the destination. Use the GetDisplayName method to get the file path.
The other parameter, punkItems is more complex. It is documented like this:
Pointer to the IUnknown of the IShellItemArray, IDataObject, or IEnumShellItems object which represents the group of items to be copied. You can also point to an IPersistIDList object to represent a single item, effectively accomplishing the same function as IFileOperation::CopyItem.
This is telling you that there could be an IShellItemArray, IDataObject, IEnumShellItems or a IPersistIDList object behind the IUnknown interface that you receive. And that there could be multiple items in that single object, each one to be copied to the destination folder. You will need to query punkItems for each possible interface in turn until you find out which of these possibilities you have to deal with. And then handle each one with special code. In order to test this you'll need to write code that calls CopyItems with each of the possible interfaces. You'll find out how to do all of this from the documentation of each of the four interfaces. If you don't already know shell programming and COM well, expect to do so by the time you complete this work.
Finally, I doubt that this is a very good way to detect file copying. Files are copied using many different APIs. And IFileOperation.CopyItems is but one of them. If you only hook IFileOperation.CopyItems then you'll miss a lot of file copy operations.
Is it legal/proper c++0x to leave an object moved for the purpose of move-construction in a state that can only be destroyed? For instance:
class move_constructible {...};
int main()
{
move_constructible x;
move_constructible y(std::move(x));
// From now on, x can only be destroyed. Any other method will result
// in a fatal error.
}
For the record, I'm trying to wrap in a c++ class a c struct with a pointer member which is always supposed to be pointing to some allocated memory area. All the c library API relies on this assumption. But this requirement prevents to write a truly cheap move constructor, since in order for x to remain a valid object after the move it will need its own allocated memory area. I've written the destructor in such a way that it will first check for NULL pointer before calling the corresponding cleanup function from the c API, so that at least the struct can be safely destroyed after the move.
Yes, the language allows this. In fact it was one of the purposes of move semantics. It is however your responsibility to ensure that no other methods get called and/or provide proper diagnostics. Note, usually you can also use at least the assignment operator to "revive" your variable, such as in the classical example of swapping two values.
See also this question
What is a "Handle" when discussing resources in Windows? How do they work?
It's an abstract reference value to a resource, often memory or an open file, or a pipe.
Properly, in Windows, (and generally in computing) a handle is an abstraction which hides a real memory address from the API user, allowing the system to reorganize physical memory transparently to the program. Resolving a handle into a pointer locks the memory, and releasing the handle invalidates the pointer. In this case think of it as an index into a table of pointers... you use the index for the system API calls, and the system can change the pointer in the table at will.
Alternatively a real pointer may be given as the handle when the API writer intends that the user of the API be insulated from the specifics of what the address returned points to; in this case it must be considered that what the handle points to may change at any time (from API version to version or even from call to call of the API that returns the handle) - the handle should therefore be treated as simply an opaque value meaningful only to the API.
I should add that in any modern operating system, even the so-called "real pointers" are still opaque handles into the virtual memory space of the process, which enables the O/S to manage and rearrange memory without invalidating the pointers within the process.
A HANDLE is a context-specific unique identifier. By context-specific, I mean that a handle obtained from one context cannot necessarily be used in any other aribtrary context that also works on HANDLEs.
For example, GetModuleHandle returns a unique identifier to a currently loaded module. The returned handle can be used in other functions that accept module handles. It cannot be given to functions that require other types of handles. For example, you couldn't give a handle returned from GetModuleHandle to HeapDestroy and expect it to do something sensible.
The HANDLE itself is just an integral type. Usually, but not necessarily, it is a pointer to some underlying type or memory location. For example, the HANDLE returned by GetModuleHandle is actually a pointer to the base virtual memory address of the module. But there is no rule stating that handles must be pointers. A handle could also just be a simple integer (which could possibly be used by some Win32 API as an index into an array).
HANDLEs are intentionally opaque representations that provide encapsulation and abstraction from internal Win32 resources. This way, the Win32 APIs could potentially change the underlying type behind a HANDLE, without it impacting user code in any way (at least that's the idea).
Consider these three different internal implementations of a Win32 API that I just made up, and assume that Widget is a struct.
Widget * GetWidget (std::string name)
{
Widget *w;
w = findWidget(name);
return w;
}
void * GetWidget (std::string name)
{
Widget *w;
w = findWidget(name);
return reinterpret_cast<void *>(w);
}
typedef void * HANDLE;
HANDLE GetWidget (std::string name)
{
Widget *w;
w = findWidget(name);
return reinterpret_cast<HANDLE>(w);
}
The first example exposes the internal details about the API: it allows the user code to know that GetWidget returns a pointer to a struct Widget. This has a couple of consequences:
the user code must have access to the header file that defines the Widget struct
the user code could potentially modify internal parts of the returned Widget struct
Both of these consequences may be undesirable.
The second example hides this internal detail from the user code, by returning just void *. The user code doesn't need access to the header that defines the Widget struct.
The third example is exactly the same as the second, but we just call the void * a HANDLE instead. Perhaps this discourages user code from trying to figure out exactly what the void * points to.
Why go through this trouble? Consider this fourth example of a newer version of this same API:
typedef void * HANDLE;
HANDLE GetWidget (std::string name)
{
NewImprovedWidget *w;
w = findImprovedWidget(name);
return reinterpret_cast<HANDLE>(w);
}
Notice that the function's interface is identical to the third example above. This means that user code can continue to use this new version of the API, without any changes, even though the "behind the scenes" implementation has changed to use the NewImprovedWidget struct instead.
The handles in these example are really just a new, presumably friendlier, name for void *, which is exactly what a HANDLE is in the Win32 API (look it up at MSDN). It provides an opaque wall between the user code and the Win32 library's internal representations that increases portability, between versions of Windows, of code that uses the Win32 API.
A HANDLE in Win32 programming is a token that represents a resource that is managed by the Windows kernel. A handle can be to a window, a file, etc.
Handles are simply a way of identifying a particulate resource that you want to work with using the Win32 APIs.
So for instance, if you want to create a Window, and show it on the screen you could do the following:
// Create the window
HWND hwnd = CreateWindow(...);
if (!hwnd)
return; // hwnd not created
// Show the window.
ShowWindow(hwnd, SW_SHOW);
In the above example HWND means "a handle to a window".
If you are used to an object oriented language you can think of a HANDLE as an instance of a class with no methods who's state is only modifiable by other functions. In this case the ShowWindow function modifies the state of the Window HANDLE.
See Handles and Data Types for more information.
A handle is a unique identifier for an object managed by Windows. It's like a pointer, but not a pointer in the sence that it's not an address that could be dereferenced by user code to gain access to some data. Instead a handle is to be passed to a set of functions that can perform actions on the object the handle identifies.
So at the most basic level a HANDLE of any sort is a pointer to a pointer or
#define HANDLE void **
Now as to why you would want to use it
Lets take a setup:
class Object{
int Value;
}
class LargeObj{
char * val;
LargeObj()
{
val = malloc(2048 * 1000);
}
}
void foo(Object bar){
LargeObj lo = new LargeObj();
bar.Value++;
}
void main()
{
Object obj = new Object();
obj.val = 1;
foo(obj);
printf("%d", obj.val);
}
So because obj was passed by value (make a copy and give that to the function) to foo, the printf will print the original value of 1.
Now if we update foo to:
void foo(Object * bar)
{
LargeObj lo = new LargeObj();
bar->val++;
}
There is a chance that the printf will print the updated value of 2. But there is also the possibility that foo will cause some form of memory corruption or exception.
The reason is this while you are now using a pointer to pass obj to the function you are also allocating 2 Megs of memory, this could cause the OS to move the memory around updating the location of obj. Since you have passed the pointer by value, if obj gets moved then the OS updates the pointer but not the copy in the function and potentially causing problems.
A final update to foo of:
void foo(Object **bar){
LargeObj lo = LargeObj();
Object * b = &bar;
b->val++;
}
This will always print the updated value.
See, when the compiler allocates memory for pointers it marks them as immovable, so any re-shuffling of memory caused by the large object being allocated the value passed to the function will point to the correct address to find out the final location in memory to update.
Any particular types of HANDLEs (hWnd, FILE, etc) are domain specific and point to a certain type of structure to protect against memory corruption.
A handle is like a primary key value of a record in a database.
edit 1: well, why the downvote, a primary key uniquely identifies a database record, and a handle in the Windows system uniquely identifies a window, an opened file, etc, That's what I'm saying.
Think of the window in Windows as being a struct that describes it. This struct is an internal part of Windows and you don't need to know the details of it. Instead, Windows provides a typedef for pointer to struct for that struct. That's the "handle" by which you can get hold on the window.,