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  • Why does void in C mean not void?

    - by Naftuli Tzvi Kay
    In strongly-typed languages like Java and C#, void (or Void) as a return type for a method seem to mean: This method doesn't return anything. Nothing. No return. You will not receive anything from this method. What's really strange is that in C, void as a return type or even as a method parameter type means: It could really be anything. You'd have to read the source code to find out. Good luck. If it's a pointer, you should really know what you're doing. Consider the following examples in C: void describe(void *thing) { Object *obj = thing; printf("%s.\n", obj->description); } void *move(void *location, Direction direction) { void *next = NULL; // logic! return next; } Obviously, the second method returns a pointer, which by definition could be anything. Since C is older than Java and C#, why did these languages adopt void as meaning "nothing" while C used it as "nothing or anything (when a pointer)"?

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  • How to implement an interface member that returns void in F#

    - by Drew Noakes
    Imagine the following interface in C#: interface IFoo { void Bar(); } How can I implement this in F#? All the examples I've found during 30 minutes of searching online show only examples that have return types which I suppose is more common in a functional style, but something I can't avoid in this instance. Here's what I have so far: type Bar() = interface IFoo with member this.Bar = void Fails with FS0010: Unexpected keyword 'void' in expression.

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  • What does this mean: warning: converting from ‘void (ClassName::*)()’ to ‘void (*)()’

    - by Brendan Long
    I have a member function in a class that has a callback, but the callback isn't strictly neccessary, so it has a default callback, which is empty. It seems to work fine, but I get an annoying warning: warning: converting from ‘void (ClassName::*)()’ to ‘void (*)()’ I'm trying to figure out what it means and how to turn it off (or fix it if I really am doing something wrong). Here's some simple code: class ClassName{ public: void doSomething(void (*callbackFunction)() = (void(*)()) &ClassName::doNothing){ callbackFunction(); } void doNothing(){} }; int main(){ ClassName x; x.doSomething(); return 0; } Note: If I do this (without explicitly casting it as a void(*)()): void doSomething(void (*callbackFunction)() = &ClassName::doNothing) I get this: main.cpp:3: error: default argument for parameter of type ‘void (*)()’ has type ‘void (ClassName::*)()’

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  • How to use void*

    - by Rondogiannis Aristophanes
    I am imlementing a simple merge function and I have got stuck, as the compiler gives me errors that I cannot explain. Here is my merge function: void merge(void *a, int beg, int middle, int end, int (*cmp)(const void*, const void* { std::stack<void*> first; std::stack<void*> second; for(int i = beg; i < middle; i++) { first.push(a+i); } for(int i = middle; i < end; i++) { second.push(a+i); } for(int i = beg; i < end; i++) { if(first.empty()) { void *tmp = second.top(); second.pop(); a+i = tmp; } else if(second.empty()) { void *tmp = first.top(); first.pop(); a+i = tmp; } else if(cmp(first.top(), second.top())) { void *tmp = first.top(); first.pop(); a+i = tmp; } else { void *tmp = second.top(); second.pop(); a+i = tmp; } } } And here is the error: sort.h: In function `void merge(void*, int, int, int, int (*)(const void*, const void*))': sort.h:9: error: pointer of type `void *' used in arithmetic sort.h:12: error: pointer of type `void *' used in arithmetic sort.h:19: error: pointer of type `void *' used in arithmetic sort.h:19: error: non-lvalue in assignment sort.h:23: error: pointer of type `void *' used in arithmetic sort.h:23: error: non-lvalue in assignment sort.h:27: error: pointer of type `void *' used in arithmetic sort.h:27: error: non-lvalue in assignment sort.h:31: error: pointer of type `void *' used in arithmetic sort.h:31: error: non-lvalue in assignment Can anyone help me? TIA.

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  • What do I return if the return type of a method is Void? (Not void!)

    - by DR
    Due to the use of Generics in Java I ended up in having to implement a function having Void as return type: public Void doSomething() { //... } and the compiler demands that I return something. For now I'm just returning null, but I'm wondering if that is good coding practice... I've also tried Void.class, void, Void.TYPE, new Void(), no return at all, but all that doesn't work at all. (For more or less obvious reasons) (See this answer for details) So what am I supposed to return if the return type of a function is Void? What's the general use of the Void class? EDIT: Just to spare you the downvotes: I'm asking about V?oid, not v?oid. The class Void, not the reserved keyword void.

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  • -(void)... does not work/appeal (iOS)

    - by user1012535
    Hi I've got a problem with my -(void) in my Xcode project for iOS. First of all here is the code ViewController.h #import <UIKit/UIKit.h> @interface ViewController : UIViewController { IBOutlet UIWebView *webview; IBOutlet UIActivityIndicatorView *active; UIAlertView *alert_start; UIAlertView *alert_error; } -(IBAction)tele_button:(id)sender; -(IBAction)mail_button:(id)sender; -(IBAction)web_button:(id)sender; -(IBAction)news_button:(id)sender; @end ViewController.m #import "ViewController.h" @implementation ViewController - (void)didReceiveMemoryWarning { [super didReceiveMemoryWarning]; // Release any cached data, images, etc that aren't in use. } #pragma mark - View lifecycle - (void)viewDidLoad { [super viewDidLoad]; //Stop Bounce for WebView for (id subview in webview.subviews) if ([[subview class] isSubclassOfClass: [UIScrollView class]]) ((UIScrollView *)subview).bounces = NO; //First Start Alert [alert_start show]; NSLog(@"first alert"); NSString *start_alert = [[NSUserDefaults standardUserDefaults] objectForKey:@"alert_start"]; if(start_alert == nil) { [[NSUserDefaults standardUserDefaults] setValue:@"1" forKey:@"alert_start"]; [[NSUserDefaults standardUserDefaults] synchronize]; UIAlertView *alert_start = [[UIAlertView alloc] initWithTitle:@"iOptibelt" message:@"On some points this application need a internet connection." delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil]; [alert_start show]; [alert_start release]; } // Do any additional setup after loading the view, typically from a nib. [webview loadRequest:[NSURLRequest requestWithURL:[NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"home-de" ofType:@"html"]isDirectory:NO]]]; NSLog(@"webview fertig"); } -(void)webViewDidStartLoad:(UIWebView *) webview { [UIApplication sharedApplication].networkActivityIndicatorVisible = YES; [active startAnimating]; NSLog(@"lade"); } -(void)webViewDidFinishLoad:(UIWebView *) webview { [UIApplication sharedApplication].networkActivityIndicatorVisible = NO; [active stopAnimating]; NSLog(@"fertig"); } -(void)webView: (UIWebView *) webview didFailLoadWithError:(NSError *)error{ NSLog(@"lade error"); UIAlertView *alert_error = [[UIAlertView alloc] initWithTitle:@"Error" message:@"Can't connect. Please check your internet Connection" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil]; [alert_error show]; [alert_error release]; }; - (IBAction)tele_button:(id)sender{ NSLog(@"it's connected!"); //Local HTML Call Button NSURLRequest *theRequest = [NSURLRequest requestWithURL:[NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"phone" ofType:@"html"]isDirectory:NO]]; [webview loadRequest:theRequest]; } - (IBAction)mail_button:(id)sender{ NSLog(@"it's connected!"); //Mail App Mail Button [[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"mailto://[email protected]"]]; } - (IBAction)web_button:(id)sender{ NSLog(@"it's connected!"); //Local HTML Button NSURLRequest *theRequest = [NSURLRequest requestWithURL:[NSURL URLWithString: @"http://optibelt.com"]]; [webview loadRequest:theRequest]; } - (IBAction)news_button:(id)sender{ NSLog(@"it's connected!"); //local Home Button NSURLRequest *theRequest = [NSURLRequest requestWithURL:[NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"home-de" ofType:@"html"]isDirectory:NO]]; [webview loadRequest:theRequest]; } - (void)viewDidUnload { [super viewDidUnload]; // Release any retained subviews of the main view. // e.g. self.myOutlet = nil; } - (void)viewWillAppear:(BOOL)animated { [super viewWillAppear:animated]; } - (void)viewDidAppear:(BOOL)animated { [super viewDidAppear:animated]; } - (void)viewWillDisappear:(BOOL)animated { [super viewWillDisappear:animated]; } - (void)viewDidDisappear:(BOOL)animated { [super viewDidDisappear:animated]; } - (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation { // Return YES for supported orientations return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown); } @end At last my 3. -(void) does not work and i ve no more idea what could be the problem.... -(void)webViewDidStartLoad:(UIWebView *) webview { [UIApplication sharedApplication].networkActivityIndicatorVisible = YES; [active startAnimating]; NSLog(@"lade"); } -(void)webViewDidFinishLoad:(UIWebView *) webview { [UIApplication sharedApplication].networkActivityIndicatorVisible = NO; [active stopAnimating]; NSLog(@"fertig"); } -(void)webView: (UIWebView *) webview didFailLoadWithError:(NSError *)error{ NSLog(@"lade error"); UIAlertView *alert_error = [[UIAlertView alloc] initWithTitle:@"Error" message:@"Can't connect. Please check your internet Connection" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil]; [alert_error show]; [alert_error release];

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  • Void in main and keeping Dos program from dying

    - by TimothyTech
    Alright, so im using bloodshed complier, first thing i should note is im using a 2001 edition C++ for the aboslute beginner, was there any changes to C++ since 2001 that would effect the validity of this book? i ask because i know php has php5 and now php6 but i havent seen any C++3. now for the reason i asked that, in this code it uses, void main (void) why would i want an arguement to be void? what does this do. But when i run the " void main (void) in my compiler it says that main must have a "int" before it. so i cant have "void" main (void); also, once the code runs through, as long as there isnt a "cin" the program closes. is this normal behavior, is there a better way to stop this besides making a cin at the end of my program.

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  • Void pointers in C

    - by avd
    I have written this qsort: void qsort(void *a[],int low,int high, int (*compare)(void*,void*)); When I call this on char *strarr[5]; It says invalid conversion from char** to void**. Why this is wrong? This is the code:http://codepad.org/Flfm2zAE

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  • How to get rid of void-pointers.

    - by Patrick
    I inherited a big application that was originally written in C (but in the mean time a lot of C++ was also added to it). Because of historical reasons, the application contains a lot of void-pointers. Before you start to choke, let me explain why this was done. The application contains many different data structures, but they are stored in 'generic' containers. Nowadays I would use templated STL containers for it, or I would give all data structures a common base class, so that the container can store pointers to the base class, but in the [good?] old C days, the only solution was to cast the struct-pointer to a void-pointer. Additionally, there is a lot of code that works on these void-pointers, and uses very strange C constructions to emulate polymorphism in C. I am now reworking the application, and trying to get rid of the void-pointers. Adding a common base-class to all the data structures isn't that hard (few days of work), but the problem is that the code is full of constructions like shown below. This is an example of how data is stored: void storeData (int datatype, void *data); // function prototype ... Customer *myCustomer = ...; storeData (TYPE_CUSTOMER, myCustomer); This is an example of how data is fetched again: Customer *myCustomer = (Customer *) fetchData (int datatype, char *key); I actually want to replace all the void-pointers with some smart-pointer (reference-counted), but I can't find a trick to automate (or at least) help me to get rid of all the casts to and from void-pointers. Any tips on how to find, replace, or interact in any possible way with these conversions?

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  • Call a void* as a function without declaring a function pointer

    - by ToxIk
    I've searched but couldn't find any results (my terminology may be off) so forgive me if this has been asked before. I was wondering if there is an easy way to call a void* as a function in C without first declaring a function pointer and then assigning the function pointer the address; ie. assuming the function to be called is type void(void) void *ptr; ptr = <some address>; ((void*())ptr)(); /* call ptr as function here */ with the above code, I get error C2066: cast to function type is illegal in VC2008 If this is possible, how would the syntax differ for functions with return types and multiple parameters?

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  • Void pointer values comparing C++

    - by user2962977
    My actual question is it really possible to compare values contained in two void pointers, when you actually know that these values are the same type? For example int. void compVoids(void *firstVal, void *secondVal){ if (firstVal < secondVal){ cout << "This will not make any sense as this will compare addresses, not values" << endl; } } Actually I need to compare two void pointer values, while outside the function it is known that the type is int. I do not want to use comparison of int inside the function. So this will not work for me as well: if (*(int*)firstVal > *(int*)secondVal) Any suggestions? Thank you very much for help!

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  • Problem using void pointer as a function argument

    - by Nazgulled
    Hi, I can't understand this result... The code: void foo(void * key, size_t key_sz) { HashItem *item = malloc(sizeof(HashItem)); printf("[%d]\n", (int)key); ... item->key = malloc(key_sz); memcpy(item->key, key, key_sz); } void bar(int num) { foo(&num, sizeof(int)); } And I do this call: bar(900011009); But the printf() output is: [-1074593956] I really need key to be a void pointer, how can I fix this?

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  • convert pointer to pointer to void pointer

    - by FihopZz
    When I'm learning to use qsort to sort an array of string, there is a question puzzled me. For example, to sort the following s char *s[] = { "Amit", "Garima", "Gaurav", "Vaibhav" }; To use the qsort, you must provide a comparison function like the following function cstring_cmp I guess in the qsort function, the type of parameter to be passed to the function cstring_cmp is char**. How to convert a char** to a void*? Why can we convert a char** to a void*? int cstring_cmp(const void *a, const void *b) { const char **ia = (const char **)a; const char **ib = (const char **)b; return -strcasecmp(*ia, *ib); /* return the negative of the normal comparison */ }

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  • Need for prefixing a function with (void)...

    - by puffadder
    Hi All, I recently came across a rather unusual coding convention wherein the call for a function returning "void" is prefixed with (void). e.g. (void) MyFunction(); Is it any different from the function call like: MyFunction(); Has it got any advantage or is it yet another needless but there coding convention of some sort?

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  • C Class Instance from Void Pointer using Ctypes

    - by g.d.d.c
    I've got a C DLL that exposes a handful of methods that return void pointers to a Class like so: void *GetLicense() { static AppLicenseImpl ipds_; return (void *) &ipds_; } In C++, after loading the DLL, I'd do this to work with it: typedef void *(* FPGetLicense)(); GetLicense_ = (FPGetLicense)GetAddress("GetLicense"); license_ = (AppLicense *) GetLicense_(); license_->GetApplicationStatus(); // Load data so that other calls don't fail I can't figure out how to parallel that in Python. This gets me the pointer: d = ctypes.cdll.LoadLibrary('license.dll') d.GetLicense.restype = ctypes.c_void_p p = d.GetLicense() # returns ptr loc, something like 8791433660848L But I obviously can't call p.GetApplicationStatus() in Python. Does anyone have a suggestion on how I'd instantiate that Class the rest of the way in Python so that I can call GetApplicationStatus()?

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  • casting unused return values to void

    - by markh44
    int fn(); void whatever() { (void) fn(); } Is there any reason for casting an unused return value to void, or am I right in thinking it's a complete waste of time? Follow up: Well that seems pretty comprehensive. I suppose it's better than commenting an unused return value since self documenting code is better than comments. Personally, I'll turn these warnings off since it's unnecessary noise. I'll eat my words if a bug escapes because of it...

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  • OpenGl glutIdleFunc(void (*func)(void))

    - by PHENOMERICAN
    I'm trying to design very simple animation in OpenGL such as rotating and translating objects. In the red book, I found that using GLUT's glutIdleFunc() is okay for a simple animation. How many times does glutIdleFunc(...) call the function in one second? Thank you.

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  • Scheme: Mysterious void in pattern match.

    - by Schemer
    Hi. I am writing a function called annotate that uses match-lambda -- often with recursive calls to annotate. Here is one of the pattern matches: (`(lambda (,<param1> . ,<params>) ,<stmts>) `(CLOSURE ENV (,<param1> . ,<params>) (lambda (ENV) ,(map annotate (map (lambda (x) (append `(,<param1> . ,<params>) (list x))) `(,<stmts>)))))) However, when this pattern is matched this is what returns: '(CLOSURE ENV (x) (lambda (ENV) ((CLOSURE ENV (x y) (lambda (ENV) ((+ x y)))))) #<void>) Specifically I can't figure out where "void" is coming from. In fact, if I include the line: ,(displayln (map annotate (map (lambda (x) (append `(,<param1> . ,<params>) (list x))) `(,<stmts>)))) it prints: ((CLOSURE ENV (x y) (lambda (ENV) ((+ x y))))) notably without "void". If someone could tell me what the problem is it would be greatly appreciated. Thanks.

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  • C++: casting to void* and back

    - by MInner
    * ---Edit - now the whole sourse* When I debug it on the end, "get" and "value" have different values! Probably, I convert to void* and back to User the wrong way? #include <db_cxx.h> #include <stdio.h> struct User{ User(){} int name; int town; User(int a){}; inline int get_index(int a){ return town; } //for another stuff }; int main(){ try { DbEnv* env = new DbEnv(NULL); env->open("./", DB_CREATE | DB_INIT_MPOOL | DB_THREAD | DB_INIT_LOCK | DB_INIT_TXN | DB_RECOVER | DB_INIT_LOG, 0); Db* datab = new Db(env, 0); datab->open(NULL, "db.dbf", NULL, DB_BTREE, DB_CREATE | DB_AUTO_COMMIT, 0); Dbt key, value, get; char a[10] = "bbaaccd"; User u; u.name = 1; u.town = 34; key.set_data(a); key.set_size(strlen(a) + 1 ); value.set_data((void*)&u); value.set_size(sizeof(u)); get.set_flags(DB_DBT_MALLOC); DbTxn* txn; env->txn_begin(NULL, &txn, 0); datab->put(txn, &key, &value, 0); datab->get(txn, &key, &get, 0); txn->commit(0); User g; g = *((User*)&get); printf("%d", g.town); getchar(); return 0; }catch (DbException &e){ printf("%s", e.what()); getchar(); } solution create a kind of "serializator" what would convert all POD's into void* and then will unite these pieces PS Or I'd rewrite User into POD type and everything will be all right, I hope. Add It's strange, but... I cast a defenetly non-pod object to void* and back (it has std::string inside) and it's all right (without sending it to the db and back). How could it be? And after I cast and send 'trough' db defenetly pod object (no extra methods, all members are pod, it's a simple struct {int a; int b; ...}) I get back dirted one. What's wrong with my approach? Add about week after first 'add' Damn... I've compiled it ones, just for have a look at which kind of dirt it returnes, and oh! it's okay!... I can't ! ... AAh!.. Lord... A reasonable question (in 99.999 percent of situations right answer is 'my', but... here...) - whos is this fault? My or VSs?

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  • Issue with dynamic array Queue data structure with void pointer

    - by Nazgulled
    Hi, Maybe there's no way to solve this the way I'd like it but I don't know everything so I better ask... I've implemented a simple Queue with a dynamic array so the user can initialize with whatever number of items it wants. I'm also trying to use a void pointer as to allow any data type, but that's the problem. Here's my code: typedef void * QueueValue; typedef struct sQueueItem { QueueValue value; } QueueItem; typedef struct sQueue { QueueItem *items; int first; int last; int size; int count; } Queue; void queueInitialize(Queue **queue, size_t size) { *queue = xmalloc(sizeof(Queue)); QueueItem *items = xmalloc(sizeof(QueueItem) * size); (*queue)->items = items; (*queue)->first = 0; (*queue)->last = 0; (*queue)->size = size; (*queue)->count = 0; } Bool queuePush(Queue * const queue, QueueValue value, size_t val_sz) { if(isNull(queue) || isFull(queue)) return FALSE; queue->items[queue->last].value = xmalloc(val_sz); memcpy(queue->items[queue->last].value, value, val_sz); queue->last = (queue->last+1) % queue->size; queue->count += 1; return TRUE; } Bool queuePop(Queue * const queue, QueueValue *value) { if(isEmpty(queue)) return FALSE; *value = queue->items[queue->first].value; free(queue->items[queue->first].value); queue->first = (queue->first+1) % queue->size; queue->count -= 1; return TRUE; } The problem lies on the queuePop function. When I call it, I lose the value because I free it right away. I can't seem to solve this dilemma. I want my library to be generic and modular. The user should not care about allocating and freeing memory, that's the library's job. How can the user still get the value from queuePop and let the library handle all memory allocs/frees?

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  • casting void* to float* creates only zeros

    - by Paperflyer
    I am reading an audio file using CoreAudio (Extended Audio File Read Services). The audio data gets converted to 4-byte float and handed to me as a void* buffer. It can be played with Audio Queue Services, so its content is correct. Next, I want to draw a waveform and thus need access to the actual samples. So, I cast void* audioData to float*: Float32 *floatData = (Float32 *)audioData; When accessing this data however, I only get 0.0 regardless of the index. Float32 value = floatData[index]; // Is always zero for any index Am I doing something wrong with the cast?

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  • Sending items in a LINQ sequence to a method that returns void

    - by generalt
    Hello all. Often while I'm dealing with LINQ sequences, I want to send each item to a method returning void, avoiding a foreach loop. However, I haven't found an elegant way to do this. Today, I wrote the following code: private StreamWriter _sw; private void streamToFile(List<ErrorEntry> errors) { if (_sw == null) { _sw = new StreamWriter(Path.Combine (Path.GetDirectoryName(_targetDatabasePath), "errors.txt")); } Func<ErrorEntry, bool> writeSelector = (e) => { _sw.WriteLine(getTabDelimititedLine(e)); return true; }; errors.Select(writeSelector); _sw.Flush(); } As you can see, I write a lambda function that just returns true, and I realize that the Select method will return a sequence of booleans- I'll just ignore that sequence. However, this seems a little bit noobish and jank. Is there any elegant way to do this? Or am I just misapplying LINQ? Thanks.

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