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• Converting C# void* to byte[]

  - by Yurik

  In C#, I need to write T[] to a stream, ideally without any additional buffers. I have a dynamic code that converts T[] (where T is a no-objects struct) to a void* and fixes it in memory, and that works great. When the stream was a file, I could use native Windows API to pass the void * directly, but now I need to write to a generic Stream object that takes byte[]. Can anyone suggest a hack way to create a dummy array object which does not actually have any heap allocations, but rather points to an already existing (and fixed) heap location. This is the pseudo-code that I need: void Write(Stream stream, T[] buffer) { fixed( void* ptr = &buffer ) // done with dynamic code generation { int typeSize = sizeof(T); // done as well byte[] dummy = (byte[]) ptr; // <-- how do I create this fake array? stream.Write( dummy, 0, buffer.Length*typeSize ); } }

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• How to gain Access to member variables of a class using void pointer but Not Object

  - by mahesh

  Hi, I am trying to access member variables of a class without using object. please let me know how to go about. class TestMem { int a; int b; public: TestMem(){} void TestMem1() { a = 10; b = 20; } }; void (TestMem::*pMem)(); int main(int argc, char* argv[]) { TestMem o1; pMem = &(TestMem::TestMem1); void *p = (void*)&pMem; // How to access a & b member variables using variable p getch(); return 0; }

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• function pointers callbacks C

  - by robUK

  Hello, I have started to review callbacks. I found this link: http://stackoverflow.com/questions/142789/what-is-a-callback-in-c-and-how-are-they-implemented which has a good example of callback which is very similar to what we use at work. However, I have tried to get it to work, but I have many errors. #include <stdio.h> /* Is the actual function pointer? */ typedef void (*event_cb_t)(const struct event *evt, void *user_data); struct event_cb { event_cb_t cb; void *data; }; int event_cb_register(event_ct_t cb, void *user_data); static void my_event_cb(const struct event *evt, void *data) { /* do some stuff */ } int main(void) { event_cb_register(my_event_cb, &my_custom_data); struct event_cb *callback; callback->cb(event, callback->data); return 0; } I know that callback use function pointers to store an address of a function. But there is a few things that I find I don't understand. That is what is meet by "registering the callback" and "event dispatcher"? Many thanks for any advice,

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• C++: why a self pointer of a struct automatically changes to void*

  - by Stone

  struct ptr{ int node; ptr *next; ptr(){} ptr(int _node, ptr *_next){ node=_node; next=_next; } }; struct list_t{ ptr *sht; int size; void push(int node){ size++; sht=new ptr(node,sht); } }shthead[100001], comp[200001], tree[200001]; The struct ptr is a smart pointer, be used as a linked list. But when I debug the code in gdb, I found that the ptr*'s were all converted to void*. GDB output: (gdb) pt ptr type = struct ptr { int node; void *next; public: ptr(void); ptr(int, void *); } However, I can still see the data of the struct if I covert them back to ptr* in gdb. What's the reason for this please?

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• Event Dispatching, void pointer and alternatives

  - by PeeS

  i have my event dispatching / handling functionality working fine, but there is one issue that i need to resolve. Long story short, here are the details. // The event structure struct tEventMessage { // Type of the event int Type; // (void*) Allows those to be casted into per-Type objects void *pArgument1; void *pArgument2; }; I am sending events from different modules in my engine by using the above structure, which requires a pointer to an argument. All messages are queued, and then dispatched on the next ::Tick(). It works fine, untill i try to send something that doesn't exist in next ::Tick, for example: When a mouse click is being handled, it calculates the click coordinates in world space. This is being sent with a pointer to a vector representing that position, but after my program quits that method, this pointer gets invalid obviously, cause local CVector3 is destructed: CVector2 vScreenSpacePosition = vAt; CVector3 v3DPositionA = CVector3(0,0,0); CVector3 v3DPositionB = CVector3(0,0,0); // Screen space to World space calculation for depth zNear v3DPositionA = CMath::UnProject(vScreenSpacePosition, m_vScreenSize, m_Level.GetCurrentCamera()->getViewMatrix(), m_Level.GetCurrentCamera()->getProjectionMatrix(), -1.0 ); // Screen space to World space calculation for depth zFar v3DPositionB = CMath::UnProject(vScreenSpacePosition, m_vScreenSize, m_Level.GetCurrentCamera()->getViewMatrix(), m_Level.GetCurrentCamera()->getProjectionMatrix(), 1.0); // Send zFar position and ScreenSpace position to the handlers // Obviously both vectors won't be valid after this method quits.. CEventDispatcher::Get()->SendEvent(CIEventHandler::EVENT_SYSTEM_FINGER_DOWN, static_cast<void*>(&v3DPositionB), static_cast<void*>(&vScreenSpacePosition)); What i want to ask is, if there is any chance i could make my tEventMessage more 'template', so i can handle sending objects like in the above situation + use what is already implemented? Can't figure it out at the moment.. What else can be done here to allow me to pass some locally available data ? Please can somebody shed a bit of light on this please?

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• Which is preferred: foo(void) or foo() in C++

  - by George2

  I have seen two styles of defining conversion operator overload in C++, operator int* (void) const operator int*() const Question 1. I think the two styles (whether add void or not) have the same function, correct? Question 2. Any preference which is better?

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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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• Understanding C++ pointers (when they point to a pointer)

  - by Stephano

  I think I understand references and pointers pretty well. Here is what I (think I) know: int i = 5; //i is a primitive type, the value is 5, i do not know the address. int *ptr; //a pointer to an int. i have no way if knowing the value yet. ptr = &i; //now i have an address for the value of i (called ptr) *ptr = 10; //go get the value stored at ptr and change it to 10 Please feel free to comment or correct these statements. Now I'm trying to make the jump to arrays of pointers. Here is what I do not know: char **char_ptrs = new char *[50]; Node **node_ptrs = new Node *[50]; My understanding is that I have 2 arrays of pointers, one set of pointers to chars and one to nodes. So if I wanted to set the values, I would do something like this: char_ptrs[0] = new char[20]; node_ptrs[0] = new Node; Now I have a pointer, in the 0 position of my array, in each respective array. Again, feel free to comment here if I'm confused. So, what does the ** operator do? Likewise, what is putting a single * next to the instantiation doing (*[50])? (what is that called exactly, instantiation?)

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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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• Copying pointers in C++

  - by tunnuz

  Hello, I have a class A containing two pointers to objects of another class B. I want to initialize one pointer or the other depending on which one is passed to init(), which also takes other parameters. My situation is thus the following: class A { public: A(); init(int parameter, int otherParameter, B* toBeInitialized); protected: B* myB; B* myOtherB; }; Now my point is that I want to call init() as: init(640, 480, this->myB); or init(640, 480, this->myOtherB); Now, my init is implemented as: void init( int parameter, int otherParameter, B* toBeInitialized ) { toBeInitialized = someConstructorFunction(parameter, otherParameter); } The problem is that the two pointers are not initialized, I suspect that toBeInitialized is overwritten, but the original parameter is not modified. I am doing something wrong? Should I use references to pointers? Thank you Tommaso

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• How to use shared_ptr for COM interface pointers

  - by Seefer

  I've been reading about various usage advice relating to the new c++ standard smart pointers unique_ptr, shared_ptr and weak_ptr and generally 'grok' what they are about when I'm writing my own code that declares and consumes them. However, all the discussions I've read seem restricted to this simple usage situation where the programmer is using smart in his/her own code, with no real discussion on techniques when having to work with libraries that expect raw pointers or other types of 'smart pointers' such as COM interface pointers. Specifically I'm learning my way through C++ by attempting to get a standard Win32 real-time game loop up and running that uses Direct2D & DirectWrite to render text to the display showing frames per second. My first task with Direct2D is in creating a Direct2D Factory object with the following code from the Direct2D examples on MSDN: ID2D1Factory* pD2DFactory = nullptr; HRESULT hr = D2D1CreateFactory(D2D1_FACTORY_TYPE_SINGLE_THREADED, &pD2DFactory); pD2DFactory is obviously an 'out' parameter and it's here where I become uncertain how to make use of smart pointers in this context, if indeed it's possible. My inexperienced C++ mind tells me I have two problems: With pD2DFactory being a COM interface pointer type, how would smart_ptr work with the Add() / Release() member functions for a COM object instance? Are smart pointers able to be passed to functions in situations where the function is using an 'out' pointer parameter technique? I did experiment with the alternative of using _com_ptr_t in the comip.h header file to help with pointer lifetime management and declared the pD2DFactory pointer with the following code: _com_ptr_t<_com_IIID<pD2DFactory, &__uuidof(pD2DFactory)>> pD2DFactory = nullptr; and it appears to work so far but, as you can see, the syntax is cumbersome :) So, I was wondering if any C++ gurus here could confirm whether smart pointers are able to help in cases like this and provide examples of usage, or point me to more in-depth discussions of smart pointer usage when needing to work with other code libraries that know nothing of them. Or is it simply a case of my trying to use the wrong tool for the job? :)

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• Automatically generate table of function pointers in C.

  - by jeremytrimble

  I'm looking for a way to automatically (as part of the compilation/build process) generate a "table" of function pointers in C. Specifically, I want to generate an array of structures something like: typedef struct { void (*p_func)(void); char * funcName; } funcRecord; /* Automatically generate the lines below: */ extern void func1(void); extern void func2(void); /* ... */ funcRecord funcTable[] = { { .p_func = &func1, .funcName = "func1" }, { .p_func = &func2, .funcName = "func2" } /* ... */ }; /* End automatically-generated code. */ ...where func1 and func2 are defined in other source files. So, given a set of source files, each of which which contain a single function that takes no arguments and returns void, how would one automatically (as part of the build process) generate an array like the one above that contains each of the functions from the files? I'd like to be able to add new files and have them automatically inserted into the table when I re-compile. I realize that this probably isn't achievable using the C language or preprocessor alone, so consider any common *nix-style tools fair game (e.g. make, perl, shell scripts (if you have to)). But Why? You're probably wondering why anyone would want to do this. I'm creating a small test framework for a library of common mathematical routines. Under this framework, there will be many small "test cases," each of which has only a few lines of code that will exercise each math function. I'd like each test case to live in its own source file as a short function. All of the test cases will get built into a single executable, and the test case(s) to be run can be specified on the command line when invoking the executable. The main() function will search through the table and, if it finds a match, jump to the test case function. Automating the process of building up the "catalog" of test cases ensures that test cases don't get left out (for instance, because someone forgets to add it to the table) and makes it very simple for maintainers to add new test cases in the future (just create a new source file in the correct directory, for instance). Hopefully someone out there has done something like this before. Thanks, StackOverflow community!

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• boost::serialization of mutual pointers

  - by KneLL

  First, please take a look at these code: class Key; class Door; class Key { public: int id; Door *pDoor; Key() : id(0), pDoor(NULL) {} private: friend class boost::serialization::access; template <typename A> void serialize(A &ar, const unsigned int ver) { ar & BOOST_SERIALIZATION_NVP(id) & BOOST_SERIALIZATION_NVP(pDoor); } }; class Door { public: int id; Key *pKey; Door() : id(0), pKey(NULL) {} private: friend class boost::serialization::access; template <typename A> void serialize(A &ar, const unsigned int ver) { ar & BOOST_SERIALIZATION_NVP(id) & BOOST_SERIALIZATION_NVP(pKey); } }; BOOST_CLASS_TRACKING(Key, track_selectively); BOOST_CLASS_TRACKING(Door, track_selectively); int main() { Key k1, k_in; Door d1, d_in; k1.id = 1; d1.id = 2; k1.pDoor = &d1; d1.pKey = &k1; // Save data { wofstream f1("test.xml"); boost::archive::xml_woarchive ar1(f1); // !!!!! (1) const Key *pK = &k1; const Door *pD = &d1; ar1 << BOOST_SERIALIZATION_NVP(pK) << BOOST_SERIALIZATION_NVP(pD); } // Load data { wifstream i1("test.xml"); boost::archive::xml_wiarchive ar1(i1); // !!!!! (2) A *pK = &k_in; B *pD = &d_in; // (2.1) //ar1 >> BOOST_SERIALIZATION_NVP(k_in) >> BOOST_SERIALIZATION_NVP(d_in); // (2.2) ar1 >> BOOST_SERIALIZATION_NVP(pK) >> BOOST_SERIALIZATION_NVP(pD); } } The first (1) is a simple question - is it possible to pass objects to archive without pointers? If simply pass objects 'as is' that boost throws exception about duplicated pointers. But I'm confused of creating pointers to save objects. The second (2) is a real trouble. If comment out string after (2.1) then boost will corectly load a first Key object (and init internal Door pointer pDoor), but will not init a second Door (d_in) object. After this I have an inited *k_in* object with valid pointer to Door and empty *d_in* object. If use string (2.2) then boost will create two Key and Door objects somewhere in memory and save addresses in pointers. But I want to have two objects *k_in* and *d_in*. So, if I copy a values of memory objects to local variables then I store only addresses, for example, I can write code after (2.2): d_in.id = pD->id; d_in.pKey = pD->pKey; But in this case I store only a pointer and memory object remains in memory and I cannot delete it, because *d_in.pKey* will be unvalid. And I cannot perform a deep copy with operator=(), because if I write code like this: Key &operator==(const Key &k) { if (this != &k) { id = k.id; // call to Door::operator=() that calls *pKey = *d.pKey and so on *pDoor = *k.pDoor; } return *this; } then I will get a something like recursion of operator=()s of Key and Door. How to implement proper serialization of such pointers?

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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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• C++ arithmetic with pointers

  - by user69514

  I am trying to add the following: I have an array of double pointers call A. I have another array of double pointers call it B, and I have an unsigned int call it C. So I want to do: A[i] = B[i] - C; how do I do it? I did: A[i] = &B[i] - C; I don't think I am doing this correctly.

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• Getting my head around the practical applications of strong and weak pointers in Objective-C

  - by Chris Wilson

  I've just read the accepted excellent answer to this question that clarifies the conceptual differences between strong and weak pointers in Objective-C, and I'm still trying to understand the practical differences. I come from a C++ background where these concepts don't exist, and I'm having trouble figuring out where I would use one vs the other. Could someone please provide a practical example, using Objective-C code, that illustrates the different uses of strong and weak pointers?

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• Why no switch on pointers?

  - by meeselet

  For instance: #include <stdio.h> void why_cant_we_switch_him(void *ptr) { switch (ptr) { case NULL: printf("NULL!\n"); break; default: printf("%p!\n", ptr); break; } } int main(void) { void *foo = "toast"; why_cant_we_switch_him(foo); return 0; } gcc test.c -o test test.c: In function 'why_cant_we_switch_him': test.c:5: error: switch quantity not an integer test.c:6: error: pointers are not permitted as case values Just curious. Is this a technical limitation? EDIT People seem to think there is only one constant pointer expression. Is that is really true, though? For instance, here is a common paradigm in Objective-C (it is really only C aside from NSString, id and nil, which are merely a pointers, so it is still relevant — I just wanted to point out that there is, in fact, a common use for it, despite this being only a technical question): #include <stdio.h> #include <Foundation/Foundation.h> static NSString * const kMyConstantObject = @"Foo"; void why_cant_we_switch_him(id ptr) { switch (ptr) { case kMyConstantObject: // (Note that we are comparing pointers, not string values.) printf("We found him!\n"); break; case nil: printf("He appears to be nil (or NULL, whichever you prefer).\n"); break; default: printf("%p!\n", ptr); break; } } int main(void) { NSString *foo = @"toast"; why_cant_we_switch_him(foo); foo = kMyConstantObject; why_cant_we_switch_him(foo); return 0; } gcc test.c -o test -framework Foundation test.c: In function 'why_cant_we_switch_him': test.c:5: error: switch quantity not an integer test.c:6: error: pointers are not permitted as case values It appears that the reason is that switch only allows integral values (as the compiler warning said). So I suppose a better question would be to ask why this is the case? (though it is probably too late now.)

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• What are function pointers good for ?

  - by gramm

  Hi, I have trouble seing the utility of the function pointers. I guess it may be useful in some case (it exists, after all), but I can't think of a case where it's better or unavoidable to use a function pointer. Could you give some example of good use of function pointers (in C or C++)? Many thanks :)

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• What is the point of function pointers?

  - by gramm

  Hi, I have trouble seing the utility of the function pointers. I guess it may be useful in some cases (they exist, after all), but I can't think of a case where it's better or unavoidable to use a function pointer. Could you give some example of good use of function pointers (in C or C++)? Many thanks :)

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• int main() v/s int main (void)

  - by yCalleecharan

  Hi, In C, what's the difference between: int main() { return 0; } and int main(void) { return 0; } Thanks a lot... Update: If main is not called recursively, then main() is the right way to go?

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• What does "javascript:void(0)" mean?

  - by Shore

  <a href="javascript:void(0)" id="loginlink">login</a> I've seen such hrefs many times, but I don't know what exactly that means.

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• Vector of pointers to base class, odd behaviour calling virtual functions

  - by Ink-Jet

  I have the following code #include <iostream> #include <vector> class Entity { public: virtual void func() = 0; }; class Monster : public Entity { public: void func(); }; void Monster::func() { std::cout << "I AM A MONSTER" << std::endl; } class Buddha : public Entity { public: void func(); }; void Buddha::func() { std::cout << "OHMM" << std::endl; } int main() { const int num = 5; // How many of each to make std::vector<Entity*> t; for(int i = 0; i < num; i++) { Monster m; Entity * e; e = &m; t.push_back(e); } for(int i = 0; i < num; i++) { Buddha b; Entity * e; e = &b; t.push_back(e); } for(int i = 0; i < t.size(); i++) { t[i]->func(); } return 0; } However, when I run it, instead of each class printing out its own message, they all print the "Buddha" message. I want each object to print its own message: Monsters print the monster message, Buddhas print the Buddha message. What have I done wrong?

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