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  • When does invoking a member function on a null instance result in undefined behavior?

    - by GMan
    This question arose in the comments of a now-deleted answer to this other question. Our question was asked in the comments by STingRaySC as: Where exactly do we invoke UB? Is it calling a member function through an invalid pointer? Or is it calling a member function that accesses member data through an invalid pointer? With the answer deleted I figured we might as well make it it's own question. Consider the following code: #include <iostream> struct foo { void bar(void) { std::cout << "gman was here" << std::endl; } void baz(void) { x = 5; } int x; }; int main(void) { foo* f = 0; f->bar(); // (a) f->baz(); // (b) } We expect (b) to crash, because there is no corresponding member x for the null pointer. In practice, (a) doesn't crash because the this pointer is never used. Because (b) dereferences the this pointer (this->x = 5;), and this is null, the program enters undefined behavior. Does (a) result in undefined behavior? What about if both functions are static?

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  • C++ this as thread parameter, variables unavailable

    - by brecht
    I have three classes: class Rtss_Generator { int mv_transfersize; } class Rtss_GenSine : Rtss_Generator class Rtss_GenSineRpm : Rtss_GenSine Rtss_GenSine creates a thread in his constructer, it is started immediatly and the threadfunction is off-course declared static, waiting for an event to start calculating. the problem: all the variables that are accessed through the gen-pointer are 0, but it does not fail. Also, this-address in the constructer and the gen-pointer-address in the thread are the same, so the pointer is ok. this code is created and compile in visual studio 6.0 service pack 2003 ORIGINAL CODE no thread Rtss_GenSine::getNextData() { //[CALCULATION] //mv_transferSize is accessible and has ALWAYS value 1600 which is ok } NEW CODE Rtss_GenSine::Rtss_GenSine() { createThread(NULL, threadFunction, (LPVOID) this,0,0); } Rtss_GenSine::getNextData() { SetEvent(startCalculating); WaitForSingleObject(stoppedCalculaing, INFINITE); } DWORD Rtss_GenSine::threadFunction(LPVOID pParam) { Rtss_GenSine* gen = (Rtss_GenSine*) pParam; while(runThread) { WaitForSingleObject(startCalculating, INFINITE); ResetEvent(startCalculating) //[CALCULATION] //gen->mv_transferSize ---> it does not fail, but is always zero //all variables accessed via the gen-pointer are 0 setEvent(stoppedCalculaing) } }

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  • Using a Function returning apointer as LValue

    - by Amrish
    Why cant I used a function returning a pointer as a lvalue? For example this one works int* function() { int* x; return x; } int main() { int* x = function(); x = new int(9); } but not this int* function() { int* x; return x; } int main() { int* x; function() = x; } While I can use a pointer variable as a lvalue, why can't I use a function returning a pointer as a lvalue? Also, when the function returns a refernce, instead of a pointer, then it becomes a valid lvalue.

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  • C++: Pointers and scope

    - by oh boy
    int* test( ) { int a = 5; int* b = &a; return b; } Will the result of test be a bad pointer? As far as I know a should be deleted and then b would become a messed up pointer, right? How about more complicated things, not an int pointer but the same with a class with 20 members or so?

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  • Modulo operator in Objective-C returns the wrong result

    - by Greg Maletic
    I'm a little freaked out by the results I'm getting when I do modulo arithmetic in Objective-C. -1 % 3 is coming out to be -1, which isn't the right answer: according to my understanding, it should be 2. -2 % 3 is coming out to -2, which also isn't right: it should be 1. Is there another method I should be using besides the % operator to get the correct result?

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  • Function Pointer

    - by Shaista Naaz
    How is that function pointer better than if-else or switch case? Is it because function pointer helps callback functions and thus promotes asynchronous implementation?

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  • PHP array pointer craziness

    - by JMan
    I'm trying to create a "GetCurrentLevel" method that takes a point value as an input and returns what "Level" that corresponds to. I'm storing the Level = Points mapping in an array, but the array pointer is not moving logically when I use it a foreach loop. I've added echo statements for debugging. Here's my class definition: class Levels extends Model { protected $_map = array ( 'None' => 0, 'Bronze' => 50, 'Silver' => 200, 'Gold' => 500 ); public function __construct() { parent::__construct(); } public function GetCurrentLevel($points) { foreach ($this->_map as $name => $threshold) { echo "Level Name: $name<br/>"; echo "Level Threshold: $threshold<br/>"; echo "Current Level: " . key($this->_map) . "<br/>"; echo "Current Threshold: " . current($this->_map) . "<br/>"; if ($points < $threshold) /* Threshold is now above the points, so need to go back one level */ { $previousThreshold = prev($this->_map); echo "Previous Threshold: $previousThreshold<br/>"; echo "Final Level: " . key($this->_map) . "<br/>"; return key($this->_map); } echo "Go to next level <br/>"; } } And here is what I see when I call GetCurrentLevel(60): Level Name: None Level Threshold: 0 Current Level: Bronze //* Looks like foreach immediately moves the array pointer *// Current Threshold: 50 Go to next level Level Name: Bronze Level Threshold: 50 Current Level: Bronze //* WTF? Why hasn't the array pointer moved? *// Current Threshold: 50 Go to next level Level Name: Silver Level Threshold: 200 Current Level: Bronze //* WTF? Why hasn't the array pointer moved? *// Current Threshold: 50 Previous Threshold: 0 Final Level: None But the "Final Level" should be 'Bronze' since 60 points is above the 50 points needed for a Bronze medal, but below the 200 points needed for a Silver medal. Sorry for the long post. Thanks for your help!

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  • question regarding pointer in c language

    - by din
    char *sample = "String Value"; &sample is a pointer to the pointer of "String Value" is the above statement right? If the above statement right, what is the equivalent of &sample if my declaration is char sample[] = "String Value"

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  • python object to native c++ pointer

    - by Lodle
    Im toying around with the idea to use python as an embedded scripting language for a project im working on and have got most things working. However i cant seem to be able to convert a python extended object back into a native c++ pointer. So this is my class: class CGEGameModeBase { public: virtual void FunctionCall()=0; virtual const char* StringReturn()=0; }; class CGEPYGameMode : public CGEGameModeBase, public boost::python::wrapper<CGEPYGameMode> { public: virtual void FunctionCall() { if (override f = this->get_override("FunctionCall")) f(); } virtual const char* StringReturn() { if (override f = this->get_override("StringReturn")) return f(); return "FAILED TO CALL"; } }; Boost wrapping: BOOST_PYTHON_MODULE(GEGameMode) { class_<CGEGameModeBase, boost::noncopyable>("CGEGameModeBase", no_init); class_<CGEPYGameMode, bases<CGEGameModeBase> >("CGEPYGameMode", no_init) .def("FunctionCall", &CGEPYGameMode::FunctionCall) .def("StringReturn", &CGEPYGameMode::StringReturn); } and the python code: import GEGameMode def Ident(): return "Alpha" def NewGamePlay(): return "NewAlpha" def NewAlpha(): import GEGameMode import GEUtil class Alpha(GEGameMode.CGEPYGameMode): def __init__(self): print "Made new Alpha!" def FunctionCall(self): GEUtil.Msg("This is function test Alpha!") def StringReturn(self): return "This is return test Alpha!" return Alpha() Now i can call the first to functions fine by doing this: const char* ident = extract< const char* >( GetLocalDict()["Ident"]() ); const char* newgameplay = extract< const char* >( GetLocalDict()["NewGamePlay"]() ); printf("Loading Script: %s\n", ident); CGEPYGameMode* m_pGameMode = extract< CGEPYGameMode* >( GetLocalDict()[newgameplay]() ); However when i try and convert the Alpha class back to its base class (last line above) i get an boost error: TypeError: No registered converter was able to extract a C++ pointer to type class CGEPYGameMode from this Python object of type Alpha I have done alot of searching on the net but cant work out how to convert the Alpha object into its base class pointer. I could leave it as an object but rather have it as a pointer so some non python aware code can use it. Any ideas?

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  • Converting a pointer C# type to F#??

    - by Brendon
    Hello all I am just a beginner in programing i wish covert some code from C# to F#, I have encotered this code: "float[] v1=new float[10]" I need to use this pointer to pass to the function: "ComputeBuffer bufV1 = new ComputeBuffer(Context, ComputeMemoryFlags.ReadWrite | ComputeMemoryFlags.UseHostPointer, v1);" If i creat an array in F# like this: "let v1 = [| 1.0..10.0 |]" and call now the funaction like this: "let bufV1 = new ComputeBuffer(Context, ComputeMemoryFlags.ReadWrite ||| ComputeMemoryFlags.UseHostPointer, v1)" Is it an error?? How do i pass a pointer??

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  • How to cast a pointer of memory block to std stream

    - by Shahrooz Kia
    I have programed an application on windows XP and in Visual Studio with c++ language. In that app I used LoadResource() API to load a resource for giving a file in the resource memory. It returned a pointer of memory block and I wanna cast the pointer to the std stream to use for compatibility. Could anyone help me?

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  • Using Int32 or what you need

    - by Sir Psycho
    Should you use Int32 in places where you know the value is not going to be higher than 32,767? I'd like to keep memory down, hoever, using casts everywhere just to perform simple arithmetic is getting annoying. short a = 1; short result = a + 1; // Error short result = (short)(a + 1); // works but looks ugly when does lots of times What would be better for overall application performance?

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  • Call c++ function pointer from c#

    - by Sam
    Is it possible to call a c(++) static function pointer like this typedef int (*MyCppFunc)(void* SomeObject); from c#? void CallFromCSharp(MyCppFunc funcptr, IntPtr param) { funcptr(param); } I need to be able to callback from c# into some old c++ classes. C++ is managed, but the classes are not ref classes (yet). So far I got no idea how to call a c++ function pointer from c#, is it possible?

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  • C++ class pointer

    - by JHollanti
    I know that you can get a reference to a static method like this: typedef void (*pointer)(); pointer p = &MyClass::MyMethod; But is there a way to get a reference to the class itself?

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  • pointer&dynamic memory

    - by gcc
    how many methods are there taking input by using with pointer and dynamic memory input 3 1 2 n k l 2 1 2 p 4 55 62 * # x x is stop character and first input 3 is for another variable int hakko; only hakko use first input the others will be hold in one pointer and input size not determined

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  • My mouse pointer disappears on "system surfaces" , like background picture, menus, background for all kind of system menus

    - by Siegfried
    I have an annoying problem with Ubuntu 11.10, my mouse pointer disappears on the Ubuntu surface ( not in programs like firefox, libre office ..) The mouse pointer is there, but its size is only one pixel and in the white background of e.g. "system" it is not seen at all, although with "ctrl" I can see where it should be. I have not found any place where I can change the size or colors of the mouse pointer. Can anybody help me?

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  • setcontext and makecontext to call a generic function pointer

    - by Simone Margaritelli
    In another question i had the problem to port the code unsigned long stack[] = { 1, 23, 33, 43 }; /* save all the registers and the stack pointer */ unsigned long esp; asm __volatile__ ( "pusha" ); asm __volatile__ ( "mov %%esp, %0" :"=m" (esp)); for( i = 0; i < sizeof(stack); i++ ){ unsigned long val = stack[i]; asm __volatile__ ( "push %0" :: "m"(val) ); } unsigned long ret = function_pointer(); /* restore registers and stack pointer */ asm __volatile__ ( "mov %0, %%esp" :: "m" (esp) ); asm __volatile__ ( "popa" ); To a 64bit platform and many guys told me i should use the setcontext and makecontext functions set instead due to the calling conversion differences between 32 and 64 bits and portability issues. Well, i really can't find any useful documentation online, or at least not the kind i need to implement this, so, how can i use those functions to push arguments onto the stack, call a generic function pointer, obtain the return value and then restore the registers?

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  • Copy a LinkedList that has a Random Pointer in it

    - by Bragaadeesh
    Hi, First of all this is not a homework, this is an interview question that I got from a company I attended today. You have a singly linked list with the Node structure as the following class Node{ int data; Node next; Node random; } You have a typical singly linked list of length n. The random pointer in each node in the linkedlist randomly points to some Node within the linked list. The Question is to create a copy of the linked list efficiently into a different LinkedList. I said that I will first calculate the Random pointer's position in the linked list and store it in an array. Then create a new linked list normally. Then iterate through the linked list by setting the random pointer where they belong by reading the values stored from the array. I know its a very brute force technique and the interviewer asked me to come up with a better solution but I couldnt. Please can someone answer this? I can explain if the question is not clear.

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  • [c++] accessing the hidden 'this' pointer

    - by Kyle
    I have a GUI architecture wherein elements fire events like so: guiManager->fireEvent(BUTTON_CLICKED, this); Every single event fired passes 'this' as the caller of the event. There is never a time I dont want to pass 'this', and further, no pointer except for 'this' should ever be passed. This brings me to a problem: How can I assert that fireEvent is never given a pointer other than 'this', and how can I simplify (and homogenize) calls to fireEvent to just: guiManager->fireEvent(BUTTON_CLICKED); At this point, I'm reminded of a fairly common compiler error when you write something like this: class A { public: void foo() {} }; class B { void oops() { const A* a = new A; a->foo(); } }; int main() { return 0; } Compiling this will give you ../src/sandbox.cpp: In member function ‘void B::oops()’: ../src/sandbox.cpp:7: error: passing ‘const A’ as ‘this’ argument of ‘void A::foo()’ discards qualifiers because member functions pass 'this' as a hidden parameter. "Aha!" I say. This (no pun intended) is exactly what I want. If I could somehow access the hidden 'this' pointer, it would solve both issues I mentioned earlier. The problem is, as far as I know you can't (can you?) and if you could, there would be outcries of "but it would break encapsulation!" Except I'm already passing 'this' every time, so what more could it break. So, is there a way to access the hidden 'this', and if not are there any idioms or alternative approaches that are more elegant than passing 'this' every time?

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  • NULL pointer comparison fails

    - by Ilya
    Hello, I'm initializing in a class a pointer to be NULL. Afterwards I check if it is NULL in the same class. But it's not always 0x0. Sometimes it's 0x8 or 0xfeffffff or 0x3f800000 or 0x80 or other strange stuff. In most case the pointer is 0x0 but sometimes it gets altered somehow. I'm sure that I'm not changing it anywhere in my code. Is there a way it gets changed by "itself"? Here's my code: MeshObject::MeshObject() { mesh.vertexColors = NULL; } MeshObject::MeshObject(const MeshObject &_copyFromMe) { SimpleLog("vertexColors pointer: %p", _copyFromMe.mesh.vertexColors); if (_copyFromMe.mesh.vertexColors != NULL) { SimpleLog("vertexColors"); this->mesh.vertexColors = new tColor4i[_copyFromMe.mesh.vertexCount]; memcpy(this->mesh.vertexColors, _copyFromMe.mesh.vertexColors, _copyFromMe.mesh.vertexCount * sizeof(tColor4i) ); } } My application crashes, because vertexColors wasn't initialized and is being copied. However it is NULL and shouldn't be copied. Thanks.

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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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  • why no implicit conversion from pointer to reference to const pointer.

    - by user316606
    I'll illustrate my question with code: #include <iostream> void PrintInt(const unsigned char*& ptr) { int data = 0; ::memcpy(&data, ptr, sizeof(data)); // advance the pointer reference. ptr += sizeof(data); std::cout << std::hex << data << " " << std::endl; } int main(int, char**) { unsigned char buffer[] = { 0x11, 0x11, 0x11, 0x11, 0x22, 0x22, 0x22, 0x22, }; /* const */ unsigned char* ptr = buffer; PrintInt(ptr); // error C2664: ... PrintInt(ptr); // error C2664: ... return 0; } When I run this code (in VS2008) I get this: error C2664: 'PrintInt' : cannot convert parameter 1 from 'unsigned char *' to 'const unsigned char *&'. If I uncomment the "const" comment it works fine. However shouldn't pointer implicitly convert into const pointer and then reference be taken? Am I wrong in expecting this to work? Thanks!

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