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  • dangling pointer, reason for value change after free()?

    - by Aman Jain
    In the following code segment, after free(x), why does y becomes 0? As per my understanding, the memory in the heap that was being pointed to by x, and is still being pointed by y, hasn't been allocated to someone else, so how can it change to 0? And moreover, I don't think it is free(x) that changed it to 0. Any comments? #include <stdio.h> int main ( int argc, char *argv[] ) { int *y = NULL; int *x = NULL; x = malloc(4); *x = 5; y = x; printf("[%d]\n", *y); //prints 5 free(x); printf("[%d]\n", *y); //why doesn't print 5?, prints 0 instead return 0; }

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  • Exposing boost::scoped_ptr in boost::python

    - by Rupert Jones
    Hello, I am getting a compile error, saying that the copy constructor of the scoped_ptr is private with the following code snippet: class a {}; struct s { boost::scoped_ptr<a> p; }; BOOST_PYTHON_MODULE( module ) { class_<s>( "s" ); } This example works with a shared_ptr though. It would be nice, if anyone knows the answer. Thanks

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  • Getting a seg fault, having trouble with classes and variables.

    - by celestialorb
    Ok, so I'm still learning the ropes of C++ here so I apologize if this is a simple mistake. I have this class: class RunFrame : public wxFrame { public: RunFrame(); void OnKey(wxKeyEvent& keyEvent); private: // Configuration variables. const wxString *title; const wxPoint *origin; const wxSize *size; const wxColour *background; const wxColour *foreground; const wxString *placeholder; // Control variables. wxTextCtrl *command; // Event table. DECLARE_EVENT_TABLE() }; ...then in the OnKey method I have this code: void RunFrame::OnKey(wxKeyEvent& keyEvent) { // Take the key and process it. if(WXK_RETURN == keyEvent.GetKeyCode()) { bool empty = command -> IsEmpty(); } // Propogate the event through. keyEvent.Skip(); } ...but my program keeps seg faulting when it reaches the line where I attempt to call the IsEmpty method from the command variable. My question is, "Why?" In the constructor of the RunFrame class I can seemingly call methods for the command variable in the same way I'm doing so in the OnKey method...and it compiles correctly, it just seg faults on me when it attempts to execute that line. Here is the code for the constructor if necessary: RunFrame::RunFrame() : wxFrame(NULL, wxID_ANY, wxT("DEFAULT"), wxDefaultPosition, wxDefaultSize, wxBORDER_NONE) { // Create the styling constants. title = new wxString(wxT("RUN")); origin = new wxPoint(0, 0); size = new wxSize(250, 25); background = new wxColour(33, 33, 33); foreground = new wxColour(255, 255, 255); placeholder = new wxString(wxT("command")); // Set the styling for the frame. this -> SetTitle(*title); this -> SetSize(*size); // Create the panel and attach the TextControl to it. wxPanel *panel = new wxPanel(this, wxID_ANY, *origin, *size, wxBORDER_NONE); // Create the text control and attach it to the panel. command = new wxTextCtrl(panel, wxID_ANY, *placeholder, *origin, *size); // Set the styling for the text control. command -> SetBackgroundColour(*background); command -> SetForegroundColour(*foreground); // Connect the key event to the text control. command -> Connect(wxEVT_CHAR, wxKeyEventHandler(RunFrame::OnKey)); // Set the focus to the command box. command -> SetFocus(); } Thanks in advance for any help you can give! Regards, celestialorb

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  • declaring a 2D array of pointer objects

    - by Tyler Stennette
    I'm having a tough time figuring out how to instantiate a 2D array of pointer objects. Here is how I'm doing it: Pieces* chessBoard[9][9]; When I want to set it to an actual object pointer, I'm doing the following: chessBoard[1][1] = new Rook(p1Rook); Rook is a class that inherits attributes from the Pieces class and p1Rook is a char variable set to 'R'. This class also implements virtual functions (not pure virtual) from Pieces such as move() or getPiece() that are unique to the particular chess piece. However, when I compile my program, I get the following error: ChessBoard.cpp:69: error: expected type-specifier before ‘Rook’ ChessBoard.cpp:69: error: cannot convert ‘int*’ to ‘Pieces*’ in assignment Can someone please explain what I should change to get rid of this rather annoying persistent error? I would appreciate it.

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  • error: invalid type argument of '->' (have 'struct node')

    - by Roshan S.A
    Why cant i access the pointer "Cells" like an array ? i have allocated the appropriate memory why wont it act like an array here? it works like an array for a pointer of basic data types. #include<stdio.h> #include<stdlib.h> #include<ctype.h> #define MAX 10 struct node { int e; struct node *next; }; typedef struct node *List; typedef struct node *Position; struct Hashtable { int Tablesize; List Cells; }; typedef struct Hashtable *HashT; HashT Initialize(int SIZE,HashT H) { int i; H=(HashT)malloc(sizeof(struct Hashtable)); if(H!=NULL) { H->Tablesize=SIZE; printf("\n\t%d",H->Tablesize); H->Cells=(List)malloc(sizeof(struct node)* H->Tablesize); should it not act like an array from here on? if(H->Cells!=NULL) { for(i=0;i<H->Tablesize;i++) the following lines are the ones that throw the error { H->Cells[i]->next=NULL; H->Cells[i]->e=i; printf("\n %d",H->Cells[i]->e); } } } else printf("\nError!Out of Space"); } int main() { HashT H; H=Initialize(10,H); return 0; } The error I get is as in the title-error: invalid type argument of '->' (have 'struct node').

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  • how to cast c++ smart pointer up and down

    - by user217428
    two clients communicate to each other on top of a message layer in the message body, I need include a field pointing to any data type From client A, I send the field as a shared_ptr to the message layer. I define this field as a shared_ptr in the message layer. But how can I convert this field back to shared_ptr in client B? Or should I define shared_ptr in message layer as something else? Thanks

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  • Mutual class instances in C++

    - by SepiDev
    Hi guys. What is the issue with this code? Here we have two files: classA.h and classB.h classA.h: #ifndef _class_a_h_ #define _class_a_h_ #include "classB.h" class B; //???? class A { public: A() { ptr_b = new B(); //???? } virtual ~A() { if(ptr_b) delete ptr_b; //???? num_a = 0; } int num_a; B* ptr_b; //???? }; #endif //_class_a_h_ classB.h: #ifndef _class_b_h_ #define _class_b_h_ #include "classA.h" class A; //???? class B { public: B() { ptr_a = new A(); //???? num_b = 0; } virtual ~B() { if(ptr_a) delete ptr_a; //???? } int num_b; A* ptr_a; //???? }; #endif //_class_b_h_ when I try to compile it, the compiler (g++) says: classB.h: In constructor ‘B::B()’: classB.h:12: error: invalid use of incomplete type ‘struct A’ classB.h:6: error: forward declaration of ‘struct A’ classB.h: In destructor ‘virtual B::~B()’: classB.h:16: warning: possible problem detected in invocation of delete operator: classB.h:16: warning: invalid use of incomplete type ‘struct A’ classB.h:6: warning: forward declaration of ‘struct A’ classB.h:16: note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined.

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  • Whats the problem with int *p=23;

    - by piemesons
    Yesterday in my interview I was asked this question. (At that time I was highly pressurized by so many abrupt questions). int *p; *p=23; printf('%d',*p); Is there any problem with this code? I explained him that you are trying to assign value to a pointer to whom memory is not allocated. But the way he reacted, it was like I am wrong. Although I got the job but after that he said Mohit think about this question again. I don't know what he was trying to say. Please let me know is there any problem in my answer?

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  • Normal pointer vs Auto pointer (std::auto_ptr)

    - by AKN
    Code snippet (normal pointer) int *pi = new int; int i = 90; pi = &i; int k = *pi + 10; cout<<k<<endl; delete pi; [Output: 100] Code snippet (auto pointer) Case 1: std::auto_ptr<int> pi(new int); int i = 90; pi = &i; int k = *pi + 10; //Throws unhandled exception error at this point while debugging. cout<<k<<endl; //delete pi; (It deletes by itself when goes out of scope. So explicit 'delete' call not required) Case 2: std::auto_ptr<int> pi(new int); int i = 90; *pi = 90; int k = *pi + 10; cout<<k<<endl; [Output: 100] Can someone please tell why it failed to work for case 1?

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  • Getting Segmentation Fault in C++, but why?

    - by Carlos
    I am getting segmentation fault in this code but i cant figure out why. I know a segmentation fault happens when a pointer is NULL, or when it points to a random memory address. #include <iostream> #include <fstream> #include <cmath> using namespace std; //**************************** CLASS ******************************* class Database { struct data{ string city; float latitude, longitude; data *link; }*p; public: Database(); void display(); void add(string cityName, float lat, float lon); private: string cityName; float lat, lon; }; //************************** CLASS METHODS ************************** Database::Database() { p = NULL; } void Database::add(string cityName, float lat, float lon){ data *q, *t; if(p == NULL){ p = new data; p -> city = cityName; p -> latitude = lat; p -> longitude = lon; p -> link = NULL; } else{ q = p; while(q -> link != NULL){ q = q -> link; } t = new data; t -> city = cityName; t -> latitude = lat; t -> longitude = lon; q -> link = t; } } void Database::display() { data *q; cout<<endl; for( q = p ; q != NULL ; q = q->link ) cout << endl << q -> city; } //***************************** MAIN ******************************* //*** INITIALIZATION *** Database D; void loadDatabase(); //****** VARIABLES ***** //******* PROGRAM ****** int main() { loadDatabase(); D.display(); } void loadDatabase() { int i = 0; string cityName; float lat, lon; fstream city; city.open("city.txt", ios::in); fstream latitude; latitude.open("lat.txt", ios::in); fstream longitude; longitude.open("lon.txt", ios::in); while(!city.eof()){ //************************************ city >> cityName; //* * latitude >> lat; //Here is where i think is the problem longitude >> lon; //* * D.add(cityName, lat, lon); //************************************ } city.close(); latitude.close(); longitude.close(); } This is the error am actually getting in console

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  • Write pointer to file in C

    - by Sergey
    I have a stucture: typedef structure student { char *name; char *surname; int age; } Student; I need to write it to binary file. Student *s = malloc(sizeof(*s)); I fill my structure with data and then i write in to the file: fwrite(s, sizeof(*s), 1, fp); In my file doesnt exist a name and surname, it have an adresses of char*. How can i write to file a word, not an adresses?

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  • Help with data retrieval MACRO

    - by Andrei Ciobanu
    Hello, given the following structure: struct nmslist_elem_s { nmptr data; struct nmslist_elem_s *next; }; typedef struct nmslist_elem_s nmslist_elem; Where: typedef void* nmptr; Is it possible to write a MACRO that retrieves the data from the element and cast it to the right type: MACRO(type, element) that expands to *((type*)element->data). For example for int, i would need something like this: *((int*)(element->data)) .

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  • Create thread is not accepting the member functon

    - by prabhakaran
    I am trying to create a class for network programming. This will create a general purpose socket with thread. But when I tried to creat the thread using createthread(). The third argument is producing errors. And from the net I came to know that I can't use the member functions as a argument to the createthread(). Is there any thing by which I can achieve this.

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  • bubble sort on array of c structures not sorting properly

    - by xmpirate
    I have the following program for books record and I want to sort the records on name of book. the code isn't showing any error but it's not sorting all the records. #include "stdio.h" #include "string.h" #define SIZE 5 struct books{ //define struct char name[100],author[100]; int year,copies; }; struct books book1[SIZE],book2[SIZE],*pointer; //define struct vars void sort(struct books *,int); //define sort func main() { int i; char c; for(i=0;i<SIZE;i++) //scanning values { gets(book1[i].name); gets(book1[i].author); scanf("%d%d",&book1[i].year,&book1[i].copies); while((c = getchar()) != '\n' && c != EOF); } pointer=book1; sort(pointer,SIZE); //sort call i=0; //printing values while(i<SIZE) { printf("##########################################################################\n"); printf("Book: %s\nAuthor: %s\nYear of Publication: %d\nNo of Copies: %d\n",book1[i].name,book1[i].author,book1[i].year,book1[i].copies); printf("##########################################################################\n"); i++; } } void sort(struct books *pointer,int n) { int i,j,sorted=0; struct books temp; for(i=0;(i<n-1)&&(sorted==0);i++) //bubble sort on the book name { sorted=1; for(j=0;j<n-i-1;j++) { if(strcmp((*pointer).name,(*(pointer+1)).name)>0) { //copy to temp val strcpy(temp.name,(*pointer).name); strcpy(temp.author,(*pointer).author); temp.year=(*pointer).year; temp.copies=(*pointer).copies; //copy next val strcpy((*pointer).name,(*(pointer+1)).name); strcpy((*pointer).author,(*(pointer+1)).author); (*pointer).year=(*(pointer+1)).year; (*pointer).copies=(*(pointer+1)).copies; //copy back temp val strcpy((*(pointer+1)).name,temp.name); strcpy((*(pointer+1)).author,temp.author); (*(pointer+1)).year=temp.year; (*(pointer+1)).copies=temp.copies; sorted=0; } *pointer++; } } } My Imput The C Programming Language X Y Z 1934 56 Inferno Dan Brown 1993 453 harry Potter and the soccers stone J K Rowling 2012 150 Ruby On Rails jim aurther nil 2004 130 Learn Python Easy Way gmaps4rails 1967 100 And the output ########################################################################## Book: Inferno Author: Dan Brown Year of Publication: 1993 No of Copies: 453 ########################################################################## ########################################################################## Book: The C Programming Language Author: X Y Z Year of Publication: 1934 No of Copies: 56 ########################################################################## ########################################################################## Book: Ruby On Rails Author: jim aurther nil Year of Publication: 2004 No of Copies: 130 ########################################################################## ########################################################################## Book: Learn Python Easy Way Author: gmaps4rails Year of Publication: 1967 No of Copies: 100 ########################################################################## ########################################################################## Book: Author: Year of Publication: 0 No of Copies: 0 ########################################################################## We can see the above sorting is wrong? What I'm I doing wrong?

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  • How to read some bytes from BYTE*

    - by chekalin-v
    I have BYTE pointer. For example the length of this BYTE array is 10. How can I read 4 bytes from 3 position BYTE array? Now I doing it so BYTE *source = "1234567890\0"; BYTE* tmp = new BYTE[4+1](); for(int i=0; i<4; i++) { tmp[i] = source[i+3]; }

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  • Pointer arithmetic.

    - by Knowing me knowing you
    Having code: int** a = new int*[2]; a[0] = new int(1); a[1] = new int(2); cout << "a[0] " << a[0] << '\n'; cout << "a[1] " << a[1] << '\n'; cout << "a[2] " << a[2] << '\n'; cout << "a[0] + 1 " << a[0] + 1 << '\n';//WHY THIS ISN'T == a[1] ? cout << "*(a + 1): " << *(a + 1) << '\n'; //WHY THIS IS == a[1] ? cout << "a[0] - a[1] " << static_cast<int>(a[0] - a[1])<< '\n';//WHY THIS IS == 16 not 4? cout << sizeof(int**); Questions are included right next to relevant lines in code.

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  • c++ File input/output

    - by Myx
    Hi: I am trying to read from a file using fgets and sscanf. In my file, I have characters on each line of the while which I wish to put into a vector. So far, I have the following: FILE *fp; fp = fopen(filename, "r"); if(!fp) { fprintf(stderr, "Unable to open file %s\n", filename); return 0; } // Read file int line_count = 0; char buffer[1024]; while(fgets(buffer, 1023, fp)) { // Increment line counter line_count++; char *bufferp = buffer; ... while(*bufferp != '\n') { char *tmp; if(sscanf(bufferp, "%c", tmp) != 1) { fprintf(stderr, "Syntax error reading axiom on " "line %d in file %s\n", line_count, filename); return 0; } axiom.push_back(tmp); printf("put %s in axiom vector\n", axiom[axiom.size()-1]); // increment buffer pointer bufferp++; } } my axiom vector is defined as vector<char *> axiom;. When I run my program, I get a seg fault. It happens when I do the sscanf. Any suggestions on what I'm doing wrong?

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  • How to attach boost::shared_ptr (or another smart pointer) to reference counter of object's parent?

    - by Checkers
    I remember encountering this concept before, but can't find it in Google now. If I have an object of type A, which directly embeds an object of type B: class A { B b; }; How can I have a smart pointer to B, e. g. boost::shared_ptr<B>, but use reference count of A? Assume an instance of A itself is heap-allocated I can safely get its shared count using, say, enable_shared_from_this.

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  • Passing a template func. as a func. ptr to an overloaded func. - is there a way to compile this code

    - by LoudNPossiblyRight
    Just a general c++ curiosity: This code below shouldn't compile because it's impossible to know which to instantiate: temp(const int&) or temp(const string&) when calling func(temp) - this part i know. What i would like to know is if there is anything i can do to the line marked PASSINGLINE to get the compiler to deduce that i want FPTR1 called and not FPTR2 ? #include<iostream> using std::cout; using std::endl; /*FPTR1*/ void func(void(*fptr)(const int&)){ fptr(1001001);} /*FPTR2*/ void func(void(*fptr)(const string&)){ fptr("1001001"); } template <typename T> void temp(const T &t){ cout << t << endl; } int main(){ /*PASSINGLINE*/ func(temp); return 0; } Thank you.

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  • Another dynamic memory allocation bug.

    - by m4design
    I'm trying to allocate memory for a multidimensional array (8 rows, 3 columns). Here's the code for the allocation (I'm sure the error is clear for you) char **ptr = (char **) malloc( sizeof(char) * 8); for (i = 0; i < 3; i++) ptr[i] = (char *) malloc( sizeof(char) * 3); The crash happens when I reference this: ptr[3][0]; Unhandled exception at 0x0135144d in xxxx.exe: 0xC0000005: Access violation writing location 0xabababab. Are there any recommended references/readings for this kind of subject? Thanks.

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  • Embedding Python and adding C functions to the interpreter

    - by monoceres
    I'm currently writing an applications that embedds the python interpreter. The idea is to have the program call user specified scripts on certain events in the program. I managed this part but now I want the scripts to be able to call functions in my program. Here's my code so far: #include "python.h" static PyObject* myTest(PyObject* self,PyObject *args) { return Py_BuildValue("s","123456789"); } static PyMethodDef myMethods[] = {{"myTest",myTest},{NULL,NULL}}; int main() { Py_Initialize(); Py_InitModule("PROGRAM",myMethods); PyRun_SimpleString("print PROGRAM.myTest()"); Py_Finalize(); } Thanks!

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  • Dangling pointer

    - by viswanathan
    How can the below code result in a dangling pointer. { char *cPointer = malloc ( some constant number ); /* some code */ free ( cPointer ); cPointer = NULL; /* some code */ }

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  • How to determine the end of an integer array when manipulating with integer pointer?

    - by AKN
    Here is the code: int myInt[] ={ 1, 2, 3, 4, 5 }; int *myIntPtr = &myInt[0]; while( *myIntPtr != NULL ) { cout<<*myIntPtr<<endl; myIntPtr++; } Output: 12345....<junks>.......... For Character array: (Since we have a NULL character at the end, no problem while iterating) char myChar[] ={ 'A', 'B', 'C', 'D', 'E', '\0' }; char *myCharPtr = &myChar[0]; while( *myCharPtr != NULL ) { cout<<*myCharPtr<<endl; myCharPtr++; } Output: ABCDE My question is since we say to add NULL character as end of the strings, we rule out such issues! If in case, it is rule to add 0 to the end of integer array, we could have avoided this problem. What say?

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