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  • Inheritance - initialization problem

    - by dumbquestion
    I have a c++ class derived from a base class in a framework. The derived class doesn't have any data members because I need it to be freely convertible into a base class and back - the framework is responsible for loading and saving the objects and I can't change it. My derived class just has functions for accessing the data. But there are a couple of places where I need to store some temporary local variables to speed up access to data in the base class. mydata* MyClass::getData() { if ( !m_mydata ) { // set to NULL in the constructor m_mydata = some_long_and complex_operation_to_get_the_data_in_the_base() } return m_mydata; } The problem is if I just access the object by casting the base class pointer returned from the framework to MyClass* the ctor for MyClass is never called and m_mydata is junk. Is there a way of only initializing the m_mydata pointer once?

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  • jQuery $.post & $.append & IE6

    - by Jim
    I'm having a weird problem with jQuery and IE6. Script works on IE7+ and with all other browsers I have tried it. I can't post the full script, but what it does is this: $.post("file.php",{'foo':'bar'},function(data){ $('#target').append(data) }) When I run the code in IE6, #target just shows ? and a white char with a hole in the middle. I have no idea what this second char is. My initial thought was that this was some sort of content-type problem because the file.php just echoes answer without any header information. I added Content-type: text/html with header() but didn't help. Any suggestions?

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  • Codesample with bufferoverflow (gets method). Why does it not behave as expected?

    - by citronas
    This an extract from an c program that should demonstrate a bufferoverflow. void foo() { char arr[8]; printf(" enter bla bla bla"); gets(arr); printf(" you entered %s\n", arr); } The question was "How many input chars can a user maximal enter without a creating a buffer overflow" My initial answer was 8, because the char-array is 8 bytes long. Although I was pretty certain my answer was correct, I tried a higher amount of chars, and found that the limit of chars that I can enter, before I get a segmentation fault is 11. (Im running this on A VirtualBox Ubuntu) So my question is: Why is it possible to enter 11 chars into that 8 byte array?

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  • Best Practice for CouchDB Document Versioning

    - by Groundwater
    Following my question here I am exmploring ideas for a generic approach to document versioning in CouchDB. While I imagine there may be no canonical approach, I had the following idea and am looking for feedback. I would like to maintain readable document ids as much as possible, so a document existing at /document1 would contain a pointer document to all existing versions on the system. The actual revision documents would be at something like /document1/308ef032a3801a where 308ef032a3801a is some random number or hash. Example The pointer document { "_id" : "document1", "versions" : [ "document1/308ef032a3801a" ] } The version document { "_id" : "document1/308ef032a3801a", ... actual content }

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  • Is concatenating with an empty string to do a string conversion really that bad?

    - by polygenelubricants
    Let's say I have two char variables, and later on I want to concatenate them into a string. This is how I would do it: char c1, c2; // ... String s = "" + c1 + c2; I've seen people who say that the "" + "trick" is "ugly", etc, and that you should use String.valueOf or Character.toString instead. I prefer this construct because: I prefer using language feature instead of API call if possible In general, isn't the language usually more stable than the API? If language feature only hides API call, then even stronger reason to prefer it! More abstract! Hiding is good! I like that the c1 and c2 are visually on the same level String.valueOf(c1) + c2 suggests something is special about c1 It's shorter. Is there really a good argument why String.valueOf or Character.toString is preferrable to "" +? Trivia: in java.lang.AssertionError, the following line appears 7 times, each with a different type: this("" + detailMessage);

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  • Output unicode strings in Windows console app

    - by Andrew
    Hi I was trying to output unicode string to a console with iostreams and failed. I found this: Using unicode font in c++ console app and this snippet works. SetConsoleOutputCP(CP_UTF8); wchar_t s[] = L"èéøÞ???Sæca"; int bufferSize = WideCharToMultiByte(CP_UTF8, 0, s, -1, NULL, 0, NULL, NULL); char* m = new char[bufferSize]; WideCharToMultiByte(CP_UTF8, 0, s, -1, m, bufferSize, NULL, NULL); wprintf(L"%S", m); However, I did not find any way to output unicode correctly with iostreams. Any suggestions?

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  • STL deque accessing by index is O(1)?

    - by jasonline
    I've read that accessing elements by position index can be done in constant time in a STL deque. As far as I know, elements in a deque may be stored in several non-contiguous locations, eliminating safe access through pointer arithmetic. For example: abc-defghi-jkl-mnop The elements of the deque above consists of a single character. The set of characters in one group indicate it is allocated in contiguous memory (e.g. abc is in a single block of memory, defhi is located in another block of memory, etc.). Can anyone explain how accessing by position index can be done in constant time, especially if the element to be accessed is in the second block? Or does a deque have a pointer to the group of blocks? Update: Or is there any other common implementation for a deque?

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  • Write to a binary file?

    - by rick irby
    Here is data structure w/ variables: struct Part_record { char id_no[3]; int qoh; string desc; double price: }; --- (Using "cin" to input data) --- Part_record null_part = {" ", 0," ",0.0}; --- --- file.seekg( -(long)sizeof(Part_record), ios::cur); file.write( ( char *)&part, sizeof(Part_record) ); The three variables, qoh, Id_no & price, write out correctly, but the "desc" variable is not right. Do I need to initialize Part_record some other way? It should be 20 characters in length. If you have enough info here, pls share your advice,thanks.

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  • Would this constructor be acceptable practice?

    - by Robb
    Let's assume I have a c++ class that have properly implemented a copy constructor and an overloaded = operator. By properly implemented I mean they are working and perform a deep copy: Class1::Class1(const Class1 &class1) { // Perform copy } Class1& Class1::operator=(const Class1 *class1) { // perform copy return *this; } Now lets say I have this constructor as well: Class1::Class1(Class1 *class1) { *this = *class1; } My question is would the above constructor be acceptable practice? This is code that i've inherited and maintaining.

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  • C : Memory layout of C program execution

    - by pavun_cool
    Hi All , I wanted know how the kernel is providing memory for simple C program . For example : #include<stdio.h> #include<malloc.h> int my_global = 10 ; main() { char *str ; static int val ; str = ( char *) malloc ( 100 ) ; scanf ( "%s" , str ) ; printf( " val:%s\n",str ) ; } See, In this program I have used static , global and malloc for allocating dynamic memory So , how the memory lay out will be ... ? Any one give me url , which will have have details information about this process..

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  • Multi-threaded random_r is slower than single threaded version.

    - by Nixuz
    The following program is essentially the same the one described here. When I run and compile the program using two threads (NTHREADS == 2), I get the following run times: real 0m14.120s user 0m25.570s sys 0m0.050s When it is run with just one thread (NTHREADS == 1), I get run times significantly better even though it is only using one core. real 0m4.705s user 0m4.660s sys 0m0.010s My system is dual core, and I know random_r is thread safe and I am pretty sure it is non-blocking. When the same program is run without random_r and a calculation of cosines and sines is used as a replacement, the dual-threaded version runs in about 1/2 the time as expected. #include <pthread.h> #include <stdlib.h> #include <stdio.h> #define NTHREADS 2 #define PRNG_BUFSZ 8 #define ITERATIONS 1000000000 void* thread_run(void* arg) { int r1, i, totalIterations = ITERATIONS / NTHREADS; for (i = 0; i < totalIterations; i++){ random_r((struct random_data*)arg, &r1); } printf("%i\n", r1); } int main(int argc, char** argv) { struct random_data* rand_states = (struct random_data*)calloc(NTHREADS, sizeof(struct random_data)); char* rand_statebufs = (char*)calloc(NTHREADS, PRNG_BUFSZ); pthread_t* thread_ids; int t = 0; thread_ids = (pthread_t*)calloc(NTHREADS, sizeof(pthread_t)); /* create threads */ for (t = 0; t < NTHREADS; t++) { initstate_r(random(), &rand_statebufs[t], PRNG_BUFSZ, &rand_states[t]); pthread_create(&thread_ids[t], NULL, &thread_run, &rand_states[t]); } for (t = 0; t < NTHREADS; t++) { pthread_join(thread_ids[t], NULL); } free(thread_ids); free(rand_states); free(rand_statebufs); } I am confused why when generating random numbers the two threaded version performs much worse than the single threaded version, considering random_r is meant to be used in multi-threaded applications.

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  • JNI AttachCurrentThread NULLs the jenv

    - by Damg
    Hello all, I'm currently in the process of adding JNI functionality into a legacy delphi app. In a single-threaded environment everything works fine, but as soon as I move into multi-threaded environment, things start to become hairy. My problem is that calling JavaVM^.AttachCurrentThread( JavaVM, @JEnv, nil ); returns 0, but puts the JEnv pointer to nil. I have no idea why jvm.dll should return a NULL pointer. Is there anything I am missing? Thank you in advance -- damg PS: * Environment: WinXP + JDK 1.6 * Using JNI.pas from http://www.pacifier.com/~mmead/jni/delphi/

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  • test cases for testing a strtok-alike function [C++]

    - by Neeraj
    Consider the following class definition: class StrToTokens { StrToTokens(const char* str, const char* delimiters = "\t\r\n"); //constructor string getNextToken(); void reset(); bool empty(); } Can someone list some good testcases to test the above class. A few I could think of are: empty string, empty delimiters, repeated delimiters, consecutive delimiters, string with only delimiters. However, the interviewer expected some more(better ones). Can you help out. Thanks.

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  • reading a line, tokenizing and assigning to struct in C

    - by Dervin Thunk
    line is fgets'd, and running in a while loop with counter n, d is a struct with 2 char arrays, p and q. Basically, in a few words, I want to read a line, separate it into 2 strings, one up until the first space, and the rest of the line. I clean up afterwards (\n from the file becomes \'0'). The code works, but is there a more idiomatic way to do this? What errors am I running into "unknowingly"? int spc = strcspn(line," "); strncpy(d[n].p, line, spc); d[n].p[spc+1]='\0'; int l = strlen(line)-spc; strncpy(d[n].q, line+spc+1, l); char* nl = strchr(d[n].q, '\n'); if(nl){ *nl='\0'; } n++; Thanks.

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  • Statically initialize anonymous union in C++

    - by wpfwannabe
    I am trying to statically initialize the following structure in Visual Studio 2010: struct Data { int x; union { char ch; const Data* data; }; }; The following is fails with error C2440: 'initializing' : cannot convert from 'Data *' to 'char'. static Data d1; static Data d = {1, &d1}; I have found references to some ways this can be initialized properly but none of them work in VS2010. Any ideas?

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  • Sort a list of pointers.

    - by YuppieNetworking
    Hello all, Once again I find myself failing at some really simple task in C++. Sometimes I wish I could de-learn all I know from OO in java, since my problems usually start by thinking like Java. Anyways, I have a std::list<BaseObject*> that I want to sort. Let's say that BaseObject is: class BaseObject { protected: int id; public: BaseObject(int i) : id(i) {}; virtual ~BaseObject() {}; }; I can sort the list of pointer to BaseObject with a comparator struct: struct Comparator { bool operator()(const BaseObject* o1, const BaseObject* o2) const { return o1->id < o2->id; } }; And it would look like this: std::list<BaseObject*> mylist; mylist.push_back(new BaseObject(1)); mylist.push_back(new BaseObject(2)); // ... mylist.sort(Comparator()); // intentionally omitted deletes and exception handling Until here, everything is a-ok. However, I introduced some derived classes: class Child : public BaseObject { protected: int var; public: Child(int id1, int n) : BaseObject(id1), var(n) {}; virtual ~Child() {}; }; class GrandChild : public Child { public: GrandChild(int id1, int n) : Child(id1,n) {}; virtual ~GrandChild() {}; }; So now I would like to sort following the following rules: For any Child object c and BaseObject b, b<c To compare BaseObject objects, use its ids, as before. To compare Child objects, compare its vars. If they are equal, fallback to rule 2. GrandChild objects should fallback to the Child behavior (rule 3). I initially thought that I could probably do some casts in Comparator. However, this casts away constness. Then I thought that probably I could compare typeids, but then everything looked messy and it is not even correct. How could I implement this sort, still using list<BaseObject*>::sort ? Thank you

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  • C++ operator overloading doubt

    - by avd
    I have a code base, in which for Matrix class, these two definitions are there for () operator: template <class T> T& Matrix<T>::operator() (unsigned row, unsigned col) { ...... } template <class T> T Matrix<T>::operator() (unsigned row, unsigned col) const { ...... } One thing I understand is that the second one does not return the reference but what does const mean in the second declaration. Also which function is called when I do say mat(i,j)

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  • Passing enums to functions in C++

    - by rocknroll
    Hi all, I have a header file with all the enums listed (#ifndef #define #endif construct has been used to avoid multiple inclusion of the file) that I use in multiple cpp files in my application.One of the enums in the files is enum StatusSubsystem {ENABLED,INCORRECT_FRAME,INVALID_DATA,DISABLED}; There are functions in the application delcared as ShowStatus(const StatusSubsystem&); Earlier in the application when I made calls to the above function like ShowStatus(INCORRECT_FRAME); my application used to compile perfectly. But after some code was added The compilation halts giving the following error: File.cpp:71: error: invalid conversion from `int' to `StatusSubsystem' File.cpp:71: error: initializing argument 1 of `void Class::ShowStatus(const StatusSubsystem&) I checked the code for any conflicting enums in the new code and it looked fine. My Question is what is wrong with the function call that compiler shows as erroneous? For your reference the function definition is: void Class::ShowStatus(const StatusSubsystem& eStatus) { QPalette palette; mStatus=eStatus;//store current Communication status of system if(eStatus==DISABLED) { //select red color for label, if it is to be shown disabled palette.setColor(QPalette::Window,QColor(Qt::red)); mLabel->setText("SYSTEM"); } else if(eStatus==ENABLED) { //select green color for label,if it is to be shown enabled palette.setColor(QPalette::Window,QColor(Qt::green)); mLabel->setText("SYSTEM"); } else if(eStatus==INCORRECT_FRAME) { //select yellow color for label,to show that it is sending incorrect frames palette.setColor(QPalette::Window,QColor(Qt::yellow)); mLabel->setText("SYSTEM(I)"); } //Set the color on the Label mLabel->setPalette(palette); } A strange side effect of this situation is it compiles when I cast all the calls to ShowStatus() as ShowStatus((StatusSubsystem)INCORRECT_FRAME); Though this removes any compilation error, but a strange thing happens. Though I make call to INCORRECT_FRAME above but in function definition it matches with ENABLED. How on earth is that possible? Its like while passing INCORRECT_FRAME by reference, it magically converts to ENABLED, which should be impossible. This is driving me nuts. Can you find any flaw in what I am doing? or is it something else? The application is made using C++,Qt-4.2.1 on RHEL4. Thanks.

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  • Constructing a function call in C

    - by 0x6adb015
    Given that I have a pointer to a function (provided by dlsym() for example) and a linked list of typed arguments, how can I construct a C function call with those arguments? Example: struct param { enum type { INT32, INT64, STRING, BOOL } type; union { int i32; long long i64; char *str; bool b; } value; struct param *next; }; int call_this(int (*function)(), struct param *args) { int result; /* magic here that calls function(), which has a prototype of f(int, long long, char *, bool); , when args consist of a linked list of INT32, INT64, STRING, BOOL types. */ return result; } The OS is Linux. I would like the solution to be portable across MIPS, PPC and x86 (all 32 bits) architecture, using GCC as the compiler. Thanks!

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  • Should downcasting be avoided while using a class hierarchy in C++?

    - by neuviemeporte
    Let's say I'm writing an application which works with projects, and exposes different functionality depending on the type of the project. I have a hierarchy of classes for the different types of projects: class AbstractProject { }; class ProjectA : public AbstractProject { }; class ProjectB : public AbstractProject { }; class ProjectC : public AbstractProject { }; Now, I was planning to have an AbstractProject *_currentProject pointer as a member in the application's main class, pop up a dialog box on startup and based on the selection, do: _currentProject = new ProjectB(); // e.g. Later, I'll have to downcast the pointer to the specific type to utilize the functionality specific to different Project-s. Somehow this makes me feel uneasy. Is there a Better Way of doing this?

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  • matlab: putting a circled number onto a graph

    - by Jason S
    I want to put a circled number on a graph as a marker near (but not on) a point. Sounds easy, but I also want to be invariant of zoom/aspect ratio changes. Because of this invariant, I can't draw a circle as a line object (without redrawing it upon rescale); if I use a circle marker, I'd have to adjust its offset upon rescale. The simplest approach I can think of is to use the Unicode or Wingdings characters ① ② ③ etc. in a string for the text() function. But unicode doesn't seem to work right, and the following sample only works with ① and not for the other numbers (which yield rectangle boxes): works: clf; text(0.5,0.5,char(129),'FontName','WingDings') doesn't work (should be a circled 2): clf; text(0.5,0.5,char(130),'FontName','WingDings') What gives, and can anyone suggest a workaround?

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  • recv receiving not whole data sometime

    - by milo
    hi all, i have following issue: here is the chunk of code: void get_all_buf(int sock, std::string & inStr) { int n = 1; char c; char temp[1024*1024]; bzero(temp, sizeof(temp)); n = recv(sock, temp, sizeof(temp), 0); inStr = temp; }; but sometimes recv returning not whole data (data length always less then sizeof(temp)), only it's part. write side always sends me whole data (i got it with sniffer). what matter? thx. P.S. i know, good manner suggests me to check n (if (n < 0) perror ("error while receiving data), but it doesn't matter now - it's not reason of my problem. P.S.2 i've forgot - it's blocking socket.

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  • c++ normalizing data sizes across systems

    - by Bocochoco
    I have a struct with three variables: two unsigned ints and an unsigned char. From my understanding, a c++ char is always 1 byte regardless of what operating system it is on. The same can't be said for other datatypes. I am looking for a way to normalize POD's so that when saved into a binary file, the resulting file is readable on any operating system that the code is compiled for. I changed my struct to use a 1-byte alignment by adding #pragma as follows: #pragma pack(push, 1) struct test { int a; } #pragma pack(pop) but that doesn't necessarily mean that int a is exactly 4 bytes on every os, I don't think? Is there a way to ensure that a file saved from my code will always be readable?

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  • Removing first two elements of a string array in C

    - by sandeep p
    How can I remove first two elements of a string array? I have a code which is something like this. char *x[10]; .............. .............. .............. char *event[20]; event[0]=strtok(x[i]," "); event[1]=strtok(NULL," "); event[2]=strtok(NULL," "); event[3]=strtok(NULL," "); event[4]=strtok(NULL," "); event[5]=strtok(NULL," "); for(i=2;i<length;i++) { strcpy(event[i-2],event[i]); } I observed that only event[0] has proper values. I printed the contents of event[][] before for loop and it displays correctly. Could you please tell me why this is wrong? and a possible solution?

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