C++ Gurus

• 

  SOLVED Error allocating memory
  • jenya7  

  4
  0
  Votes
  4
  Posts
  60
  Views

  

  and you shouldn't use malloc() in c++ anyways. const size_t intCount = 1000U; int* intPtr = new int[intCount]; . . . delete [] intPtr;
• 

  UNSOLVED Lazy C coder challege , just for fun
  • AnneRanch  

  2
  0
  Votes
  2
  Posts
  43
  Views

  

  This looks like it might be promising: https://doc.qt.io/qt-5/qtglobal.html#qInstallMessageHandler
• 

  UNSOLVED stream inserter order/aggregation...
  • Kent-Dorfman  

  12
  0
  Votes
  12
  Posts
  104
  Views

  

  OK. interleaved in the plethora of answers. Therein is some wisdom that clarifies my assertions. Thanks. Embedded doesn't necessarily infer single process here. use case will be both multi-process, AND multi-thread within service daemons (embedded linux)...so thread safety is important. Other thing that was missing from my knowledgebase was pre-c++17 behaviour...left-to-right not being specified before that. Building a temporary stream object and manually flushing or endl seems to address my issues, but I was trying to determine if it was a requirement for proper behaviour...it seems to be. Thanks!
• 

  SOLVED what is difference between destructor and virtual destructor in c++?
  • Qt embedded developer  

  2
  0
  Votes
  2
  Posts
  31
  Views

  

  Similar to virtual and non-virtual functions. Holding a pointer to a base class and calling a function on it calls the method of said class only. If it's virtual then a lookup is done in the virtual table to call a method of the actual object behind the pointer. For virtual destructors it works similar - the destructor is called for the actual object's class, not for the type specified by the pointer (as they may differ) if it's virtual, otherwise, you're going to get partial destruction. The rule of thumb is - if the class has virtual functions or the object must be destroyed through a base class pointer, then you need to make the destructor of the base class virtual. [edit: fixed typo SGaist]
• 

  UNSOLVED How to use cache memory in c++ ?
  • Qt embedded developer  

  4
  0
  Votes
  4
  Posts
  36
  Views

  

  @Qt-embedded-developer Well, especially since it has nothing to do with Qt, there is plenty to read up on out there from Google cache memory in c++. You are likely to have to do a lot of your own library writing/usage, and I don't think it will usable with Qt library code which does not particularly use any cache. Do you really want to do any cache stuff yourself? I wonder how much benefit you think you will really achieve over just leaving C++ to its own devices...?
• 

  UNSOLVED Inlining Failed AVX-512
  • ofmrew  

  7
  0
  Votes
  7
  Posts
  44
  Views

  

  @ofmrew said in Inlining Failed AVX-512: -mnative Correct spelling of this options is -march=native
• 

  UNSOLVED How to get the list of all updated and Not updated row of Data base from server ?
  • Qt embedded developer  

  4
  0
  Votes
  4
  Posts
  38
  Views

  

  @Qt-embedded-developer As clear as mud. So what is your actual question? Is it a SQL/SQLite question about doing a SELECT, or something else? (Not that I even understand how SQLite is involved here or where it runs or how it can return 1 row less than your SELECT , or anything else.) You really need to explain what kind of an answer you are looking for....
• 

  UNSOLVED How to get total Heap and Utilized Heap by application developed in c++ ?
  • Qt embedded developer  

  5
  0
  Votes
  5
  Posts
  56
  Views

  

  @Kent-Dorfman yes But i am not getting the right answer. As i mention i need answer in context of C++ and Based on LINUX os
• 

  UNSOLVED The program has unexpectedly finished.
  • Rajashekara_NR  

  17
  0
  Votes
  17
  Posts
  35
  Views

  

  @Rajashekara_NR The same code in the destructor cannot handle both those cases. You can delete if something was newed, not otherwise. You cannot delete if it might be a stack variable. Either stipulate in your "contract" that Ambientlight requires a newed Serviceproxy which it then takes ownership of and deletes, or require the caller/outside world to deal with the Serviceproxy's lifetime. Or do it with QObject parentage.
• 

  UNSOLVED Passing parmaterts to slot - what are the options ?
  • AnneRanch  

  4
  0
  Votes
  4
  Posts
  59
  Views

  

  @AnneRanch Sorry - I quoted a source , I did not invent the "half lambda". nothing to feel sorry about. I actually may need to pass more then just index... Ehm, your example showed how to capture one variable this I showed you how to capture 2 variables - this and index- and gave you a link to the documentation.... You know, there are two types of people in this world: Those who can extrapolate from incomplete data
• 

  This topic is deleted!
  • AnneRanch  

  1
  0
  Votes
  1
  Posts
  7
  Views

  No one has replied

• 

  UNSOLVED Multiple inheritace - missing "class name" ?
  • AnneRanch  

  11
  0
  Votes
  11
  Posts
  131
  Views

  

  @AnneRanch said in Multiple inheritace - missing "class name" ?: Great advice , but HOW do I solve my problem ? Here you go: https://en.wikipedia.org/wiki/Object_composition#Aggregation
• 

  This topic is deleted!
  • mahmud99  

  1
  0
  Votes
  1
  Posts
  3
  Views

  No one has replied

• 

  SOLVED when pushback function of std::vector make program crash? How to handle it?
  • Qt embedded developer  

  9
  0
  Votes
  9
  Posts
  91
  Views

  

  Thank you all dear for answers...
• 

  SOLVED constant functions in Qt Classes
  • Swati777999  

  14
  0
  Votes
  14
  Posts
  97
  Views

  

  @Kent-Dorfman I'm with you :) Originally there wasn't any void either!
• 

  SOLVED What is Inline declaration?
  • Swati777999  

  5
  0
  Votes
  5
  Posts
  43
  Views

  

  @J-Hilk said in What is Inline declaration?: for more information: https://en.cppreference.com/w/cpp/language/inline This link has better examples. Thanks for sharing!
• 

  SOLVED Need for Forward declaration
  • Swati777999  

  3
  0
  Votes
  3
  Posts
  29
  Views

  

  @Swati777999 Forward declarations can help to break circular dependency: in header a you include header b and in header b you include header a -> circular dependency. You solve this using forward declarations, then you do not have to include the header file (you, of course, need to include it in cpp file, but this does not lead to circular dependency). Forward declarations also help to speed up compalation time, because you reduce the amount of includes in your header files. Each time you include a header file the preprocessor replaces the include statement with the content of the header file, but if the header file includes other header files then the amount of included text increases, which makes build times longer.
• 

  UNSOLVED Manage Exception on Applciation based in QT and c++ languge
  • Fiore  

  10
  0
  Votes
  10
  Posts
  97
  Views

  

  There is not much information to go by to find out your real problem. Nevertheless, there are two things that come to mind: If your application is multithreaded you should note that GUI calls have to be managed within the GUI thread. This means that if your exception is caught in a different thread you cannot directly call showMessage(). Doing so will (or at least might) crash your app. You don't show the code inside the try block. Maybe you have wrong expectations about the use of signals/slots together with exceptions. Exceptions inside the slots cannot be safely handled outside the slot. You can try to put a try/catch around the main event loop. This will show if the exceptions take a path you don't expect. Here the catch block cannot use the GUI anymore because there is no functioning GUI left once you are outside the main event loop. If you are coming from a managed language you should maybe think about changing your style when writing C++. There is an ongoing discussion in the C++ community about how to handle errors properly. So far, the only thing (almost) everyone agrees on is that exceptions should only be use for exceptional behavior and not for error handling. If, e.g., a file cannot be found this is expected and not exceptional. There are only very few places in C++ where exceptions are thrown, like out-of-memory or dynamic cast on a reference. The likeliest reason why you want to catch exceptions is that you throw them yourself. Then you should maybe look for other ways to report and handle errors. Exceptions don't play nice with the asynchronous nature of signals and slots.
• 

  SOLVED Do member `const`s save memory?
  • JonB  

  14
  0
  Votes
  14
  Posts
  92
  Views

  

  @JonB said in Do member `const`s save memory?: First, may I say I am well aware of how, say, char * or constexpr would have worked instead. I picked the example I did (silly as it is) precisely because it is not one of those type of constructs, Yes, thinking it through I did appreciate about it must be a properly-allocated member of the class in each instance. So I do get why. In addition to @Chris-Kawa's comment, const is for you and you alone, it's not a storage specifier. The compiler doesn't and can't optimize anything about it, as you can strip it at any time (mutable members and const_cast come to mind). @JonB said in Do member `const`s save memory?: Oh, BTW, you said "on the stack", but that's only if my instances are on the stack. If I new them it's occupying space on the heap, surely? Yes, it's slang for "auto-storage", where the size is known at compile time (as opposed to direct heap allocations). @JoeCFD said in Do member `const`s save memory?: If I remember it correctly, struct is converted to class public part when it is compiled. Therefore, there is no more reason to use struct in C++ except for C code. struct and class have no meaning when you compile something, so no, nothing is converted into anything. Objects are pieces of memory with a specific data layout, whilst methods go into the static portion (.text section) of the binary. Virtual methods have an additional level of indirection through the virtual table, which is pointed by the vptr contained somewhere in the actual object data (usually the first, hidden, data field). PS. @Chris-Kawa said in Do member `const`s save memory?: If you don't want to modify it use const char* fred = "abc" instead of an array. That should be const char * const fred = "abc", otherwise the compiler is going to (rightfully) complain about it.
• 

  SOLVED when to use casting in c++ instead class global pointer?
  • Qt embedded developer  

  6
  0
  Votes
  6
  Posts
  67
  Views

  

  global pointers to members of another glass are generally a bad idea because accessing the class without going thru the MOC can have undesired behaviour. inter-class comms should be via signal/slot rather than direct handle to the target class instance.
• 

  SOLVED SOLVED convernt __LINE__ to text in qDebug
  • AnneRanch  

  3
  0
  Votes
  3
  Posts
  25
  Views

  

  Thanks , I knew the solution was very simple. SOLVED
• 

  SOLVED what is meaning of this statement app::print::WaybillCollection_Report in c++ ?
  • Qt embedded developer  

  6
  0
  Votes
  6
  Posts
  43
  Views

  

  @Qt-embedded-developer i got the answer why this get used like my statement because of multiple namespace . namespace app { namespace print { typedef enum _PrintType { Unknown = 0, Paper_Feed, Ticket_Print, StationList_Report, WaybillCollection_Report, TripWiseCollection_Report, StationWiseFare_Report, StageWiseFare_Report,
• 

  SOLVED is it possible to call class in destructor ? if not then why ?
  • Qt embedded developer  

  4
  0
  Votes
  4
  Posts
  33
  Views

  

  @Qt-embedded-developer Yes this will of course prevent the leak. However there's no need to do this if you allocate on stack instead of heap: WaybillCollectionReportFlow waybillCollectionReportFlow; waybillCollectionReportFlow.execute(); That's it, no new or delete or leak.
• 

  UNSOLVED How to split string by 2 tocken in c++?
  • Qt embedded developer  

  4
  0
  Votes
  4
  Posts
  53
  Views

  

  @Qt-embedded-developer I'm not going to work through what your code actually does, and how it compares to the output, but you have: token = strtok(str, s); ... token1 = strtok(str, s1); What do you think this is actually going to do? Once you have called strtok(str, ...); you should only call strtok(NULL, ...); to get another token out of str. strtok(str, ...); alters (writes a \0 into) str. This could be a problem for you. More pertinently: strtok(NULL, ...) can only work on the single, last string you called strtok(str, ...) on. Your code seems to go: strtok(str, s); strtok(str, s1); strtok(NULL, s1); // then you return to the *outer* loop and go strtok(NULL, s); That last line is now operating on the latest strtok(str, s1) executed, not on the original strtok(str, s) which I believe is what you think it is doing. Maybe that's the cause of your output. In any case: don't use strtok() in modern programs. Either use a re-entrant strtok_r() which at least allows multiple separate string to tokenised without interfering with each other. Even then it's bad because it still writes into the string. Or, much better as @jsulm says, use C++ methods instead. You ought no longer be using char []s, char *s, strtok() and so on in your code. That was for C, now we are C++. P.S. Instead of C code and strtok(), in Qt just use QStrings and QString::split() to do your tokenisation work. You won't have these problems that way.
• 

  UNSOLVED what to do when your select query consist so many rows at a time ? How to reduce its impact on heap memory ?
  • Qt embedded developer  

  7
  0
  Votes
  7
  Posts
  53
  Views

  

  @Qt-embedded-developer Apart from the suggestions for limiting rows returned, make sure you only request those columns you really need. I don't know whether the option has any effect when using SQLite, but do try setting QSqlQuery::setForwardOnly(bool forward). It could make a big difference to memory usage (and speed).
• 

  SOLVED What care i need to take when my 2 different thread access same database ?
  • Qt embedded developer  

  15
  0
  Votes
  15
  Posts
  82
  Views

  

  @Qt-embedded-developer said in What care i need to take when my 2 different thread access same database ?: but it make my ui stuck to wait for release mutex lock Well, then analyse and fix your code - you have a dead-lock
• 

  SOLVED what is meaning of below c++ map means ?
  • Qt embedded developer  

  2
  0
  Votes
  2
  Posts
  33
  Views

  

  @Qt-embedded-developer said in what is meaning of below c++ map means ?: In below code i am not understanding the concept of the below statement : sqliteManagerMap.insert(std::pair<int,SqliteManagerBGSync>(threadId,new SqliteManagerBGSync()))* can any body explain me it theoretically. what this statement do ? As described in std:map documentation => https://www.cplusplus.com/reference/map/map/insert/ It insert a new item in to map.
• 

  SOLVED Incompatible pointer type when referencing the method.
  reference error pdfium • • artwaw  

  14
  0
  Votes
  14
  Posts
  77
  Views

  

  @mpergand thank you but: I stated previously that I work with PDFium this time, as Poppler doesn't offer edits; I managed to work around without using static method: maybe not the nicest solution around but works. At any rate I made a note of your comment in case I need to revisit this godawful file format known as PDF in the future.
• 

  SOLVED How to free file descriptor of same file get accessed by two different thread simultaneously ?
  • Qt embedded developer  

  2
  0
  Votes
  2
  Posts
  40
  Views

  

  @Qt-embedded-developer said in How to free file descriptor of same file get accessed by two different thread simultaneously ?: FuelCheck ObjFuelCheck; ObjFuelCheck.gauge_init(); Why do you instantiate and init ObjFuelCheck inside the loop?! Same for all the other stuff created inside the loop (like openning the file). Do it once before you enter the loop... Also, you really should not write into same file from two threads! This will endup in a mess. If you want to do so for whatever reason, open the file only once and synchronise the access to it (use a QMutexLocker).
• 

  SOLVED How to reduce use of heap section in c++?
  • Qt embedded developer  

  13
  0
  Votes
  13
  Posts
  137
  Views

  

  @mpergand said in How to reduce use of heap section c++?: sqlite3_free I am not using sqlite3_prepare_v2 i am using sqlite3_exec for execute the sqlite query. And also i am not using the sqlite3_free(errMsg). I have seen that there is as we continue execute queries my heap continuously get increasing .
• 

  This topic is deleted!
  • tonyteotonyteo  

  1
  0
  Votes
  1
  Posts
  6
  Views

  No one has replied

• 

  UNSOLVED Boolean in C
  • SaintBroseph  

  34
  0
  Votes
  34
  Posts
  251
  Views

  

  @JonB said in Boolean in C: Are you also claiming to be a descendant of Henry VIII? :) Claiming is such a strict definition. It is an interesting possibility that historians play with, and I cannot discount how much my cousins look like Henry in his later years (according to paintings)...and we do know that grammy Mary was quite the party girl.
• 

  UNSOLVED hacking std::copy to use pointers
  • Kent-Dorfman  

  11
  0
  Votes
  11
  Posts
  249
  Views

  

  I just don't get where it fails from this discussion: #include <cstdint> #include <algorithm> #include <cassert> int main(int argc, char *argv[]) { constexpr size_t length = 0x20U; uint8_t* ptr = new uint8_t[length]; for(size_t count=0; count<length; ++count) ptr[count] = count; uint8_t* dest = new uint8_t[length]; std::copy(ptr, ptr+length, dest); for(size_t count=0; count<length; ++count){ assert(ptr[count] == dest[count]); } delete[] ptr; delete[] dest; return 0; } I tested in both c++11 and c++17.
• 

  This topic is deleted!
  • inlichtet  

  1
  0
  Votes
  1
  Posts
  4
  Views

  No one has replied

• 

  UNSOLVED Qt MSVC supported versions for windows 10?
  • Nethzz  

  5
  0
  Votes
  5
  Posts
  67
  Views

  

  @jsulm Thanks a lot!!! I have installed visual studio and it fixed...
• 

  UNSOLVED malloc / new issue
  • Match0um  

  4
  0
  Votes
  4
  Posts
  183
  Views

  

  @Match0um where is this declared? int bufferSize; if declared as member variable during initialization use like this.. bufferSize = new int(); //don't use int
• 

  UNSOLVED Headers in c++
  • Thank You  

  7
  0
  Votes
  7
  Posts
  100
  Views

  

  Or simply use a proper Configuration class (which internally uses QSettings for example) where you can retrieve all those information from?
• 

  UNSOLVED How to tell MOC to exclude gcc related includes to avoid problem like:Parse error at "std"?
  • Alex Luya  

  3
  0
  Votes
  3
  Posts
  84
  Views

  

  @jeremy_k Thanks,I want to exclude all gcc headers which is not created by myself,but comes from GCC.
• 

  UNSOLVED qt moc error:usr/include/c++/10/bits/fs_fwd.:39: Parse error at "std"
  • Alex Luya  

  3
  0
  Votes
  3
  Posts
  77
  Views

  

  @JoeCFD Thanks,I have all qmake.stash deleted,but the problem remains,and I noticed complete moc statement got printed out like this: /usr/lib/qt5/bin/moc -DJSON_DIAGNOSTICS=0 -DJSON_USE_IMPLICIT_CONVERSIONS=1 "-DvtkRenderingCore_AUTOINIT_INCLUDE=\"/home/alex/workspace/MTESS/build/CMakeFiles/vtkModuleAutoInit_be7301261a49b13d6a9b1d9e110eacd8.h\"" -I/home/alex/workspace/MTESS/src -I/home/alex/workspace/MTESS/vendor/ProjectStructure -I/home/alex/workspace/MTESS/vendor/imgui -I/home/alex/workspace/MTESS/vendor/imgui/imgui -I/usr/include/x86_64-linux-gnu/qt5 -I/usr/local/qwt-6.2.0/include -I/usr/local/include/opencascade -I/lib -I/usr/local/include -I/usr/include/c++/11 -I/usr/include/x86_64-linux-gnu/c++/11 -I/usr/include/c++/11/backward -I/usr/lib/llvm-14/lib/clang/14.0.0/include -I/usr/include/x86_64-linux-gnu -I/usr/include --include /home/alex/workspace/MTESS/build/MTESS_autogen/moc_predefs.h --output-dep-file -o /home/alex/workspace/MTESS/build/MTESS_autogen/MLM4FWGUYC/moc_NearFieldProcessUtil.cpp /home/alex/workspace/MTESS/src/Layers/ViewLayer/Process/NearFieldProcessUtil.h And copy this to run in bash,it will give: usr/include/c++/11/bits/fs_fwd.:39: Parse error at "std" If removing: -I/usr/local/include -I/usr/include/c++/11 -I/usr/include/x86_64-linux-gnu/c++/11 -I/usr/include/c++/11/backward Then run: /usr/lib/qt5/bin/moc -DJSON_DIAGNOSTICS=0 -DJSON_USE_IMPLICIT_CONVERSIONS=1 "-DvtkRenderingCore_AUTOINIT_INCLUDE=\"/home/alex/workspace/MTESS/build/CMakeFiles/vtkModuleAutoInit_be7301261a49b13d6a9b1d9e110eacd8.h\"" -I/home/alex/workspace/MTESS/src -I/home/alex/workspace/MTESS/vendor/ProjectStructure -I/home/alex/workspace/MTESS/vendor/imgui -I/home/alex/workspace/MTESS/vendor/imgui/imgui -I/usr/include/x86_64-linux-gnu/qt5 -I/usr/local/qwt-6.2.0/include -I/usr/local/include/opencascade -I/lib -I/usr/lib/llvm-14/lib/clang/14.0.0/include -I/usr/include/x86_64-linux-gnu -I/usr/include --include /home/alex/workspace/MTESS/build/MTESS_autogen/moc_predefs.h --output-dep-file -o /home/alex/workspace/MTESS/build/MTESS_autogen/MLM4FWGUYC/moc_NearFieldProcessUtil.cpp /home/alex/workspace/MTESS/src/Layers/ViewLayer/Process/NearFieldProcessUtil.h Expected file: moc_NearFieldProcessUtil.cpp got generated successfully,so question becomes: How to tell MOC to exclude gcc related includes?
• 

  UNSOLVED QRenderCaptureReply returns a screenshot with a background and no model
  • qingcheng  

  2
  0
  Votes
  2
  Posts
  63
  Views

  

  Hi, Please do not post the same question in multiple sub-forum. One is enough. Duplicate Closing this one.