Can't read file data twice using QAxObject

Question

I read excel file using QAxBase and QAxObject.

I got global variable QAxObject* db_workbook; where I store pointer to some workbook(don't know why it called like that, but whatever) in excel. I need it because of throught it I need to get excel file data in few functions, not just only one.

When readExcelFile method executes fine, in test_function() an exception

enter image description here

appears at this line sheet_N = db_workbook->querySubObject("Worksheets(int)", 1);

Why does it happens and how to fix it?

Code part is here

//
QAxObject* db_workbook;

//for read what we need to
void importdb_module::readExcelFile(QAxObject* excel, QString& file_path){

    if(initExcel(excel)){

        QAxObject* workbooks = excel->querySubObject("WorkBooks");
        workbooks->dynamicCall("Open (const QString&)", file_path);
        QAxObject* workbook = excel->querySubObject("ActiveWorkBook");
        db_workbook = workbook;//global ptr points same adress now
        QAxObject* worksheets = workbook->querySubObject("WorkSheets");

        //test getting sheet num 1 name
        QAxObject* sheet_hh = workbook->querySubObject("Worksheets(int)", 1);

        QString sheet_name = sheet_hh->property("Name").toString();

        qDebug()<<sheet_name<<"TEST!";//here everything works fine
    }
}

void importdb_module::test_function(){
    QAxObject* sheet_N;
    //ERROR IS HERE!
    sheet_N = db_workbook->querySubObject("Worksheets(int)", 1);

    QString sheet_name = sheet_N->property("Name").toString();

    qDebug()<<sheet_name;
}


//executes on button click
void importdb_module::testExlOp(QString &_path){
    QAxObject* excel;
    QStringList spreadsheet_list; //get spreadsheet list when opening file
    QString path = _path;//gonna use GUI choose

    if(initExcel(excel)){
        if (readExcelFile(excel, path)){
            //
            test_function();
            excel->dynamicCall("Quit(void)");
            }else{
            //error output
            QMessageBox::information(0, "", "Error");
        }
    }


    delete excel;
}

Thank you!

What happens if you repeat the `QAxObject* sheet_hh = workbook->querySubObject("Worksheets(int)", 1);` query with a second variable immediately after? What happens if this line appears before the line `QAxObject* worksheets = workbook->querySubObject("WorkSheets");`? — abiessu, Oct 29 '13 at 19:57
This just looks like a giant memory leak. Surely there's a delete call *somewhere*? Which kills that global variable. — Hans Passant, Oct 29 '13 at 20:03
@dauphic not, I checked it before, look http://i.imgur.com/L8X3wbH.png http://i.imgur.com/jIvYrZ1.png — DanilGholtsman, Oct 29 '13 at 20:05
@abiessu sorry i cant check it right now, deep night here, got no VS and Qt in my laptop (I made post text and screenshots few hours before posting it) — DanilGholtsman, Oct 29 '13 at 20:09
@DanilGholtsman: Yes, your `db_workbook` is pointing to garbage. It's not a valid object; you can tell by looking at `__vfptr` and noticing that it isn't pointing to valid memory. — Collin Dauphinee, Oct 29 '13 at 20:10
@dauphic hmm, I'm pretty sure that I didn't delete any related stuff. — DanilGholtsman, Oct 29 '13 at 20:13
@dauphic: we have enough code I think, there is no place for a delete between the lines `if (readExcelFile(excel, path)){` and `test_function();` in the main function except possibly for the `qDebug()` bit. — abiessu, Oct 29 '13 at 20:17

score 1 · Accepted Answer · answered Oct 29 '13 at 21:54

1

Probably you have a dangling pointer. Use QPointer<QAxObject> instead of a naked pointer. It will reset itself to null when the instance of QAxObject gets destructed.

Generally speaking, in this day and age, you're not supposed to be using naked pointers for anything that's not implicitly owned by something else. This means that for QObjects that have parents you don't need to do use smart pointers (although it doesn't hurt any). Generally speaking, smart pointers don't hurt. Use them.

answered Oct 29 '13 at 21:54

Kuba hasn't forgotten Monica

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Oh, never heard about such things before. I changed global var declaration `QPointer database_workbook;` but I got still same error. Well, do I need to use `QPointer` instaed of every `QAxObject` pointer? – DanilGholtsman Oct 30 '13 at 03:19
Well, I fixed it, your answer really helped. Such stuff is so complicated and I am so stupid :\ – DanilGholtsman Oct 30 '13 at 17:42

Can't read file data twice using QAxObject

1 Answers1