could somebody please explain const to me?

sorry for posting what i am afraid is a pure c++-problem here, but at least it is about a qt-program:
i have a loop going through a query and a QMap where i want to store the type and name of each column that is found. so i am trying to put the result of QSqlField::type() into my QMap, but this gives an error because it is const. Why is that? after all i had expected i am actually only copying the value into my QMap so no need to care about the original being const?
the code looks like this:

Code:

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while (query.next()) {
        QSqlRecord rec = query.record();
        for (int col=0;col<rec.count();col++) {
            QSqlField field = rec.field(col);
            if (row == 0) {
                QMap<QString,QVariant>* currCol = &columns.value(col);
                currCol->insert("type",field.type());  // first error
                currCol->insert("name",field.name()); // second error
            }
            qDebug() << "working on row " << row << ", column " << col;
        }
        tableData.value(row) = rec; //third error
        row++;
    }

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and in the .h i have:

Code:

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QMap<int,QSqlRecord> tableData;
QMap<int,QMap<QString,QVariant> > columns;

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the errors i am getting are all like this:

Quote:

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passing `const QString' as `this' argument of `QString& QString::operator=(char)' discards qualifiers

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Please post the exact error messages and tell us which lines they correspond to.

Quote:

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Originally Posted by mikro

Code:

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QMap<QString,QVariant>* currCol = &columns.value(col);
currCol->insert("type",field.type());  // first error
currCol->insert("name",field.name()); // second error

---

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According to trolltech's docs value() method of QMap class returns const value:

Code:

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const T value ( const Key & key ) const

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so you cannot assign address of this value to pointer to non-const objects.

EDIT:
There is also another problem. What value() returns is temporary object. It's not advisable to take and use address of temporary object.

BTW: "Edit" button doesn't work under Opera.

yes, that was the problem - damn... i had that one before. the problem was, that at first the lines were

Code:

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columns.value(col).value("type") = field.type();
columns.value(col).value("name") = field.name();

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and since i splitt that a little to make it better readable i didn't look at the line above anymore, especially as i understood the errormessage to be complaining about field.type() being a const QString not about the part on the left side being const. obviously this is also the problem with the third error. Thanks

nothing specific to the problem but as an explaination of 'const', (among many other things)

http://www.parashift.com/c++-faq-lite/

K