QMap function: check it contains isbn number

Question

I am trying to write a function to determine if a given isbn number is in a QMap list, I see I'm not using contains properly, not sure if this would even logically work?

bool TextbookMap::foundTextBook(QString isbn) const {
    bool found = false;
    ConstIterator itr = constBegin();
    for ( ; itr != constEnd(); ++itr)
    {
         if (itr.contains(isbn))
             found = true;
    }
    return found;
}

Answer 1

You do not need to iterate the map - this is what contains() already does for you. Simply use

bool TextbookMap::foundTextBook(const QString& isbn) const {
   return contains(isbn);
}

(I assume that you have derived TextbookMap from QMap)

The below code prints

false
true

---

class Textbook {
};

class TextbookMap : public QMap<QString, Textbook*> {
public:
    bool foundTextBook(const QString& isbn) const;
};

bool TextbookMap::foundTextBook(const QString& isbn) const {
    return contains(isbn);
}

int main(int argc, char ** argv) {
    TextbookMap map;
    map.insert("1234", new Textbook());
    map.insert("5678", new Textbook());
    qDebug() << map.foundTextBook("01234");
    qDebug() << map.foundTextBook("1234");

    return 0;
}

In this example, you do not even need to implement a separate method - you could also use map.contains() directly. But it depends on your concrete requirements whether it makes sense to encapsulate the contains() method like this. On a side note, I usually try to avoid deriving from container classes and use delegation instead.