How do I reuse prepared statements with QSqlQuery?

Question

So part of the purpose of using (and reusing) prepared statements is that the database driver performs less work. Coming from Perl, I am used to preparing a SQL query and storing a reference to that prepared query so that I can later bind some values and execute the query, many times if necessary.

With Qt C++ (Qt 5.1) I try to do this:

    class MyClass {
            [...]
            QSqlDatabase db;
            QSqlQuery insert_query;
    };

    MyClass::MyClass() {
            db = QSqlDatabase::addDatabase("QSQLITE");
            db.setDatabaseName("whatever");
            db.open();

            insert_query = QSqlQuery(db);
            insert_query.prepare("insert into players (firstname, lastname) values(:firstname, :lastname)"));
    }

    void MyClass::MyMeth(QString firstname, QString lastname) {
            insert_query.bindValue(":firstname", firstname); //COMPILE ERROR
    }

error: no matching member function for call to 'bindValue'

note: candidate function not viable: 'this' argument has type 'const QSqlQuery', but method is not marked const

But I want to bind new values in the prepared query outside of the MyClass constructor. I found this, but I am suspicious that it is cargo-cult because calling QSqlQuery::prepare("query") more than once is effectively a noop if "query" remains the same (even if the QSqlQuery object is different). Is this true with some drivers? Otherwise, what am I missing? How am I supposed to reuse prepared queries?

fxam · Accepted Answer · 2013-09-23 01:35:44Z

3

Your compiler error has nothing to do with the re-usedfulness of QSqlQuery. Did you declare MyMeth as const? Remove the const, it's preventing call to the non-const bindValue().

Change

void MyMeth(QString firstname, QString lastname) const;

to

void MyMeth(QString firstname, QString lastname);

answered Sep 23, 2013 at 1:35

fxam

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1 Comment

njahnke Over a year ago

Bingo! I missed this because I forgot I was in an overridden method with const inherited. I need bind the values in another method I control the signature of.

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