Everything is data. Occasionally, when programming day-to-day objects and functions, you get a reminder that everything from the keys you press to the code it makes to the bits it reads is data.

But enough, say you want to use C++ with SQLite in a sane, object-oriented way. More specifically, you want to use sqlite3_exec to execute a SQL query. Here’s an example:

int rc = sqlite3_exec(db, sql, callback, data, &errMsg);

What is each one of these parameters? The first, db, is your opened database, sql is a const char * query you want to run, callback is a static function pointer, data is a peice of data you want to send to callback, and errMsg is what it sounds like. Okay, all’s well, lets define that callback.

static int callback(void *param, int argc, char **argv, char **azColName)
{
    return 0;
}

Damnit. This is going to be called once for every row, I don’t know when it’s going to end, and it’s static, throwing any hope for object-orientation out the window.

Or does it? Switch gears for a moment: What’s one of the hallmarks of object-oriented programming? I’m talking about the keyword this or self. What this is is difficult to describe to others because you’ll often find yourself saying something like, “this is this, like you know, this is this”. But this can be thought of as a hidden parameter passed along with every message sent to an object which can access the object that the message was sent to.

Well if this is what we’re after, this is a hidden parameter, and we’ve got a void * we can put whatever we want in, how about we just reconstruct our instance of an object using C++ casting?

First, make your request:

char* customer::getPerson(int id)
{
    ...
    rc = sqlite3_exec(db, sql, callback, this, &errMsg);
    ...
}

Then, define your C callback with C++ casting to turn person into this:

static int callback(void *param, int argc, char **argv, char **azColName)
{
    Person * person = reinterpret_cast<Person *>(param);
    return person->cppCallback(argc, argv, azColName);
}

Finally, profit:

int Person::cppCallback(int argc, char **argv, char **azColName)
{
    ...
}

Thanks to this StackOverflow post for the insight.