How do a pass a database variable to a function in Rust? - rust

Question

I'm just starting to look at Rust. I wanted to experiment with a database, and found the sqlite repo which is good to have to experiment with.
I would like to know the "correct" way to pass the sqlite database variable to a function. The error messages that I was initially getting from the compiler appeared to indicate that when I passed the Db variable from main() to the function, it was gone, so I returned it. Although this appears to work, it doesn't seem to me that it would be the normal way. While I'm not a believer in a large number of Global variables, I attempted to create a Global variables, but I couldn't discover how to do that.
Below is the test program. Please note that I am not yet using the Rust naming conventions, but it is very-early days
The main lines in question are :
oDb1 = fCreateTable(oDb1);
fn fCreateTable(oDb1:sqlite::database::Database) -> sqlite::database::Database {
and what is the alternative and why is it necessary (in this instance) to return it?
Example program:
extern mod sqlite;
fn main() {
let mut oDb1:sqlite::database::Database;
oDb1 = fOpenDb();
oDb1 = fCreateTable(oDb1) ;
let mut iInsertTot: int = 0;
while iInsertTot < 25 {
let oDbExec = oDb1.exec("INSERT INTO test (sname, iborn) VALUES ('xxxxx', 1973)");
if (! oDbExec.is_ok()) {
fail!(fmt!("Insert Nr. %d Failed!", iInsertTot+1));
}
iInsertTot += 1;
}
println (fmt!("Inserts completed = %d", iInsertTot));
}
fn fOpenDb() -> sqlite::database::Database {
let oDbOpen = sqlite::open("test.db");
if oDbOpen.is_err() {
fail!(fmt!("Error opening test.db: %?", oDbOpen));
}
println(fmt!("Database Open OK? %?", oDbOpen.is_ok()));
oDbOpen.unwrap()
}
fn fCreateTable(oDb1:sqlite::database::Database) -> sqlite::database::Database {
let mut oDbExec = oDb1.exec("drop table if exists test");
println(fmt!("Drop Table OK? %?", oDbExec.is_ok()));
if (!oDbExec.is_ok()) {
fail!("Drop-table failed");
}
oDbExec = oDb1.exec("CREATE TABLE test (ikey INTEGER PRIMARY KEY not null,
sname text, iborn int)");
println(fmt!("Create OK? %?", oDbExec.is_ok()));
if !oDbExec.is_ok() {
fail!("Create Table failed");
}
oDb1
}

Answer 1

sqlite::database::Database implements Drop, meaning it has a destructor, meaning it is never copied and always moved: fCreateTable(oDb1) moves the database object out of oDb1: Now there's nothing left in oDb1! Of course, you can put back something. For example, when you return the database from fCreateTable, you again move - back into fCreateTable.
But this is a silly dance. Just don't move the database in the first place, borrowed a pointer to it:
fn main() {
let oDb1 = fOpenDb();
fCreateTable(&oDb1);
...
}
fn fCreateTable(oDb1: &sqlite::database::Database) {
...
}