如何将数据库变量传递给Rust中的函数？

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) {
    ...
}