Parameter type may not live long enough

Question

I have a simple program where I am trying to implement a polymorphic account type:

enum AccountType {
    INVALID,
    TYPE1,
    TYPE2,
}

trait Account {
          


        

Answer 1

The compiler's suggestion actually works. If you write add_account as follows:

fn add_account<A: Account + 'static>(&mut self, account: A) {
    self.accounts.push(Box::new(account));
}

your code compiles. (Incidentally, you need &mut self, not &self here)

Answer 2

I'll try to give a more thorough answer: the issue has to do with the definition of the accounts member of Accounts. Vec<Box<Account>> in this context is equivalent to Vec<Box<Account + 'static>>, i.e. the box can't contain any references to data on the stack. On the other hand, the declaration of add_account doesn't restrict the lifetime of the type: it's equivalent to fn add_account<'a, A: Account + 'a>(&self, account: A) {.

The solution is to make sure the type A lives long enough. The simplest approach is to just add the A: 'static bound suggested in the error message (fn add_account<A: Account + 'static>(&self, account: A) {).

If you don't want to copy the Account data, you can do something more complicated, like this:

struct Accounts<'a> {
    accounts: Vec<&'a Account + 'a>
}

impl<'a> Accounts<'a> {
    fn new() -> Accounts<'a> {
        Accounts { accounts: Vec::new() }
    }

    fn add_account<A: Account + 'a>(&mut self, account: &'a A) {
        self.accounts.push(Box::new(account));
    }
}

At this point, though, you have a data structure which is probably more general than you actually need.