C++ method that can/cannot return a struct

I have a C++ struct and a method:

struct Account
{
    unsigned int id;
    string username;
    ...
};


Account GetAccountById(unsigned int id) const { }

I can return an Account struct if the account exists, but what to do if there's no account?

I thought of having:

• An "is valid" flag on the struct (so an empty one can be returned, with that set to false)
• An additional "is valid" pointer (const string &id, int *is_ok) that's set if the output is valid
• Returning an Account* instead, and returning either a pointer to a struct, or NULL if it doesn't exist?

Is there a best way of doing this?