Is it possible to use global variables in Rust

Question

I know that in general  global-variables are to be avoided  Nevertheless  I think in a practical sense  it is sometimes desirable  in situations where the variable is integral to the program  to use them   In order to learn Rust  I m currently writing a database test program using sqlite3 and the Rust sqlite3 package on GitHub  Consequently  that necessitates  in my test-program   as an alternative to a global variable   to pass the database variable between functions of which there are about a dozen  An example is below    Is it possible and feasible and desirable to use global variables in Rust  Given the example below  can I declare and use a global variable      extern crate sqlite   fn main         let db  sqlite  Connection   open database         if  insert data  amp db  insert max            return            I tried the following  but it doesn t appear to be quite right and resulted in the errors below  I tried also with an unsafe block    extern crate sqlite   static mut DB  Option lt sqlite  Connection gt    None   fn main         DB   sqlite  open  test db   expect  Error opening test db        println   Database Opened OK         create table        println   Completed          Create Table fn create table         let sql    CREATE TABLE IF NOT EXISTS TEMP2  ikey INTEGER PRIMARY KEY NOT NULL        match DB exec sql            Ok      gt  println   Table created            Err err    gt  println   Exec of Sql failed      nSql      err  sql             Errors that resulted from compile   error E0308   mismatched types  -- gt  src main rs 6 10     6       DB   sqlite  open  test db   expect  Error opening test db                                                                         expected enum  std  option  Option   found struct  sqlite  Connection          note  expected type  std  option  Option lt sqlite  Connection gt                found type  sqlite  Connection   error  no method named  exec  found for type  std  option  Option lt sqlite  Connection gt   in the current scope   -- gt  src main rs 16 14      16       match DB exec sql

User · Answer

I am new to Rust  but this solution seems to work    macro use  extern crate lazy static   use std  sync   Arc  Mutex    lazy static        static ref GLOBAL  Arc lt Mutex lt GlobalType gt            Arc  new Mutex  new GlobalType  new         Another solution is to declare a crossbeam channel tx rx pair as an immutable global variable  The channel should be bounded and can only hold 1 element  When you initialize the global variable  push the global instance into the channel  When using the global variable  pop the channel to acquire it and push it back when done using it  Both solutions should provide a safe approach to using global variables

User · Answer

It s possible but no heap allocation allowed directly  Heap allocation is performed at runtime  Here are a few examples   static SOME INT  i32   5  static SOME STR   amp  static str    A static string   static SOME STRUCT  MyStruct   MyStruct       number  10      string   Some string      static mut db  Option lt sqlite  Connection gt    None   fn main         println        SOME INT       println        SOME STR       println        SOME STRUCT number       println        SOME STRUCT string        unsafe           db   Some open database              struct MyStruct       number  i32      string   amp  static str

User · Answer

Look at the const and static section of the Rust book   You can use something as follows   const N  i32   5     or  static N  i32   5    in global space   But these are not mutable  For mutability  you could use something like   static mut N  i32   5    Then reference them like   unsafe       N    1       println   N       N

User · Answer

You can use static variables fairly easily as long as they are thread-local    The downside is that the object will not be visible to other threads your program might spawn  The upside is that unlike truly global state  it is entirely safe and is not a pain to use - true global state is a massive pain in any language  Here s an example   extern mod sqlite   use std  cell  RefCell   thread local  static ODB  RefCell lt sqlite  database  Database gt    RefCell  new sqlite  open  test db      fn main         ODB with  odb cell            let odb   odb cell borrow mut               code that uses odb goes here             Here we create a thread-local static variable and then use it in a function  Note that it is static and immutable  this means that the address at which it resides is immutable  but thanks to RefCell the value itself will be mutable    Unlike regular static  in thread-local  static      you can create pretty much arbitrary objects  including those that require heap allocations for initialization such as Vec  HashMap and others   If you cannot initialize the value right away  e g  it depends on user input  you may also have to throw Option in there  in which case accessing it gets a bit unwieldy    extern mod sqlite   use std  cell  RefCell   thread local  static ODB  RefCell lt Option lt sqlite  database  Database gt  gt    RefCell  New None     fn main         ODB with  odb cell               assumes the value has already been initialized  panics otherwise         let odb   odb cell borrow mut   as mut   unwrap               code that uses odb goes here

User · Answer

Heap allocations are possible for static variables if you use the lazy static macro as seen in the docs  Using this macro  it is possible to have statics that require code to be executed at runtime in order to be initialized  This includes anything requiring heap allocations  like vectors or hash maps  as well as anything that requires function calls to be computed      Declares a lazily evaluated constant HashMap  The HashMap will be evaluated once and    stored behind a global static reference   use lazy static  lazy static  use std  collections  HashMap   lazy static        static ref PRIVILEGES  HashMap lt  amp  static str  Vec lt  amp  static str gt  gt              let mut map   HashMap  new            map insert  quot James quot   vec   quot user quot    quot admin quot             map insert  quot Jim quot   vec   quot user quot             map           fn show access name   amp str        let access   PRIVILEGES get name       println   quot          quot   name  access      fn main         let access   PRIVILEGES get  quot James quot        println   quot James       quot   access        show access  quot Jim quot

[global-variables] Is it possible to use global variables in Rust?

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