是否有可能在Rust中使用全局变量？ [英] Is it possible to use global variables in Rust?

问题描述

我知道，一般来说，全局变量是应该避免的。尽管如此，我认为从实际意义上讲，它有时是可取的（在变量是程序不可缺少的情况下）。

我目前正在使用sqlite3和GitHub上的Rust / sqlite3包编写数据库测试程序。因此，必须（在我的测试程序中）（作为全局变量的一种替代方法）在数据库变量之间传递数据库变量。一个例子如下。

1. 在Rust中使用全局变量是否可行也是可行和可行的？




2. 给出下面的例子，我可以声明并使用一个全局变量吗？

  extern crate sqlite; 
 
 fn main（）{
 let db：sqlite :: Connection = open_database（）; 
 
 if！insert_data（& db，insert_max）{
 return; 
 
 
 $ / code $ / pre 
 
我尝试了以下，但它没有出现（我尝试了 unsafe  block）：
  extern crate sqlite; 
 
 static mut DB：Option< sqlite :: Connection> =无; 
 
 fn main（）{
 DB = sqlite :: open（test.db）。expect（Error open test.db）; 
 println！（Database Opened OK）; 
 
 create_table（）; 
 println！（已完成）; 
 
 $ b $ //创建表
 fn create_table（）{
 let sql =CREATE TABLE IF NOT EXISTS TEMP2（ikey INTEGER PRIMARY KEY NOT NULL）; 
匹配DB.exec（sql）{
 Ok（_）=> println！（创建表），
 Err（err）=> println！（Exec of Sql failed：{} \\\
Sql = {}，err，sql），
} 
} 
  
编译产生的错误： 
 $ b 
 错误[E0308]：不匹配的类型
  - > src / main.rs：6：10 
 | 
 6 | DB = sqlite :: open（test.db）。expect（Error open test.db）; 
 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^预期enum`std :: option :: Option`，找到struct`sqlite :: Connection` 
 | 
 = note：期望的类型`std :: option :: Option< sqlite :: Connection>`
找到的类型`sqlite :: Connection` 
 
错误：在当前作用域中为`std :: option :: Option< sqlite :: Connection>`类型找到的exec` 
  - > src / main.rs：16：14 
 | 
 16 |匹配DB.exec（sql）{
 | ^^^^ 
  
 
 
解决方案
这是可能的，但不允许堆分配直。堆分配在运行时执行。这里有几个例子： 
 
 
  static SOME_INT：i32 = 5; 
 static SOME_STR：&'static str =一个静态字符串; 
 static SOME_STRUCT：MyStruct = MyStruct {
 number：10，
 string：Some string，
}; 
 static mut db：Option< sqlite :: Connection> =无; 
 
 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：&'static str，
} 
  
 
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(&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<sqlite::Connection> = 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(_) => println!("Table created"),
        Err(err) => println!("Exec of Sql failed : {}\nSql={}", err, sql),
    }
}
Errors that resulted from compile:
error[E0308]: mismatched types
 --> 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<sqlite::Connection>`
             found type `sqlite::Connection`

error: no method named `exec` found for type `std::option::Option<sqlite::Connection>` in the current scope
  --> src/main.rs:16:14
   |
16 |     match DB.exec(sql) {
   |              ^^^^

解决方案
It's possible but no heap allocation allowed directly. Heap allocation is performed at runtime. Here are few examples:
static SOME_INT: i32 = 5;
static SOME_STR: &'static str = "A static string";
static SOME_STRUCT: MyStruct = MyStruct {
    number: 10,
    string: "Some string",
};
static mut db: Option<sqlite::Connection> = 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: &'static str,
}


                        
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