是否可以在 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，应该避免使用全局变量.尽管如此，我认为在实际意义上，有时(在变量对程序不可或缺的情况下)使用它们是可取的.

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.

为了学习 Rust，我目前正在使用 GitHub 上的 sqlite3 和 Rust/sqlite3 包编写数据库测试程序.因此，这需要(在我的测试程序中)(作为全局变量的替代方案)在大约有十几个函数之间传递数据库变量.下面是一个例子.

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.

1. 在 Rust 中使用全局变量是否可行、可行和可取?

1. Is it possible and feasible and desirable to use global variables in Rust?

给出下面的例子，我可以声明和使用一个全局变量吗?

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;
    }
}

我尝试了以下操作，但似乎不太正确并导致以下错误(我也尝试使用 unsafe 块):

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 : {}
Sql={}", err, sql),
    }
}

编译导致的错误:

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) {
   |              ^^^^

推荐答案

是可以的，但是不能直接分配堆.堆分配在运行时执行.以下是一些示例:

It's possible, but heap allocation is not allowed directly. Heap allocation is performed at runtime. Here are a 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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