`import`语句的用法 [英] Usage of the `import` statement

问题描述

有人可以向我解释 import 语句是如何工作的吗？例如我有一个 myapp / app / models 包中的$ c> User ：

pre > 包模型

类型用户结构{
//导出字段
}


在 myapp / app / controllers Users >包：

 包控制器
 
导入（
 _myapp / app / models 
github.com/revel/revel
）
 
类型用户struct {
 * revel.Controller 
} 
 
 func（c Users）HandleSubmit（user * User）revel.Result {
 //此处代码
} 
  

这给了我以下错误：

undefined：User

我尝试将导入更改为以下代码：

 导入（
。advorts / app / models
github.com/revel/revel
）
  

但是得到这个错误：

undefined：myapp / app / controllers.User

我也不明白。那么， import的区别是什么？ something和导入something？如何正确导入我的模型在我的情况？

解决方案

有一组类型，函数，变量等。让我们称它们为实体。每个实体都可以被导出（其名称以大写字母开头）或未导出（名称以小写字母开头）。



包只能访问导出另一包装的物品。要做到这一点，它需要 import 它，这将使导出的实体可以使用包名作为标识符。示例：



  importgithub.com/revel/revel
  

将获得 revel 包的所有导出实体，这些包将可用 revel 。前缀。在 revel.Controller 中，它是 revel Controller 类型c $ c> package。



您可以通过在导入路径前添加所需的标识符来为包标识符设置别名。示例：



  import revgithub.com/revel/revel
  

将导入所有 revel entite，其标识符为 rev 。因此 revel.Controller 变为 rev.Controller 。如果您有多个具有相同名称的包或具有荒谬长名称的包，它非常有用。



作为奖励，您可以通过别名匿名导入包它到了空白标识符：



  import _github.com/revel/revel
  

它将导入包，但不会让您访问导出的实体。这对于需要导入但不能访问的驱动程序非常有用。一个常见的例子是数据库驱动程序，它们将自己注册到数据库/ sql 包，因此您不需要直接访问它们。

作为额外的奖励，您还可以在本地导入一个包，并使用。标识符进行别名。然后，导出的实体将无需标识符即可使用，就像您将它们定义在同一个包中一样。

如何正确导入包是由您决定的。一般惯例是永远别名，如果你可以管理它，隐藏你不需要访问但仍然需要导入的包（数据库驱动程序），就这些了。你真的从不需要在本地导入一个包，即使有些教程或框架是为了简单起见。

Can someone explain me how the import statement works ?

For example I have a type User in the myapp/app/models package:

package models

type User struct {
    // exportod fields
}

I have a type Users in the myapp/app/controllers package:

package controllers

import (
    _ "myapp/app/models"
    "github.com/revel/revel"
)

type Users struct {
    *revel.Controller
}

func (c Users) HandleSubmit(user *User) revel.Result {
    // Code here
}

This gives me the following error:

undefined: User

I tried to change the imports to the following code:

import (
    . "advorts/app/models"
    "github.com/revel/revel"
)

But get this error:

undefined: "myapp/app/controllers".User

Which I don't understand either. So, what is the difference between import . "something" and import "something" ? How to properly import my model in my case ?

解决方案

Each package has a set of types, functions, variables, etc. Let's call them entities. Each entity can be either exported (its name start with an Uppercase letter), or unexported (its name start with a lowercase letter).

A package can only access the exported entites of another package. To do this, it needs to import it, which will make the exported entites available with the package name as identifier. Example:

import "github.com/revel/revel"

will get all exported entites of the revel package, which will be available using revel. prefix. As in revel.Controller, which is the Controller type defined in the revel package.

You can alias a package identifier by prefixing the import path with the wanted identifier. Example:

import rev "github.com/revel/revel"

will import all revel entites with the identifier rev. So revel.Controller becomes rev.Controller. It is useful if you have multiple package with the same name, or a package with an absurdly long name.

As a bonus, you can import a package anonymously, by aliasing it to the blank identifier:

import _ "github.com/revel/revel"

which will import the package, but not give you access to the exported entities. It is useful for things like drivers, which you need to import but never access. A frequent example is the database drivers, which register themselves to the database/sql package so you never need to access them directly.

And as a bonus' bonus, you can also import locally a package, by aliasing it with the . identifier. The exported entites will then be available without identifier, as if you defined them in the same package.

How to properly import your packages is up to you. The general convention is to never alias if you can manage it, to hide the package that you don't need to access but still need to import (database drivers), and that's all. You really never need to import locally a package, even if some tutorials or frameworks do it for simplicity's sake.

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