Detailed explanation of ThinkPHP functions: D method

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D method

D method should be the most commonly used method. It is used to instantiate custom model classes. It is an encapsulation of Model class instantiation by the ThinkPHP framework. It also implements the singleton mode and supports cross-project and group calls. The calling format As follows:
D('[Project://][Group/]Model','Model layer name')
The return value of the method is the instantiated model object.

D method can automatically detect the model class. If a custom model class exists, the custom model class will be instantiated. If it does not exist, the Model base class will be

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instantiated, and at the same time Models that have already been instantiated will not be instantiated again. The most common usage of the
D method is to instantiate a custom model of the current project, for example:

1. // 实例化User模型
    $User = D('User');

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will import the Lib/Model/UserModel.class.php file under the current project, and then instantiate the UserModel class, so the actual code may be equivalent to the following:

1. import('@.Model.UserModel');
    $User = new UserModel();

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But if you use the D method, if the UserModel class does not exist, it will be automatically called

1. new Model('User');

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And there is no need to instantiate again when calling for the second time, which can reduce a certain amount of object instantiation overhead.

D method can support instantiating models across groups and projects, for example:

1. //实例化Admin项目的User模型
    D('Admin://User')
    //实例化Admin分组的User模型
    D('Admin/User')

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Note: To implement the cross-project calling model, you must ensure that the directory structures of the two projects are parallel.

Starting from version 3.1, due to the added support for hierarchical models, the D method can also instantiate other models, for example:

1. // 实例化UserService类
    $User = D('User','Service');
    // 实例化UserLogic类
    $User = D('User','Logic');

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# #

D('User','Service');

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will import Lib/Service/UserService.class.php and instantiate it, which is equivalent to the following code:

import('@.Service.UserService');
 $User = new UserSerivce();

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