# MongoDB\Driver\ReadConcern::isDefault

(mongodb >=1.3.0)

MongoDB\Driver\ReadConcern::isDefault — Checks if this is the default read concern

### 说明

final public MongoDB\Driver\ReadConcern::isDefault(): bool

Returns whether this is the default read concern (i.e. no options are specified). This method is primarily intended to be used in conjunction with MongoDB\Driver\Manager::getReadConcern() to determine whether the Manager has been constructed without any read concern options.

The driver will not include a default read concern in its read operations (e.g. MongoDB\Driver\Manager::executeQuery()) in order order to allow the server to apply its own default. Libraries that access the Manager's read concern to include it in their own read commands should use this method to ensure that default read concerns are left unset.

### 参数

此函数没有参数。

### 返回值

Returns `true` if this is the default read concern and `false` otherwise.

### 错误／异常

- Throws MongoDB\Driver\Exception\InvalidArgumentException on argument parsing errors.

### 示例

示例 #1 MongoDB\Driver\ReadConcern::isDefault() example

`<?php
$rc = new MongoDB\Driver\ReadConcern(null);
var_dump($rc->isDefault());
$rc = new MongoDB\Driver\ReadConcern(MongoDB\Driver\ReadConcern::MAJORITY);
var_dump($rc->isDefault());
$manager = new MongoDB\Driver\Manager('mongodb://127.0.0.1/?readConcernLevel=majority');
$rc = $manager->getReadConcern();
var_dump($rc->isDefault());
$manager = new MongoDB\Driver\Manager('mongodb://127.0.0.1/');
$rc = $manager->getReadConcern();
var_dump($rc->isDefault());
?>`

以上示例会输出：

    bool(true)
    bool(false)
    bool(false)
    bool(true)

### 参见

- MongoDB\Driver\Manager::getReadConcern() - Return the ReadConcern for the Manager
- » Read Concern reference

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