php构造函数似乎不起作用

Question

I have this class:

class template {

    private $_db;

    public function __construct(){

        $this->_db = \MysqliDb::getInstance();
    }


    public function get(){

        return $this->_db->rawQuery("SHOW COLUMNS FROM rank LIKE 'view_template'");
    }
}

But, when I execute the method get, I get this error message:

Fatal error:  Call to a member function rawQuery() on a non-object in {dir/to/my/file} on line 50

Line 50 is return $this->_db->rawQuery("SHOW COLUMNS FROM rank LIKE 'view_template'");

The weird thing is that it works fine if I move the code from __construct to the get method, like this:

class template {

    private $_db;

    public function get(){

        $this->_db = \MysqliDb::getInstance();
        return $this->_db->rawQuery("SHOW COLUMNS FROM rank LIKE 'view_template'");
    }
}

What can be wrong in this case?

Answer 1

I think

\MysqliDb::getInstance()

; isn't returning a valid object at the time when the construct is called, this is why you are getting the error.

You could check the object returned in the construct to see if a valid object is returned before calling the get method

public function __construct(){ $this->_db = \MysqliDb::getInstance(); var_dump( $this->_db); }