Php-ci in the framework of $this->config, $this >input call problems

Framework PHP

Within the company's project program, the custom class directly $this->config->item () refers to the system definition class Congfig inside the method, want to ask the big God. $this->config->item () The config at this time is the property of the current class, but there is no way to find the current class has a definition of $this->config.
System file codeigniter.php has such a statement: $CFG =& load_class (' Config ', ' core '). But this is the config.php file loaded outside the custom class, why the $this->config->item () can be used directly in the current class. I hope the great God can help me to answer.

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