Ethereum 为什么要将存在的solidity合同实例作为参数传递给合同构造函数

有关以下代码段，请参见

为什么这一行（将constact实例传递给构造函数）起作用<代码>计数器（_计数器[msg.sender]）。增量（msg.sender）

我在使用混音IDE

contract CounterFactory {
 
    mapping(address => Counter) _counters;

    function createCounter() public {
        require (_counters[msg.sender] == Counter(0));
        _counters[msg.sender] = new Counter(msg.sender);
    }
    
    function increment() public {
        require (_counters[msg.sender] != Counter(0));
        Counter(_counters[msg.sender]).increment(msg.sender);
        // The following line also works as usual.
        // _counters[msg.sender].increment(msg.sender);
    }
    
    function getCount(address account) public view returns (uint256) {
        require (_counters[account] != Counter(0));
        return (_counters[account].getCount());
    }
    
    function getMyCount() public view returns (uint256) {
        return (getCount(msg.sender));
    }
 
}