如果我把 channels 这个函数里面的 Channels storage c， 改为 memory 就可以正常赋值。

但是设置为 storage 就无法进行赋值了，很疑惑改成 storage就无法赋值了。

报错信息：this variable is of storage pointer type and can be accessed without prior assignment, which would lead to undefined behaviour .

直接翻译是：此变量是存储指针类型，可以在没有事先赋值的情况下访问，这将导致未定义的行为。

pragma solidity ^0.7.6;

contract ChannelControl {

struct Channels{ 
  bool exit;  
  uint256 index;
}

mapping(uint256 => mapping(uint256=> Channels)) public ChannelsMapping;

uint256 index = 0;

function channels() public{
    Channels storage c;
    c.exit = true;
    c.index = index;
    ChannelsMapping[index][index] = c;
    index ++;
}