可见性

用于函数对外调用,相比于public,可以省gas,不可以直接内部调用,但可以this.f
但不能修饰状态变量

pragma solidity >= 0.4.0 <0.9.0;

contract States{
    int private privateData;
    int public publicData;//自动增加view的getter()
    int internal internalData;
    //int external externalData; //状态变量不能定义为external
}

contract State is States{

    int internal a1;
    //int external a2; //状态变量不能定义为external

    function internalFun() internal{
    
    }

    function externalFun() external{

    }

    constructor(){
        internalFun();
        this.externalFun();    //external,内部调用需要加this.
        //privateData = 1; //private 子类不可见
        publicData = 2;
        internalData = 3; 
        //this.externalData = 4;
    }
}