2  继承

pragma solidity 0.4.8;

// Solidity supports multiple inheritance by copying code including polymorphism.

// All function calls are virtual, which means that the most derived function is called, 
// except when the contract name is explicitly given.

// Even if a contract inherits from multiple other contracts, only a single contract 
// is created on the blockchain, the code from the base contracts is always copied into the final contract.

// In general inheritance system in solidity is very similar to Python’s, 
// especially concerning multiple inheritance.

// Lets checkout one example

// We will create a simple base class first
contract owned {
    function owned() { owner = msg.sender; }
    address owner;
}

// Now we can extende the owned using is keyword like this
contract mortal is owned {

	// here kill will have the access of owner the variable 
	// which is available in the owned contract
	// please note that owned contract's constructor will be 
	// executed at the time of mortal's initialization
    function kill() {
        selfdestruct(owner);
    }
}

// Multiple inheritance is also possible. Note that "owned" is
// also a base class of "mortal".
// in such cases the order of inheritance is important to avoid the 
// Diamond problem (which is know in multiple inheritance)
// Let's take an example.


contract User is mortal, owned {

    string public UserName;

    function User(string _name){
        UserName = _name;
    }
    // here User inherits both the cntracts owned & mortal which is resulting in diamond problem
    // Changing the order of inheritance might resolve the issue of this diamond problem.
    // The reason for this is that "User" requests "owned" to override 
    // "mortal" (by specifying mortal, owned in this order), but "mortal" itself 
    // requests to override "owned", which is a contradiction that cannot be resolved.
    // Here reverting the order will solve this problem
    // A simple rule to remember is to specify the base classes 
    // in the order from “most base-like” to “most derived”.
}