Confused with how example contract successfully calls external contract

Question

I'm trying to learn Solidity and I came across the following 2 contracts:

https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/45_pong.sol https://github.com/fivedogit/solidity-baby-steps/blob/master/contracts/45_ping.sol

The contract pong holds some value, and the contract ping is supposed to be able to access this value via the getPongvalTransactional function. I'm confused with how this function works (disclaimer: I have no background in object oriented programming).

If I understand correctly, in the ping contract, all contracts of type PongvalRetriever contain a function called getPongvalTransactional which returns -1. The pong contract address is declared to be a contract of type PongvalRetriever.

However, the pong contract also contains a function called getPongvalTransactional which returns the actual value we want. In following line of the ping contract

function getPongvalRemote() 
{
    pongval = pvr.getPongvalTransactional();
}

why does pongval equal the actual pong value instead of -1?

Answer 1

This is a technique called method overriding. Essentially, every subclass (in this case derived contract) of PongvalRetriever will contain by default a function getPongvalTransactional, which returns -1.

However, if a subclass, in this case the Pong contract, defines a method with the same name, that method "overrides" the superclass's method, so the method defined in Pong is what is actually executed.

Then, in the Ping contract, getPongvalRemote sets pongval to the return value of getPongvalTransactional, which should not be -1. The -1 is used to indicate an error, i.e. the function is for some reason not defined, so that 0 can be used as a legitimate value.

Not that pongval = pvr.getPongvalTransactional() is not making a statement about the equality of the two expressions, but is setting the value of pongval to the value of the right side of the expression.