Features of contracts

It is time to delve more deeply into contracts. Let's start with some new features and then we will go deeper into the features we have already seen.

Visibility

The visibility of a state variable or a function defines who can see it. There are four kinds of visibility: external, public, internal, and private.

By default, the visibility of functions is public and the visibility of state variables is internal. Let's see what these visibility functions mean:

• external: External functions can only be called from other contracts or via transactions. For example, we cannot call an f external function internally: f() will not work but this.f() will. We also cannot apply external visibility to state variables.
• public: Public functions and state variables can be accessed in every possible way. Compiler-generated accessor functions are all public state variables. It is not possible to create our own accessors. Actually, it generates only getters, not setters.
• internal: Internal functions...