Ch13: Classes

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Overview

Everything you have written so far has been made of the types somebody else defined: ::std::int_least32_t, ::fast_io::string, ::fast_io::str_btree_map. A class is how you define a type yourself.

A class puts data and the functions that work on that data into one thing, gives that thing a name, and then decides what the rest of the program is allowed to do with it. That last part is the point of the whole chapter: a class is not just a bag of fields, it is a small set of promises with the implementation hidden behind them.

The chapter follows one type from a first draft to a finished design: a point at the beginning, an account by the end. Each subchapter adds one idea, and the code grows with it.

You want to	Read
Put data and behaviour into one type	Ch13.1, Ch13.2
Write functions that cannot accidentally change an object	Ch13.3
Decide what the outside world may touch	Ch13.4, Ch13.5
Control how an object comes into existence	Ch13.6
Understand the invisible `this` argument	Ch13.7
Keep one variable or one function for the whole class	Ch13.8, Ch13.9
Let exactly one outsider reach the private parts	Ch13.10

A class is a promise, not a container. If a class promises “the balance is never negative”, then every function in the class has to keep that promise true, and the caller is allowed to stop checking. Encapsulation is how you make that promise believable: the data is private, so the only code that can break the promise is the code you wrote in that class.

Conventions Used in This Chapter

No int. Integer types are written ::std::int_least32_t (or ::std::int_least64_t for money), and sizes are ::std::size_t. The width of an int is not guaranteed by the language; the width of these types is. Use the type that says what you mean, not the shortest one to type.
Private data members end with _ (balance_), public data members do not (x). One glance tells you which side of the wall a name lives on.
Every member function is noexcept or throws, exactly like a free function (Ch6.16). A member function that can fail is declared throws, and the caller handles it with try { } catch throws (::std::error e) { } (Ch3.16).
Every example is a complete program and the output under it is the output you get when you run it.

Ch13: Classes

Overview

Conventions Used in This Chapter

Contents