construct/doc/STYLE.md

# How to CPP for IRCd


In the post-C++11 world it is time to leave C99+ behind and seriously consider
C++ as C proper. It has been a hard 30 year journey to finally earn that, but
now it is time. This document is the effective style guide for how Charybdis
will integrate -std=gnu++17 and how developers should approach it.


### C++ With Respect For C People


Remember your C heritage. There is nothing wrong with C, it is just incomplete.
There is also no overhead with C++, that is a myth. If you write C code in C++
it will be the same C code. Think about it like this: if C is like a bunch of
macros on assembly, C++ is a bunch of macros on C. This guide will not address
any more myths and for that we refer you [here](https://isocpp.org/blog/2014/12/myths-3).

###### Repeat the following mantra:
1. How would I do this in C?
2. Why is that dangerous, hacky, or ugly?
3. What feature does C++ offer to do it right?

This can be applied to many real patterns seen in C software which really beg
for something C++ did to make it legitimate and proper. Examples:
* Leading several structures with the same member, then casting to that leading
type to deal with the structure abstractly for container insertion. -> Think
inheritance.
* Creating a structure with a bunch of function pointers, then having a user
of the structure fill in the pointers with their own functionality. -> Think
virtual functions.
* `if` statements that check for errors and `goto` some label at the bottom of
a function under the nominal return statement. -> Think exceptions.


#### Encapsulation will be relaxed


To summarize, most structures will default to being fully public unless there
is a very pressing reason to create a private section. Such a reason is not
"the user *could* break something by touching this," instead it is "the user
*will only ever* break something by touching this."

* Do not use the keyword `class` unless your sole intent is to have the members
immediately following it be private.

* Using `class` followed by a `public:` label is nubile.


#### Direct initialization


Use `=` only for assignment to an existing object. *Break your C habit right now.*
Use bracket initialization `{}` of all variables and objects. Fall back to parens `()`
if brackets conflict with an initializer_list constructor (such as with STL containers)
or if absolutely necessary to quash warnings about conversions.

* Do not put uninitialized variables at the top of a function and assign them later.

> Quick note to preempt a confusion for C people:
> Initialization in C++ is like C but you don't have to use the `=`.
>
> ```C++
> struct user { const char *nick; };
> struct user you = {"you"};
> user me {"me"};
> ```
>

* Use Allman style for complex/long initialization statements. It's like a function
  returning the value to your new object; it is easier to read than one giant line.

> ```C++
> const auto sum
> {
>     1 + (2 + (3 * 4) + 5) + 6
> };
> ```


#### Use full const correctness


`const` correctness should extend to all variables, pointers, arguments, and
functions- not just "pointed-to" data. If it *can* be `const` then make it
`const` and relax it later if necessary.


#### Use auto


Use `auto` whenever it is possible to use it; specify a type when you must.
If the compiler can't figure out the auto, that's when you indicate the type.


#### RAII will be in full force

All variables, whether they're function-local, class-members, even globals,
must always be under some protection at all times. There must be the
expectation at *absolutely any point* including *between those points*
everything will blow up randomly and the protection will be invoked to back-out
the way you came. That is, essentially, **the juice of why we are here.**

**This is really serious business.** You have to do one thing at a time. When you
move on to the next thing the last thing has to have already fully succeeded
or fully failed. Everything is a **transaction**. Nothing in the future exists.
There is nothing you need from the future to give things a consistent state.

* The program should be effectively reversible -- should be able to "go backwards"
or "unwind" from any point. Think in terms of stacks, not linear procedures.
This means when a variable, or member (a **resource**) first comes into scope,
i.e. it is declared or accessible (**acquired**), it must be **initialized**
to a completely consistent state at that point.

>
> Imagine pulling down a window shade to hide the sun. As you pull down, the canvas
> unrolls from its spool at the top. Your goal is to hook the shade on to the nail
> at the bottom of the window: that is reaching the return statement. If you slip
> and let go, the shade will roll back up into the spool at the top: that is an
> exception.
>
> What you can't do is prepare work on the way down which needs _any_ further pulling
> to be in a consistent state and not leak. You might slip and let go at any time for
> any reason. A `malloc()` on one line and a `free()` following it is an example of
> requiring more pulling.
>
> Indeed slipping and letting go is an accident -- but the point is that *accidents
> happen*. They're not always your fault, and many times are in other parts of the
> code which are outside of your control. This is a good approach for robust and
> durable code over long-lived large-scale projects.
>


#### Exceptions will be used


Wait, you were trolling "respect for C people" right? **No.** If you viewed
the above section merely through the prism avoiding classic memory leaks, and
can foresee how to now write stackful, reversible, protected programs without
even calling free() or delete: you not only have earned the right, but you
**have** to use exceptions. This is no longer a matter of arguing for or
against `if()` statement clutter and checking return types and passing errors
down the stack.

* Object construction (logic in the initialization list, constructor body, etc)
is actual real program logic. Object construction is not something to just
prepare some memory, like initializing it to zero, leaving an instance
somewhere for further functions to conduct operations on. Your whole program
could be running - the entire universe could be running - in some member
initializer somewhere. The only way to error out of this is to throw, and it
is perfectly legitimate to do so.

* Function bodies and return types should not be concerned with error
handling and passing of such. They only cause and generate the errors.

* Try/catch style note: We specifically discourage naked try/catch blocks.
In other words, **most try-catch blocks are of the
[function-try-catch](http://en.cppreference.com/w/cpp/language/function-try-block)
variety.** The style is simply to piggyback the try/catch where another block
would have been.

> ```C++
> while(foo) try
> {
>     ...
> }
> catch(exception)
> {
> }
> ```

* We extend this demotion style of keywords to `do` as well, which should
  avoid having its own line if possible.

> ```C++
> int x; do
> {
>     ...
> }
> while((x = foo());
> ```



#### Pointers and References

* The `&` or `*` prefixes the variable name; it does not postfix the type.
This is evidenced by comma-delimited declarations. There is only one exception
to this for universal references which is described later.

> ```C++
> int a, &b{a}, *c{&b}, *const d{&b}, *const *const e{&c};
> ```

* Biblical maxim: Use references when you can, pointers when you must.

* Pass arguments by const reference `const foo &bar` preferably, non-const
reference `foo &bar` if you must.

* Use const references even if you're not referring to anything created yet.
const references can construct, contain, and refer to an instance of the type
with all in one magic. This style has no sympathy for erroneously expecting
that a const reference is not a local construction; expert C++ developers
do not make this error. See reasons for using a pointer below.

* Passing by value indicates some kind of need for object construction in
the argument, or that something may be std::move()'ed to and from it. Except
for some common patterns, this is generally suspect.

* Passing to a function with an rvalue reference argument `foo &&bar` indicates
something will be std::move()'ed to it, and ownership is now acquired by that
function.

* In a function with a template `template<class foo>`, an rvalue reference in
the prototype for something in the template `void func(foo &&bar)` is actually
a [universal reference](https://isocpp.org/blog/2012/11/universal-references-in-c11-scott-meyers)
which has some differences from a normal rvalue reference. To make this clear
our style is to move the `&&` like so `void func(foo&& bar)`. This is actually
useful because a variadic template foo `template<class... foo>` will require
the prototype `void func(foo&&... bar)`.

* Passing a pointer, or pointer arguments in general, indicates something may
be null (optional), or to explicitly prevent local const construction which is
a rare reason. Otherwise suspect.

* Avoid using references as object members, you're most likely just limiting
the ability to assign, move, and reuse the object because references cannot be
reseated; then the "~~big three~~" "big five" custom constructors have to be
created and maintained, and it becomes an unnecessary mess.


#### Miscellaneous


* Prefer "locality" rather than "centrality." In other words, we keep things
in as local of a scope or file as possible to where it is used.

* new and delete should rarely if ever be seen. This is more true than ever with
C++14 std::make_unique() and std::make_shared().

* We allow some C-style arrays, especially on the stack, even C99 dynamic sized ones;
there's no problem here, just be responsible.

* `alloca()` will not be used.

* C format strings are still acceptable. This is an IRC project, with heavy
use of strings and complex formats and all the stringencies. We even have
our own custom *protocol safe* format string library, and that should be used
where possible.

* streams and standard streams are generally avoided in this project. We could have
have taken the direction to customize C++'s stream interface to make it
performant, but otherwise the streams are generally slow and heavy. Instead we
chose a more classical approach with format strings and buffers -- but without
sacrificing type safety with our RTTI-based fmt library.

* ~~varargs are still legitimate.~~ There are just many cases when template
varargs, now being available, are a better choice; they can also be inlined.

	* Our template va_rtti is starting to emerge as a suitable replacement
	for any use of varags.

* When using a `switch` over an `enum` type, put what would be the `default` case after/outside
of the `switch` unless the situation specifically calls for one. We use -Wswitch so changes to
the enum will provide a good warning to update any `switch`.

* Prototypes should name their argument variables to make them easier to understand, except if
such a name is redundant because the type carries enough information to make it obvious. In
other words, if you have a prototype like `foo(const std::string &message)` you should name
`message` because std::string is common and *what* the string is for is otherwise opaque.
OTOH, if you have `foo(const options &options, const std::string &message)` one should skip
the name for `options &` as it just adds redundant text to the prototype.

* Consider any code inside a runtime `assert()` statement to **entirely**
disappear in optimized builds. If some implementations of `assert()` may only
elide the boolean check and thus preserve the inner statement and the effects
of its execution: this is not standard; we do not rely on this. Do not use
`assert()` to check return values of statements that need to be executed in
optimized builds.


#### Comments

* `/* */` Multi-line comments are not normally used. We reserve this for
debugging and temporary multi-line grey-outs. The goal for rarely using this
is to not impede anybody attempting to refactor or grey-out a large swath of
code.

* `//` Primary developer comment; used even on multiple lines.

* `///` Documentation comment; the same style as the single line comment; the
documentation is applied to code that follows the comment block.

* `///<` Documentation comment; this documents code preceding the comment.

##### Documentation will be pedantic, windy and even patronizing

This is considered a huge anti-pattern in most other contexts where comments
and documentation are minimal, read by experts, end up being misleading, tend
to diverge from their associated code after maintenance, etc. This project is
an exception. Consider two things:

1. This is a free and open source public internet project. The goal here
is to make it easy for many-eyeballs to understand everything. Then,
many-eyeballs can help fix comments which become misleading.

2. Most free and open source public internet projects are written in C
because C++ is complicated with a steep learning curve. It is believed
C++ reduces the amount of many-eyeballs. A huge number of contributions
to these projects come from people with limited experience working on
their "first project."

Therefor, writers of documentation will consider a reader which has
encountered IRCd as their first project, specifically in C++. Patronizing
explanations of common/standard C++ patterns and intricacies can be made.


### Art & Tableaux


* Tab style is **tabs before spaces**. Tabs set an indentation level and
then spaces format things *at that level*. This is one of the hardest styles
to get right and then enforce, but it looks the best for everyone. The point
here is that the tab-width becomes a personal setting -- nobody has to argue
whether it's worth 2 or 4 or 8 spaces... Remember, tabs are never used to
align things that would fall out of alignment if the tab-width changed.

* Only one blank line at a time. While an entire section could be devoted to
*where* to create whitespace, for now, just know to only use a single blank
line to do so. There are ways to cheat. I am a huge fan of whitespace and I
will share some of these ways. For example, a comment block may end in a
line starting with `//` with no text after it. Combined with the allowed
completely blank line after that you now have more whitespace.


### Conventions

These are things you should know when mulling over the code as a whole.
Importantly, knowing these things will help you avoid various gotchas and not
waste your time debugging little surprises. You may or may not agree with some
of these choices (specifically the lack of choices in many cases) but that's
why they're explicitly discussed here.


#### Null termination

- We don't rely on null terminated strings. We always carry around two points
of data to indicate such vectoring. Ideally this is a pair of pointers
indicating the `begin`/`end` like an STL iterator range. `string_view` et al
and the `buffer::` suite work this way.

- Null terminated strings can still be used and we even still create them in
many places on purpose just because we can.

- Null terminated creations use the BSD `strl*` style and *not* the `strn*`
style. Take note of this. When out of buffer space, such an `strl*` style
will *always* add a null to the end of the buffer. Since we almost always
have vectoring data and don't really need this null, a character of the string
may be lost. This can happen when creating a buffer tight to the length of an
expected string without a `+ 1`. This is actually the foundation of a case
to move *back* to `strn*` style but it's not prudent at this time.

- Anything named `print*` like `print(mutable_buffer, T)` always composes null
terminated output into the buffer. These functions usually return a size_t
which count characters printed *not including null*. They may return a
`string_view`/`const_buffer` of that size (never viewing the null).


#### Iteration protocols

When not using STL-iterators, you may encounter some closure/callback-based
iterator functions. Usually that's a `for_each()`. If we want to break out
of the loop, our conventions are as follows:

- *find protocol* for `find()` functions. The closure returns true to break
the loop at that element, false to continue. The `find()` function itself
then returns a pointer or reference to that element. If the end of the
iteration is reached then a `find()` usually returns `nullptr` or throws an
exception, etc.

- *test protocol* for `test()` functions (this has nothing to do with unit-
tests or development testing). This is the same logic as the find protocol
except the `test()` function itself returns true if the closure broke the
loop by returning true, or false if the end of the iteration was reached.

- *until protocol* for `until()` functions. The closure "remains true 'till
the end." When the end is reached, true is returned. The closure returns false
to break the loop, and then false is returned from until() as well.

Overloads of `for_each()` may be encountered accepting closures that return
`void` and others that return `bool`. The `bool` overloads use the
*until protocol* as that matches the same logic in a `for(; bool;)` loop.


#### nothrow is not noexcept

Often a function is overloaded with an std::nothrow_t argument or our
util::nothrow overload template. This means the function **will not throw
a specific exception expected from the overload alternative** (or set of
exceptions, etc). Any exception may still come out of that nothrow overload;
technically including the specific exception if it came from somewhere else!

When no exceptions whatsoever are expected, the `noexcept` keyword is used.


#### Indications of yielding and IO's

There is a section on how yielding and IO can occur far up the stack from a
benign-looking callsite in ctx/README. We try to make comments to indicate
these things directly in the definitions and certainly in documentation.

Some of those indications may say nothing more than `[GET]` and `[SET]` without
any other comment. That is the minimum acceptable marking for something which
will likely do read or write IO respectively to disk or even the network. In
any such case the ircd::ctx will definitely yield if that happens.