mirror of
https://github.com/matrix-construct/construct
synced 2024-09-06 08:45:20 +02:00
243 lines
9.5 KiB
Markdown
243 lines
9.5 KiB
Markdown
|
# How to CPP for Charybdis
|
||
|
|
||
|
|
||
|
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++14 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.
|
||
|
|
||
|
|
||
|
======
|
||
|
|
||
|
|
||
|
#### 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 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 then 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.
|
||
|
|
||
|
|
||
|
#### 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
|
||
|
|
||
|
|
||
|
* 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.
|
||
|
|
||
|
* 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 actually has
|
||
|
a real use, 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, or optional. 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
|
||
|
|
||
|
|
||
|
* 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.
|
||
|
|
||
|
* std::array is preferred for object members; also just generally preferred.
|
||
|
|
||
|
* 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.
|
||
|
|
||
|
* I think a better case to use our template va_rtti is starting to emerge in
|
||
|
most of our uses for 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 &, const std::string &message)` one should skip the name
|
||
|
for `options &` as it just adds redundant text to the prototype.
|