CppCon 2019: Better code: relationships

Sean Parent

  • Let’s rotate (cppcon six years ago); meme?
  • – Goal: No contradictions
  • A novice sees only the chessman; an amateur sees the board; a master sees the game

Entity notation

  • Witnessed Relationships: a relationship represented by an object
    • copyable and equality comparable
  • When an object is copied or moved any witnessed relationship is either maintained, severed, or invalidated
  • When an object is destructed any witnessed relationship is either severed, or invalidated

Safety

  • std::move()

    is an unsafe operation

C++20

  • One new feature: concepts (joke about contracts)
  • Hoare Logic (paper from 1969, C.A.R Hoare)

Equality

Applying “Design by Contract”

Contracts

  • Preconditions (require)
  • Postconditions
  • Invariants

Concepts

A compile time constraint to select an appropriate operation

Contracts assert at runtime if an an operations

A runtime constraint to select an appropriate operation is know a pattern matching

No Raw Loops

The Game

  • Architecture is the art and practice of designing and constructing structures

Timer example

Registry

Container support these:

  • Add an object and obtain receipt
  • Use receipt to retrieve the object
  • iterate

Russian Coat Check Algorithm

  • Receipts are ordered
    • Coats always appended with stub
    • Binary search to retrieve coat
      • when more than half the slot are empty; compact the coats
  • Coats are always ordered receipt stubs
  • Coats ordered by insertion

Double entry bookkeeping

  • Every transaction twice
  • 5 standard accounts: Asses, Capital, Liabilities, Revenues, and Expenses
  • This ensure the mechanical process of entering a transaction is done in two ways

Contradictions

  • unit tests expose contradictions

No Raw Synchronization Primitives

Use strong preconditions to move the issue to the caller

Setting a property

  • Code is redundant
  • – Different aspects of the same code
    • value is a*b when a changes
    • other_value is a*b when b changes
    • asking for troubles
  • Different, mutually exclusive relationships with non-local control
  • Implied last in wins relationship
  • An incidental algorithm; property convert to the correct value
  • property is not a simple property but a stream, trigger, or latch
  • or it’s just wrong

No Raw Pointers

https://sean-parent.stlab.cc for talks

CppCon 2019: Time Travel: Gradual Typing in C++

Hyrum Wright

C++ is a dynamically typed language

example with sso_string and union

If somebody can (ab)use your API, they will

Types Encode constraints

int timeout;
int number_of_pumpkins = 6*timeout;

better:

void set_deadline (absl::Duration deadline);


See “Time Programming Funamentals” by Greg Miller

Abseil Time API

  • Valid invalid operations: Duration + Duration gives Duration;
Duration + Time gives Time;
Time + Time is invalid, etc

Gradual Typing

  • python: adds annotation

Goals

  • Automated (almost) everything
  • Keep code buildable (each multiple steps)
  • Keep code readable
  • Eventually converge at a fixed point

Things that don’t work

  • Monolithic changes
  • Variable type
    • Most integers don’t represent time
  • Variable name
    • What does timeout_seconds actually mean?
  • Comments super unreliable

Types of transformations

  • Expression
    • Simplification
    • Comparison
    • Addition and Subtraction
    • Distribution
  • Variable
    • Local, global, class members
  • Inter-function
    • Parame and value

Expression: Simplification

  • Makes matching easier
  • clang-tidy code for matcher
  • code for fixer

Expresion: Comparison

Inter-function return values

Seeding Type Information

  • Identify high value targets

Reactions

  • You added a bug … oh
  • Thanks
  • (crickets)

Other applications

  • Converting to std::string_view
  • Ownership deduction (i.e. std::unique_ptr)

GitHub.com/llvm-mirror/clang-tools-extra/tree/master/clang-tidy/abseil

Questions

  • How to justify? “We should be able to change our software”

CppCon 2019: CppChat Live

John Kalb, Andrei; Herb Sutter; Phil Nash

Talking about giving a presentation

  • Recommending session
  • Giving a talk is a performance.
  • Andrei: treats each talk as different at different event. Re-does slides

Review of Andrei’s keynote

  • sort()

    algorithm needs to make many decision: attributes need to be introspected/reflect. E.g. cost of comparison; cost of move; etc

  • metaclasses are an example of this (Herb Sutter).
  • “Modules help with new features? e.g. exceptions”
  • “The dawn of a new error” by Phil Nash
  • Swift assumes out of memory is not recoverable. “C++ aspires to handle out-of-memory”
  • Looking at error handling is a great way to evaluate a language
  • Andrei:
    static_if

    is a fundamental building block just like

    if

CppCon 2019:From STL to Ranges

Jeff Garland

Intro

  • The beginning of the end – for begin and end

Basics

std::sort(a.begin(), a.end())

Instead:

ranges::sort(a);

Example with find_if

the ranges way: filter_view

for (int i : rng::filter_view (a, is_six)) { std::cout << i << “ “; }

Tristan Brindel NanoRange

Godbolt

  • supports several
  • range: something that can be iterator over
  • range algo: algorithm that takes range
  • view: lazy range thats cheap top copy
  • ranger adapter: turn range into a view

mechanics

  • 3 new names ranges, ranges::views, views = ranges::views

Why new namespaces

  • behavior and guaranteed of some algorithms are changed

What’s a range

  • iterator pair is simplest
  • sentinel and iterator can be different types
  • ranges can be infinite
  • any container with begin()/end() can be used in a range

Views are ranged with lazy evaluation

  • non-owning of elements
  • all methods
    O(1)

    for copy and assignment

range adaptors -> views from ranges

  • pipeline syntax

filter_view adaptor

for (int i = range_filter_view (vi, is_event))

No more for loops

Range Algorithm details

  • list of range algorithms

Projection parameters

  • provides first class filtering predicate
  • comes from the Adobe Source Libraries (ASL)

ranges::sort(vistuff, std::less<>{}, [](auto const &i) {return i.value;})

Views and View Adaptor details

How do ranges perform

  • Compile time: no noticeable difference
  • runtime: nothing expected over stl algorithms

Resources

wg21.link/n4810

CppCon 2019:Dawn of a new Error

Phil Nash

Exceptions

P0709 R2 Zero-overhead deterministic exceptions (static exceptions)

Swift has best error handler

Exploding return types

p03w3R7 std::expected

p0798R4 Monadic operations for std::optional

What is std::error

p1028R1

GitHub.com/ned14/status-code

error code always represent an error

p1029R1 [[move relocates]

p1144 object relocation in terms of move plus destroy

With compiler optimizations no extra overhead compared to regular return

error_code fits in two registers

auto result = try add_one (divide (42, parse_int(input));

levelofindirection.com/refs/dawn.html

boost::outcome

Question

  • What is
    intptr_t

    ?

CppCon start

CppCon 2019: Some photos

CppCon moved to Denver, Colorado (technically Aurora, CO) just a few miles from Denver Airport. 1400 people attended from Sept 16-20 and a 150 talks over the 5 days. Additionally, there were 1 and 2 day training classes the weekends before and after the conference.

The Gaylord Rocky Mountain Resort hosted CppCon. It’s a huge place that’s very conference oriented though there are some nice pools and rec areas. You are pretty isolated so plan on eating at the nice but overpriced restaurants.