John Kalb, Sean Parent, Phil Nash

Discussion about interviewing

• binary search as a take home test question
• Talking about Alex Stepanov who works at Adobe www.stlport.org/resources/StepanovUSA.html
• STL: Software Technology Lab (at Compaq/HP)

Adobe History

• Photoshop: $2B? 20-40 people
• Port to ipad read 1G file with only 40M memory

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

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”

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

…

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

  ?