CppCon 2019: De-Fragmenting C++

Herb Sutter

  • add things (more); fix things (some); simplify (least);
  • Why not reverse this?
  • adding things (eg MetaClasses) can simplify things

A tale of two compiler switches

-fno-exceptions
-fno-rtti

  • Make things static by default
  • Dynamic by opt-in

Exception Handling

  • isocpp.org 2018-02 survey; 48% ban exceptions either fully or partially (22% not allowed)
  • error codes; expected/outcome types; exceptions
  • std::error_code. std::filesystem supports both dialect

Root cause

  • Today’s exception handling is not zero-overhead. Just enabling increases size
  • throwing an exception not statically boundable space and time overhead. Violates you can’t reasonably write it better by hand
  • Lack of control

Key definition

  • what is a recoverable error
  • A function cannot do what it is supposed to do. Mirriam webster definition of “error”
  • A precondition violation is always a bug in the caller

Four coordinated proposals

  • Enable zero-overhead exception handling
  • throw few exceptions (90% of all exceptions should not be)
  • support explicit try for visble propagation

static exceptions

  • throwing values of static types means by value. No dynamic memory
  • isomorphic to error codes
  • share return channel
  • Need to be backward compatible. Opt in
  • As-if returning union(Success; Error;} + bool
  • doubles down on value semantics

Proposal

  • Add a noexcept-querable allocator proprety for “reports vs aborts” on allocation failure.
  • recommend conditional noexcept based on the relvant allocator
  • babb on GitHub.com/HerbSutter

Part 2: Run-time type information

  • Same for exceptions
  • Joe Bialek Wendesday Root cause of CVEs by patch year.
    • Type confusion increasing (e.g. static downcast)
    • static_cast perhaps because RTTI is banned
  • RTTI allowed in 68%

static_cast down-casts

  • Peter Collingbourne
  • Uses vtable ordered in memory so downcast is just a range check
  • down_cast

    5 instructions

  • fads

CppCon 2019: Modern C++ debugging tools

Greg Law

gdb

  • ptrace()
  • signals ony reach the tracee via PTRACE_COUNT
  • breakpoints and single step are SIGTRAPs
  • ^C is SIGINT
  • modify instruction causes SIGTRAP

DWARF

g++ -g3??? ; helps make inline functions not look inline; macros

  • “Optomized out” really means it’s just later
  • readelf —dump-debug
  • can actually track registers while live
  • Call frame analysys
  • Catch throw or catch
  • Libthreddb

address Sanitizer

Malloc and free are intercepted. Shadow memory keeps track

Replay

Record and replay nondeterministic steps. Just Re execute to get to current point

Static analysis

What’s The best way for a c++ programmer to make money? Inheritance

Synopsis

CppCon 2019: Naming is Hard: Let’s do Better

Kate Gregory

gregcons.com/kateblog

Naming things

  • It matters
  • It’s a learned skill

Not Naming conventions

  • camelCase, snake_case, PascalCase ….

Names carry meaning

  • expire date vs end date
  • is a date or datetime
  • It matters everywhere
  • Bad names confuse
  • Giving something the correct name may happen long after it’s first written

Naming requires empathy

  • sort/partial_sort/partial_sort_copy
  • top_n() is better name

Consitency

  • Names exist outside of your code
    • Headings n reports
    • Emails
  • Alaows call things by their proper names, everywhere
  • Don’t use same words for different things
  • Don’t accept similar English words in conversation
  • Don’t invent business works
  • Avoid pre/post and other “dependent” names
    • Unless the business uses them
  • Prefer single English words like “Save” or “Location” vs UpdateConfigFile and StorageCoOrdinates
  • Don’t mimsmatch
    • begin goes with end, …
  • Functions are verbs
  • Helper verbs: isEmpty() less ambigrous then empty()
  • isShippable() vs canShip() vs getShipStatus()
    • avoid the “-able”
  • Tools matter
    • alphabetical lists
  • Classes are Nons
    • Anything ending in “er (et al) is suspect without a noun
  • Adjectives are your friend
    • FullName not Name
  • Avoid encoding type
    • exceoptions: Date, asX
  • Maybe avoid leaking enums? e.g. instead of getStatus() == Application::OK maybe isApproved()
  • Don’t abbreviate
  • templates:
    • One? T is ok
    • Two: make meaningful
  • When to name things? When you know what it is
    • Sometimes before you even write the code
    • Never miss an opportunity to fix a name
    • Sometimes when code chnags the names need to change

Better naming

  • Care about the code you write and the people who will read it
  • Think about the purposes names server
  • Don’t be paralyzed
    • Fix it later
  • Demand good names
  • When you learn what something is, fix its name