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