Declan Valters dvalters

What is the Strict Aliasing Rule and Why do we care?

(OR Type Punning, Undefined Behavior and Alignment, Oh My!)

What is strict aliasing? First we will describe what is aliasing and then we can learn what being strict about it means.

In C and C++ aliasing has to do with what expression types we are allowed to access stored values through. In both C and C++ the standard specifies which expression types are allowed to alias which types. The compiler and optimizer are allowed to assume we follow the aliasing rules strictly, hence the term strict aliasing rule. If we attempt to access a value using a type not allowed it is classified as undefined behavior(UB). Once we have undefined behavior all bets are off, the results of our program are no longer reliable.

Unfortunately with strict aliasing violations, we will often obtain the results we expect, leaving the possibility the a future version of a compiler with a new optimization will break code we th

Effective Modern CMake

Getting Started

For a brief user-level introduction to CMake, watch C++ Weekly, Episode 78, Intro to CMake by Jason Turner. LLVM’s CMake Primer provides a good high-level introduction to the CMake syntax. Go read it now.

After that, watch Mathieu Ropert’s CppCon 2017 talk Using Modern CMake Patterns to Enforce a Good Modular Design (slides). It provides a thorough explanation of what modern CMake is and why it is so much better than “old school” CMake. The modular design ideas in this talk are based on the book [Large-Scale C++ Software Design](https://www.amazon.de/Large-Scale-Soft

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See https://github.com/lefticus/cppbestpractices/blob/master/00-Table_of_Contents.md Instead!

C++ Coding Standards Part 0: Automated Code Analysis

Automated analysis is the main advantage to working with a modern statically typed compiled language like C++. Code analysis tools can inform us when we have implemented an operator overload with a non-canonical form, when we should have made a method const, or when the scope of a variable can be reduced.

	#include <stdio.h>

	#define SQ(x) (x)*(x)

	#define M0(x,y) SQ(x)+SQ(y)<4?0:0xe0
	#define M1(x,y,x0,y0) (SQ(x)+SQ(y)<4)?M0(SQ(x)-SQ(y)+(x0),2(x)(y)+(y0)):0xc0
	#define M2(x,y,x0,y0) (SQ(x)+SQ(y)<4)?M1(SQ(x)-SQ(y)+(x0),2(x)(y)+(y0),x0,y0):0xa0
	#define M3(x,y,x0,y0) (SQ(x)+SQ(y)<4)?M2(SQ(x)-SQ(y)+(x0),2(x)(y)+(y0),x0,y0):0x80
	#define M4(x,y,x0,y0) (SQ(x)+SQ(y)<4)?M3(SQ(x)-SQ(y)+(x0),2(x)(y)+(y0),x0,y0):0x60
	#define M5(x,y,x0,y0) (SQ(x)+SQ(y)<4)?M4(SQ(x)-SQ(y)+(x0),2(x)(y)+(y0),x0,y0):0x40

	#!/bin/bash

	# This script, executed at the root of a git repository, deletes traces of every old file in this repository, index + blob on all branches
	# It can take 10-30 minutes to run and will print regular warning stating than some references are unchanged
	# time ./clear_git_repositor.bash >cleaning.log

	# We need several passes to clean files renamed multiple times (git log --find-renames prevents its deletion for each renaming)
	# MAXIMUM_PASSES should be more than the maximum number of renamings/movings for any file, if not then we might keep some traces of former files
	MAXIMUM_PASSES=10 # Maximum number of passes

	import pyproj

	# Define two projections, one for the British National Grid and one for WGS84 (Lat/Lon)
	# You can use the full PROJ4 definition or the EPSG identifier (PROJ4 uses a file that matches the two)

	#bng = Proj("+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs towgs84='446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894'")
	#wgs84 = Proj('+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs')

	bng = pyproj.Proj(init='epsg:27700')
	wgs84 = pyproj.Proj(init='epsg:4326')