In which I try to express the weird joy I felt learning about all the odd quirks of the simple task of turning bytes into characters, and characters into bytes. Slides: Interesting Characters slides

Series: Snake in Elm, Elm makes me happy, Elm Basics, Elm Unit Test, Elm JSON With up-to-date examples, and anecdotes from the last couple of years of continuing to enjoy writing web pages in Elm, here’s a new version of my Elm advert: Slides: Elm ma…

Series: 2D Shapes, drag and drop, new objects My Godot 3 game is progressing, and I am starting to think I am actually writing the level editor. Here’s how I wrote code to make new versions of existing objects (by converting them to scenes): Godot ve…

I am writing a CharsetEncoder in Java, which is my kind of fun. I was getting a mysterious error when I identified that I could not encode certain characters: Exception in thread “main” java.lang.IllegalArgumentException at java.nio.Buffer.position(Bu…

Series: 2D Shapes, drag and drop, new objects Continuing to explore the Godot 3 game engine. I want to make a game where you drag blocks around and balance them on each other, but I couldn’t find much documentation on how to drag-and-drop objects (exce…

Series: 2D Shapes, drag and drop, new objects In which I make a square and a triangle bounce on a rectangle using Godot 3: Godot version: v3.0.6.stable.official.8314054

Gimpel Software (the vendor behind PC-lint and PC-lint Plus) <have recently updated their website, and it is now dedicated entirely to PC-lint Plus. If you are considering upgrading from PC-lint 9.0 to PC-lint Plus, the relevant information is avail…

Gimpel Software (the vendor behind PC-lint and PC-lint Plus) <have recently updated their website, and it is now dedicated entirely to PC-lint Plus. If you are considering upgrading from PC-lint 9.0 to PC-lint Plus, the relevant information is avail…

LintProject Pro is a command line only product which can perform a basic per-file analysis of a C/C++ codebase using PC-lint or CppCheck. In many ways it was the proof of concept for Visual Lint, and although it has served us well, it’s getting a bit l…

LintProject Pro is a command line only product which can perform a basic per-file analysis of a C/C++ codebase using PC-lint or CppCheck. In many ways it was the proof of concept for Visual Lint, and although it has served us well, it’s getting a bit l…

Series: Snake in Elm, Elm makes me happy, Elm Basics, Elm Unit Test, Elm JSON I find JSON decoding in Elm confusing, so here are some thoughts and examples. Setup $ elm –version 0.19.0 $ mkdir myproj; cd myproj $ elm init … $ elm install elm/json …..

Quiz time! Which of the following programs can you expect to not compile? For bonus points, which are required by the C++ standard to not compile? Program 1 Program 2 Program 3 In case you’re not familiar with static_assert, it takes a constant boolean expression, and if it evaluates to false, you get a compilation … Continue reading static_assert in templates →

In this Tweet, @willkirkby posts: +!!”” that evaluates as 1 in C/C++, but no, JavaScript is the weird language C++ is indeed weird, or at least it’s very weakly typed. Let’s go through all the details of what’s going on here: Summary: Starting from the right, “” is a string literal, which gets converted to … Continue reading +!!”” →

Because the Rapsberry Pi uses a slightly older Python version, there is a special version of Graft for it. Here’s how to get it: Open a terminal window by clicking the black icon with a “>” symbol on it at the top near the left. First we need to in…

The problem with disaster planning is that it is not rehearsed. When you need to retrieve a file from backup is when you discover that your backup has been broken for three months. Modern cloud systems, based upon software defined infrastructure and…

The problem with disaster planning is that it is not rehearsed. When you need to retrieve a file from backup is when you discover that your backup has been broken for three months. Modern cloud systems, based upon software defined infrastructure and…

Have you ever wondered why you can’t list-initialize containers of non-copyable types? This is for instance not possible: If you ever wondered, or if you now are, read on! List-initialization Since C++11, you’re probably used to intitalizing containers like this: This of course also works with user defined types. Let’s say you have a class … Continue reading Why you can’t list-initialize containers of non-copyable types →

I’m running a workshop at the Egham Raspberry Jam on 21st October. The workshop will introduce my little animation language Graft. We will tell a story using animations that we created ourselves using code. The worksheet for the workshop is here: PDF o…

I want to change to change the name of a sonarqube project. This cannot be done without performing another analysis. You can just do it in SQL https://stackoverflow.com/questions/30511849/how-to-rename-a-project-in-sonarqube-5-1 but you have to be…

I want to change to change the name of a sonarqube project. This cannot be done without performing another analysis. You can just do it in SQL https://stackoverflow.com/questions/30511849/how-to-rename-a-project-in-sonarqube-5-1 but you have to be…

Elm is unstable, so upgrading to the next version can be painful. Here’s what I needed to do to upgrade from 0.18 to 0.19. Replace elm-package.json and tests/elm-package.json with elm.json – e06f5a1728 Switch to the new elm-test – b964b7c7a Re-arrange…

If you use Wunderlist and want to be able to bulk-add tasks from a text file, first install and set up wunderline. Now, to be able to right-click a text file containing one task per line on Ubuntu MATE, create a file called “wunderlist-bulk-add” in ~/….

In a previous post I described how to install Flarum locally on Ubuntu. Here is how I set up my development environment on top of that setup so I was able to write a new Flarum extension and test it on my local machine. Recap: I installed Apache and PH…

I found out on Mastodon recently that the Compose key exists, allowing you to enter special characters using easy-to-remember key sequences (e.g. “<compose>/=” gives you “≠“). To do this on Ubuntu (and probably many other systems), hold SHIFT, …

I want all requests to artificialworlds.net/rabbit-escape/levels/ to get redirected to use the https protocol, and to include “www.” at the beginning of the URL, and I found lots of Stack Overflow articles, but nothing there worked perfectly for me. He…

Here is an almost-minimal example of a systemd service file, that I use to run the Mastodon bot of my generative art playground Graft. I made a dedicated user just to run this service, and installed Graft into /home/graft/apps/graft under that username…

I am setting up a forum for sharing levels for my game Rabbit Escape, and I have decided to try and use Flarum, because it looks really usable and responsive, has features we need like liking posts and following authors, and I think it will be reasonab…

The RABBOTS are coming! Get the latest version of Rabbit Escape: Android: Rabbit Escape on F-Droid (free, no ads) Android: Rabbit Escape Free on the Play Store (free, no ads) Android: Rabbit Escape on the Play Store (Pay about 60p, identical to the f…

I have quite a few videos hosted on YouTube that I would like to upload to my new PeerTube location, but I don’t want to install all the PeerTube dependencies on my machine, so I did it all inside a Docker image. First I built and started a Docker cont…

Recent Overload journal issues contain my new articles on How to Write a Programming Language. Part 1: How to Write a Programming Language: Part 1, The Lexer Part 2: How to Write a Programming Language: Part 2, The Parser PDF of the latest issue: Overl…

Following last week’s upgrade of my Roundup instance to 1.6.0, I have now applied the Python 3 patches from issue2550960 with a few more fixes available in my py3-cmeerw branch and now have both wg21.cmeerw.net as well as issues.cmeerw.org working with…

Following last week’s upgrade of my Roundup instance to 1.6.0, I have now applied the Python 3 patches from issue2550960 with a few more fixes available in my py3-cmeerw branch and now have both wg21.cmeerw.net as well as issues.cmeerw.org working with…

wg21.cmeerw.net got a major update with Roundup being updated to 1.6.0 and the template converted to being based on jinja2 which should make it more friendly to mobile devices.

wg21.cmeerw.net got a major update with Roundup being updated to 1.6.0 and the template converted to being based on jinja2 which should make it more friendly to mobile devices.

I love the “tiling” feature in many window managers including Marco that means I can drag windows to the side of the screen and get them covering one half. However, I never use the similar feature that allows dragging a window to the top, and it often …

I tried to get back to an IRC interface to Slack using Matrix, and it had some problems. Thanks to Colin Watson’s comment on that post, I tried Daniel Beer’s slirc, and so far it seems to be working pretty well. Here’s what I did: Get a Slack legacy to…

Update: I found a better solution using slirc. I like using HexChat to talk to my colleagues, like one other guy. It is fast, and it pops up a new window when someone sends me a direct/private message. Recently, Slack shut down their IRC gateway, forci…

Groovy has lots of interesting syntax that can be used for domain-specific languages, such as Gradle build files, and Jenkinsfiles. I try to demystify the syntax tricks a bit so you have a chance to read and understand what the code is actually doing: …

On Ubuntu, cloning a repo from a machine you don’t have a certificate for will give the error: fatal: unable to access ‘https://servername’: server certificate verification failed. CAFuile /etc/ssl/certs/your_filename CRLfile: None You can …

On Ubuntu, cloning a repo from a machine you don’t have a certificate for will give the error: fatal: unable to access ‘https://servername’: server certificate verification failed. CAFuile /etc/ssl/certs/your_filename CRLfile: None You can …

Clearlake Capital-Backed Perforce Adds Enterprise-Grade Source Code Analysis to its Portfolio With Acquisition of PRQA

Clearlake Capital-Backed Perforce Adds Enterprise-Grade Source Code Analysis to its Portfolio With Acquisition of PRQA

We had an absolute blast at this year’s ACCU Conference and if you were there we imagine you did too. For us the highlight had to be the launch of #include <C++>, a new global, inclusive, and diverse community for developers interested in C++. …

We had an absolute blast at this year’s ACCU Conference and if you were there we imagine you did too. For us the highlight had to be the launch of #include <C++>, a new global, inclusive, and diverse community for developers interested in C++. …

I have 2 tables like this: > SELECT * FROM table_a; +——+——+ | id | name | +——+——+ | 1 | row1 | | 2 | row2 | +——+——+ > SELECT * FROM table_b; +——+——+——+ | id | name | aid | +——+——+——+ | …

There has been a recent stirring of attention, in the C++ community, for the practice of always placing the const modifier to the right of the thing it modifies. The practice has even been gifted a catchy name: East Const (which, I think, is what has…

There has been a recent stirring of attention, in the C++ community, for the practice of always placing the const modifier to the right of the thing it modifies. The practice has even been gifted a catchy name: East Const (which, I think, is what has…

I was looking at modifying acme tiny to support DNS-01 validation with a custom PowerDNS backend just a few days ago (in my case to get certificates for an XMPP server where there isn’t a corresponding HTTP server or the HTTP server is hosted on a diff…

I was looking at modifying acme tiny to support DNS-01 validation with a custom PowerDNS backend just a few days ago (in my case to get certificates for an XMPP server where there isn’t a corresponding HTTP server or the HTTP server is hosted on a diff…

If, like me, you are this person: (Source: xkcd.com/1782) You may want to fix the stupid :slightly_smiling_face: messages you receive from Slack via the IRC gateway. Obviously, I’d prefer they went away entirely, but it’s still better to see a characte…

I am trying to practice documentation-driven development in Pepper3, so every time I start on an area, I will write documentation explaining how it works, and include examples that are automatically verified during the build. I’ve started work on lexin…

Today I wanted to fix a Git repo that contained some bad commits (i.e. git fsck complained about them). [I wanted to do this because GitLab was not allowing me to push the bad commits.] I wanted the code to look exactly as it did before, but the histor…

Series: Examples, Questions My last post Examples of Pepper3 code was a reply to my friend’s email asking what it was all about. They replied with some questions, and I thought the questions and answers might shed some more light: Questions! Brillian…

Series: Examples, Questions I have restarted my effort to make a new programming language that fits the way I like things. I haven’t pushed any code yet, but I have made a lot of progress in my head to understand what I want. Here are some random examp…

Stop! This blog post is out of date – see Recording gameplay videos on RetroPie for a live version in github that may be more helpful. Credits: this is a slightly corrected and shortened version of How To Record A GamePlay Video From A RetroPie by sels…

(* Probably) Last week my London based C++ user group, C++ London, joined forces with SwedenCpp, based in Stockholm, for a distributed event where we shared video streams with each other. The whole thing was hosted by King, who took care of the audio…

(* Probably) Last week my London based C++ user group, C++ London, joined forces with SwedenCpp, based in Stockholm, for a distributed event where we shared video streams with each other. The whole thing was hosted by King, who took care of the audio…

I’ve been talking about Catch2 for a while – but now it’s finally here The big news for Catch2 is that it drops all support for pre-C++11 compilers. Other than meaning that some users will not be supported (you can still use Catch “Classic” (1.x) – …

I’ve been talking about Catch2 for a while – but now it’s finally here The big news for Catch2 is that it drops all support for pre-C++11 compilers. Other than meaning that some users will not be supported (you can still use Catch “Classic” (1.x) – …

is an excellent book by Tristan Gooley (isbn 978-1-473-61522-9). As usual I’m going to quote from a few pages: One of the universal truths of human observation is that we see more of what we expect to see and less of what we don’t expect to see…

is an excellent book by Tristan Gooley (isbn 978-1-473-61522-9). As usual I’m going to quote from a few pages: One of the universal truths of human observation is that we see more of what we expect to see and less of what we don’t expect to see…

is an excellent book by Niels Pflaeging (isbn 978-0-9915376-0-0). As usual I’m going to quote from a few pages: What Taylor pioneered was the idea of consistently dividing an organization between thinking people (managers) and executing people (w…

is an excellent book by Niels Pflaeging (isbn 978-0-9915376-0-0). As usual I’m going to quote from a few pages: What Taylor pioneered was the idea of consistently dividing an organization between thinking people (managers) and executing people (w…

My OpenMarket colleagues and I ran a workshop at TECH(K)NOW Day on how to write your own programming language: A big thank you to my colleagues from OpenMarket who volunteered to help: Rowan, Jenny, Zach, James and Elliot. An extra thank you to Zach …

Today I found I could not connect out through my proxy server with FileZilla It stopped working when I changed my password to something containing a double quote “. The solution? Change to use WinSCP.

I have been working on a prototype level editor for Rabbit Escape, and I’ve had trouble getting the layout I wanted: a toolbar at the side or top of the screen, and the rest a zoomable workspace. Something like this is very common in many desktop appli…

Today I was trying to make a simple tarball of a project + its dependent jar using the maven-assembly-plugin. I know this is a terrible way to do anything, but hey, just in case someone else wants to do something just as terrible, here are my pom.xml a…

I used to run Planet Code using Venus but that started breaking recently with 500 errors for SNI-enabled sites, and it looks very unmaintained. I considered some other static aggregators, but on shared hosting it’s hard to get the right Ruby version an…

is an excellent book by Miyamoto Musashi, translated by Thomas Cleary (isbn 1-57062-748-7). As usual I’m going to quote from a few pages: Preface: In common parlance, to do something with a real sword means to do it with utmost earnestness… The Bo…

is an excellent book by Miyamoto Musashi, translated by Thomas Cleary (isbn 1-57062-748-7). As usual I’m going to quote from a few pages: Preface: In common parlance, to do something with a real sword means to do it with utmost earnestness… The Bo…

If you want “tomorrow” in Python datetimes, don’t construct a datetime like this: from datetime import datetime, timedelta td = datetime.today() tm1 = datetime(td.year, td.month, td.day + 1, 14, 0, 0) # Don’t do this! Because it will work sometimes, b…

cyber-dojo now supports practising jQuery. Enjoy 🙂

cyber-dojo now supports practising jQuery. Enjoy 🙂

I have sent the following letter to LTS.Team AT oxfordshire.gov.uk in response to https://consultations.oxfordshire.gov.uk/consult.ti/OxfordRiversideRoutes/. &nbsp Hi, &nbsp I have submitted the following via the questionnaire: &nbsp The Oxpe…

I have sent the following letter to LTS.Team AT oxfordshire.gov.uk in response to https://consultations.oxfordshire.gov.uk/consult.ti/OxfordRiversideRoutes/. &nbsp Hi, &nbsp I have submitted the following via the questionnaire: &nbsp The Oxpe…

Today at the Egham Raspberry Pi Jam we did two things: 1. The Broken Levels Challenge Some nasty person came and broke our levels for our game Rabbit Escape and we need you to fix them! To play this game you will need a PC version of Rabbit Escape, ou…

C has a facility for checking dynamic assertions at run-time. It’s inside <assert.h> and its called assert. Now assert is a macro, so why isn’t it called ASSERT? I don’t know. Prior art no doubt. Anyway, assert is a dynamic runtime feature, you can only use it inside functions.

/* not inside a function, won't compile :-( */
assert(sizeof(int) * CHAR_BIT >= 32);  

That’s a pity because it would be nice if I could get the compiler to check things like this automatically at compile time. I’ve occasionally seen an attempt at a compile-time check like this…

#if sizeof(int) * CHAR_BIT < 32
#error People of Earth. Your attention please...
#endif

But this doesn’t work. The C preprocessor is a glorified text reformatter: it knows practically nothing about C. However, there is a way to write this as a compile time assertion (and moving any error trap to an earlier phase is a Good Thing)

{ yourself }

#define COMPILE_TIME_ASSERT(expr)   \
    char constraint[expr]

COMPILE_TIME_ASSERT(sizeof(int) * CHAR_BIT >= 32);

What’s is going on here? Well, the example preprocesses to…

char constraint[sizeof(int) * CHAR_BIT >= 32];

If the expression is true, (an int is at least 32 bits), the expression will have a value of one, and constraint will be an array of one char. If the assertion is false, (an int is less than 32 bits), the expression will have a value of zero, and constraint will be an empty array. That’s illegal, and you’ll get a compile time error. Viola, a compile time assertion 🙂 You can use it inside and outside a function but you can’t use it twice in the same function, as you end up with a duplicate definition. To solve that problem you could resort to some convoluted macro trickery:

#define COMPILE_TIME_ASSERT(expr)       char UNIQUE_NAME[expr]
#define UNIQUE_NAME                     MAKE_NAME(__LINE__)
#define MAKE_NAME(line)                 MAKE_NAME2(line)
#define MAKE_NAME2(line)                constraint_ ## line

But this is pretty horrible. Also, you will probably get warnings about unused variables. Take a step back for a moment and think about why it works at all. It’s because you have to specify the size of an array as a compile time constant. The formal grammar of a direct-declarator tells you this. Let’s look at some bits of grammar more closely:

Constrained arrays

 direct-declarator:
  identifier
  ( declarator )
  direct-declarator [ constant-expression opt ]
  direct-declarator ( parameter-type-list )
  direct-declarator ( identifier-list opt )

I just piggy backed on this, using the constraint that the value of the constant expression cannot (in this context) be zero. A natural question (to the curious) is are there other parts of the formal grammar that require a constant expression. The answer, of course, is yes.

Constrained enums

 enumerator:
  enumeration-constant
  enumeration-constant = constant-expression

However, I can’t use this because there are no useful constraints in this context.

Constrained bit-fields

 struct-declarator:
  declarator
  declarator opt : constant-expression

Reading the constraints of a bit field I see that if the width of a bit-field is zero the declaration cannot have a declarator. In other words this is legal…

 struct x { unsigned int : 0; };

but this is not…

 struct x { unsigned int bf : 0; };

This suggests another way to create a compile time assertion

#define COMPILE_TIME_ASSERT(expr)   \
    struct x { unsigned int bf : expr; }

COMPILE_TIME_ASSERT(sizeof(int) * CHAR_BIT >= 32);

Trying this we again get duplicate definitions, not of a variable this time, but of the type struct x. However we can fix this by creating an anonymous struct:

#define COMPILE_TIME_ASSERT(expr)   \
    struct { unsigned int bf : expr; }

This works. However, now you’ll probably get warnings about the unused untagged struct.

Constrained switch

 labelled-statement:
  identifier : statement
  case constant-expression : statement
  default : statement

It’s well known that you can’t have two case labels with the same constant. The following will not compile…

switch (0)
{
case 0:
case 0:;
}

#define COMPILE_TIME_ASSERT(pred)       \
    switch(0){case 0:case pred:;}

COMPILE_TIME_ASSERT(sizeof(int) * CHAR_BIT >= 32);

If pred evaluates to true (i.e., 1) then the case labels will be 0 and 1. Different; Ok. If pred evaluates to false (i.e., 0) then the case labels will be 0 and 0. The same; Compile time error. Viola. However, a switch statement cannot exist in the global scope. So the last piece of the puzzle is to put the compile time assertions inside a function.

#include <limits.h>

#define COMPILE_TIME_ASSERT(pred)            \  
    switch(0){case 0:case pred:;}

#define ASSERT_MIN_BITSIZE(type, size)       \
    COMPILE_TIME_ASSERT(sizeof(type) * CHAR_BIT >= size)

#define ASSERT_EXACT_BITSIZE(type, size)     \
    COMPILE_TIME_ASSERT(sizeof(type) * CHAR_BIT == size)

void compile_time_assertions(void)
{
    ASSERT_MIN_BITSIZE(char,  8);
    ASSERT_MIN_BITSIZE(int,  16);
    ASSERT_EXACT_BITSIZE(long, 32);
}

C has a facility for checking dynamic assertions at run-time. It’s inside <assert.h> and its called assert. Now assert is a macro, so why isn’t it called ASSERT? I don’t know. Prior art no doubt. Anyway, assert is a dynamic runtime feature, you can only use it inside functions.

/* not inside a function, won't compile :-( */
assert(sizeof(int) * CHAR_BIT >= 32);  

That’s a pity because it would be nice if I could get the compiler to check things like this automatically at compile time. I’ve occasionally seen an attempt at a compile-time check like this…

#if sizeof(int) * CHAR_BIT < 32
#error People of Earth. Your attention please...
#endif

But this doesn’t work. The C preprocessor is a glorified text reformatter: it knows practically nothing about C. However, there is a way to write this as a compile time assertion (and moving any error trap to an earlier phase is a Good Thing)

{ yourself }

#define COMPILE_TIME_ASSERT(expr)   \
    char constraint[expr]

COMPILE_TIME_ASSERT(sizeof(int) * CHAR_BIT >= 32);

What’s is going on here? Well, the example preprocesses to…

char constraint[sizeof(int) * CHAR_BIT >= 32];

If the expression is true, (an int is at least 32 bits), the expression will have a value of one, and constraint will be an array of one char. If the assertion is false, (an int is less than 32 bits), the expression will have a value of zero, and constraint will be an empty array. That’s illegal, and you’ll get a compile time error. Viola, a compile time assertion 🙂 You can use it inside and outside a function but you can’t use it twice in the same function, as you end up with a duplicate definition. To solve that problem you could resort to some convoluted macro trickery:

#define COMPILE_TIME_ASSERT(expr)       char UNIQUE_NAME[expr]
#define UNIQUE_NAME                     MAKE_NAME(__LINE__)
#define MAKE_NAME(line)                 MAKE_NAME2(line)
#define MAKE_NAME2(line)                constraint_ ## line

But this is pretty horrible. Also, you will probably get warnings about unused variables. Take a step back for a moment and think about why it works at all. It’s because you have to specify the size of an array as a compile time constant. The formal grammar of a direct-declarator tells you this. Let’s look at some bits of grammar more closely:

Constrained arrays

 direct-declarator:
  identifier
  ( declarator )
  direct-declarator [ constant-expression opt ]
  direct-declarator ( parameter-type-list )
  direct-declarator ( identifier-list opt )

I just piggy backed on this, using the constraint that the value of the constant expression cannot (in this context) be zero. A natural question (to the curious) is are there other parts of the formal grammar that require a constant expression. The answer, of course, is yes.

Constrained enums

 enumerator:
  enumeration-constant
  enumeration-constant = constant-expression

However, I can’t use this because there are no useful constraints in this context.

Constrained bit-fields

 struct-declarator:
  declarator
  declarator opt : constant-expression

Reading the constraints of a bit field I see that if the width of a bit-field is zero the declaration cannot have a declarator. In other words this is legal…

 struct x { unsigned int : 0; };

but this is not…

 struct x { unsigned int bf : 0; };

This suggests another way to create a compile time assertion

#define COMPILE_TIME_ASSERT(expr)   \
    struct x { unsigned int bf : expr; }

COMPILE_TIME_ASSERT(sizeof(int) * CHAR_BIT >= 32);

Trying this we again get duplicate definitions, not of a variable this time, but of the type struct x. However we can fix this by creating an anonymous struct:

#define COMPILE_TIME_ASSERT(expr)   \
    struct { unsigned int bf : expr; }

This works. However, now you’ll probably get warnings about the unused untagged struct.

Constrained switch

 labelled-statement:
  identifier : statement
  case constant-expression : statement
  default : statement

It’s well known that you can’t have two case labels with the same constant. The following will not compile…

switch (0)
{
case 0:
case 0:;
}

#define COMPILE_TIME_ASSERT(pred)       \
    switch(0){case 0:case pred:;}

COMPILE_TIME_ASSERT(sizeof(int) * CHAR_BIT >= 32);

If pred evaluates to true (i.e., 1) then the case labels will be 0 and 1. Different; Ok. If pred evaluates to false (i.e., 0) then the case labels will be 0 and 0. The same; Compile time error. Viola. However, a switch statement cannot exist in the global scope. So the last piece of the puzzle is to put the compile time assertions inside a function.

#include <limits.h>

#define COMPILE_TIME_ASSERT(pred)            \  
    switch(0){case 0:case pred:;}

#define ASSERT_MIN_BITSIZE(type, size)       \
    COMPILE_TIME_ASSERT(sizeof(type) * CHAR_BIT >= size)

#define ASSERT_EXACT_BITSIZE(type, size)     \
    COMPILE_TIME_ASSERT(sizeof(type) * CHAR_BIT == size)

void compile_time_assertions(void)
{
    ASSERT_MIN_BITSIZE(char,  8);
    ASSERT_MIN_BITSIZE(int,  16);
    ASSERT_EXACT_BITSIZE(long, 32);
}

• Events are locked?
• Exist on stack or heap?
• Can cause garbage collection?
• Meaning of this?
• Always has a default constructor?
• Default construction triggers static construction?
• Can be null?
• Use with the as operator?
• Can be locked?
• Can have a destructor?
• Default field layout?
• Can be a volatile field?
• Can have synchronized methods?
• Can be pointed to?
• Can be stackalloc’d?
• Can be sizeof’d?
• How to initialize fields?
• Inheritance differences?
• Equals behavior

Events are locked?

Events declared in a class have their += and -= access automatically locked via a lock(this) to make them thread safe (static events are locked on the typeof the class). Events declared in a struct do not have their += and -= access automatically locked. A lock(this) for a struct would not work since you can only lock on a reference type expression.

Exist on stack or heap?

Value type local instances are allocated on the stack. Reference type local instances are allocated on the heap.

Can cause garbage collection?

Creating a struct instance cannot cause a garbage collection (unless the constructor directly or indirectly creates a reference type instance) whereas creating a reference type instance can cause garbage collection.

Meaning of this?

In a class, this is classified as a value, and thus cannot appear on the left hand side of an assignment, or be used as a ref/out parameter. For example:

class Indirect
{
    //...
    public void Method(Indirect that)
    {
        RefParameter(ref this); // compile-time error
        OutParameter(out this); // compile-time error
        this = that;            // compile-time error
    }
    //...
}

struct Direct
{
    //...
    public void Reassign(Direct that)
    {
        RefParameter(ref this); // compiles ok
        OutParameter(out this); // compiles ok
        this = that;            // compiles ok
    }
    //...
}

struct Direct
{
    public Direct(int value)
    {
        Field = value;
    }

    public void Reassign(Direct that)
    {
        RefParameter(ref this); // compiles ok
        OutParameter(out this); // compiles ok
        this = that;            // compiles ok
    }

    public readonly int Field;
}

class Show
{
    static void Main()
    {
        Direct s = new Direct(42);
        Console.WriteLine(s.Field); // writes 42
        s.Reassign(new Direct(24));
        Console.WriteLine(s.Field); // writes 24
    }
}

struct Direct
{
    // as above
}

class Caller
{
    public void Method()
    {
        Console.WriteLine(d.Field); // writes 42
        d.Reassign(new Direct(24));
        Console.WriteLine(d.Field); // writes 42!
    }

    private readonly Direct d = new Direct(42);
}

class Show
{
    static void Main()
    {
        Caller c = new Caller();
        c.Method();
    }
}

Always have a default constructor?

A struct always has a built-in public default constructor.

class DefaultConstructor
{
    static void Eg()
    {
          Direct   yes = new   Direct(); // always compiles ok
        InDirect maybe = new InDirect(); // compiles if c'tor exists and is accessible
        //...
    }
}

class NonInstantiable
{
    private NonInstantiable() // ok
    {
    }
}

struct Direct
{
    private Direct() // compile-time error
    {
    }
}

Default construction triggers static constructor?

A structs static constructor is not triggered by calling the structs default constructor. It is for a class.

struct Direct
{
    static Direct()
    {
        Console.WriteLine("This is not written");
    }
}

class NotTriggered
{
    static void Main()
    {
        Direct local = new Direct();
    }
}

Can be null?

A struct instance cannot be null.

class Nullness
{
    static void Eg(Direct s, Indirect c)
    {
        if (s == null) ... // compile-time error
        if (c == null) ... // compiles ok
    }
}

Use with the as operator?

A struct type cannot be the right hand side operand of the as operator.

class Fragment
{
    static void Eg(Direct s, Indirect c)
    {
          Direct  no = s as   Direct; // compile-time error
        InDirect yes = c as InDirect; // compiles ok
        //...
    }
}

Can be locked?

A struct type expression cannot be the operand of a lock statement.

class LockStatement
{
    static void Eg(Direct s, InDirect c)
    {
        lock(s) { ... } // compile-time error
        lock(c) { ... } // compiles ok
    }
}

Can have a destructor?

A struct cannot have a destructor. A destructor is just an override of object.Finalize in disguise, and structs, being value types, are not subject to garbage collection.

struct Direct
{
    ~Direct() {} // compile-time error
}
class InDirect
{
    ~InDirect() {} // compiles ok
}

And the CIL for ~Indirect() looks like this:

.method family hidebysig virtual instance void
        Finalize() cil managed
{
  // ...
} // end of method Indirect::Finalize

Default field layout?

The default [StructLayout] attribute (which lives in the System.Runtime.InteropServices namespace) for a struct is LayoutKind.Sequential whereas the default StructLayout for a class is LayoutKind.Auto. (And yes, despite its name you can tag a class with the StructLayout attribute.) In other words the CIL for this:

public struct Direct
{
    //...
}

looks like this:

.class public sequential ansi sealed beforefieldinit Direct
       extends [mscorlib]System.ValueType
{
  //...
}

whereas the CIL for this:

public sealed class InDirect
{
    //...
}

looks like this:

.class public auto ansi sealed beforefieldinit Indirect
       extends [mscorlib]System.Object
{
    //...
}

Can be a volatile field?

You can’t declare a user-defined struct type as a volatile field but you can declare a user-defined class type as a volatile field.

class Bad
{
    private volatile Direct field; // compile-time error
}
class Good
{
    private volatile Indirect field; // compiles ok
}

Can have synchronized methods?

You can’t use the [MethodImpl(MethodImplOptions.Synchronized)] attribute on methods of a struct type (if you call the method you get a runtime TypeLoadException) whereas you can use the [MethodImpl(MethodImplOptions.Synchronized)] attribute on methods of a class type.

using System.Runtime.CompilerServices;

class Indirect
{
    [MethodImpl(MethodImplOptions.Synchronized)] // compiles and runs ok
    public void Method()
    {
        //...
    }
}

struct Direct
{
    [MethodImpl(MethodImplOptions.Synchronized)] // compiles ok, runtime TypeLoadException
    public void Method()
    {
        //...
    }
}

Can be pointed to?

Clause 25.2 of the C# standard defines an unmanaged type as any type that isn’t a reference type and doesn’t contain reference-type fields at any level of nesting. That is, one of the following:

• Any simple value type (11.1.3, eg byte, int, long, double, bool, etc).
• Any enum type.
• Any pointer type.
• Any user-defined struct-type that contains fields of unmanaged types only.

class Bad
{
    static void Main()
    {
        Indirect variable = new Indirect();
        unsafe
        {
            fixed(Indirect * ptr = &variable) // compile-time error
            {
                //...
            }
        }
    }
}

class Indirect
{
    public int fixHandle;
}
class Bad
{
    static void Main()
    {
        Indirect variable = new Indirect();
        unsafe
        {
            fixed(int * ptr = &variable.fixHandle) // compiles ok
            {
                //...
            }
        }
    }
}

struct Direct
{
    // no reference fields at any level of nesting
}
class SimpleCase
{
    static void Main()
    {
        Direct variable = new Direct();
        unsafe
        {
            Direct * ptr = &variable; // compiles ok
            //...       
        }
    }
}

So, in summary, you can never create a pointer to a class type but you sometimes create a pointer to a struct type.

Can be stackalloc’d?

You can only use stackalloc on unmanaged types. Hence you can never use stackalloc on class types. For example:

class Indirect
{
    //...
}
class Bad
{
    static void Main()
    {
        unsafe
        {
            Indirect * array = stackalloc Indirect[42]; // compile-time error
            //...
        }
    }
}

struct Direct
{
    // no reference fields at any level of nesting
}
class Good
{
    static void Main()
    {
        unsafe
        {
            Direct * array = stackalloc Direct[42]; // compiles ok
            //...
        }
    }
}

Can be sizeof’d?

You can only use sizeof on unmanaged types. Hence you can never use sizeof on class types. For example:

class Indirect
{
    //...
}
class Bad
{
    static void Main()
    {
        unsafe
        {
            int size = sizeof(Indirect); // compile-time error
            //...
        }
    }
}

struct Direct
{
    // no reference fields at any level of nesting
}
class Good
{
    static void Main()
    {
        unsafe
        {
            int size = sizeof(Direct); // compiles ok
            //...
        }
    }
}

How to initialize fields?

The fields of a class have a default initialization to zero/false/null. The fields of a struct have no default value.

struct Direct
{
    public int Field;
}

class Indirect
{
    public Indirect()
    {
    }
    //...
    public int Field;
}

class Defaults
{
    static void Main()
    {
        Direct s;
        Console.WriteLine(s.Field);  // compile-time error

        Indirect c = new Indirect();
        Console.WriteLine(c.Field); // compiles ok
    }
}

You can initialize fields in a class at their point of declaration. For example:

class Indirect
{
    //...
    private int field = 42;
}

struct Direct
{
    //...
    private int field = 42; // compile-time error
}

struct Direct
{
    public Direct(int value)
    {
        field = value;
    }
    //...
    private int field; // compiles ok
}

struct Direct
{
    public int X, Y;
}
class Example
{
    static void Main()
    {
        Direct d;
        d.X = 42;
        Console.WriteLine(d.X); // compiles ok
        Console.WriteLine(d.Y); // compile-time error
    }
}

Inheritance differences?

• a struct is implicitly sealed, a class isn’t.
• a struct can’t be abstract, a class can.
• a struct can’t call : base() in its constructor whereas a class with no explicit base class can.
• a struct can’t extend another class, a class can.
• a struct can’t declare protected members (eg fields, nested types) a class can.
• a struct can’t declare abstract function members, an abstract class can.
• a struct can’t declare virtual function members, a class can.
• a struct can’t declare sealed function members, a class can.
• a struct can’t declare override function members, a class can. The one exception to this rule is that a struct can override the virtual methods of System.Object, viz, Equals(), and GetHashCode(), and ToString().

Equals behavior?

classes inherit Object.Equals which implements identity equality whereas structs inherit ValueType.Equals which implements value equality.

using System.Diagnostics;

struct Direct
{
    public Direct(int value)
    {
        field = value;
    }
    private int field;
}

class Indirect
{
    public Indirect(int value)
    {
        field = value;
    }
    private int field;
}

class EqualsBehavior
{
    static void Main()
    {
        Direct s1 = new Direct(42);
        Direct s2 = new Direct(42);

        Indirect c1 = new Indirect(42);
        Indirect c2 = new Indirect(42);

        bool structEquality = s1.Equals(s2);
        bool classIdentity  = !c1.Equals(c2);

        Debug.Assert(structEquality);
        Debug.Assert(classIdentity);

    }
}

struct Direct
{
    public Direct(int value)
    {
        field = value;
    }

    public override bool Equals(object other)
    {
        return other is Direct && Equals((Direct)other);
    }

    public static bool operator==(Direct lhs, Direct rhs)
    {
        return lhs.Equals(rhs);
    }

    //...    

    private bool Equals(Direct other)
    {
        return field == other.field;
    }

    private int field;
}