The Visitor Pattern Explained

In my last job interview I got a (rather vague) question about traversing a tree and operating on the tree nodes. I think I’ve a lot of experience in programming but I couldn’t figure out the answer on my own. The answer the guy wanted to hear was: visitor pattern.

I had never heard of it before. So, in preparation for my next job interview, I thought I take a look at it.

While trying to figure it out, I stumbled over this quote:

The Visitor pattern is possibly the most complicated design pattern you will face. (Source)

I totally agree. (And this is probably why I’ve never heard or used it before.)

But since I’m not a quitter I went on and tamed it. So, in this article I’m going to shed some light on this mysterious design pattern.

Read more →

P/Invoke Tutorial: Pinning (Part 4)

Sometimes a C/C++ function needs to store data you pass to it for later reference. If such data is a managed object (like a string or class) you need to make sure that the garbage collector doesn’t delete it while it’s still used/stored in the native code.

That’s what pinning is for. It prevents the garbage collector from deleting and moving the object.

Read more →

Bash variable inheritances cheat sheet

Just a quick cheat sheet about how variables in Bash get inherited.


Here’s the result of a call to (see below):

From Outer (export):    yes
From Outer (no export):
From Inner (export):
From Inner (no export):

From Outer (export):    yes
From Outer (no export): yes
From Inner (export):    yes
From Inner (no export): yes


The test consists of two files: and is called by the user and internally calls – once directly and once with source.

Contents of


export FROM_OUTER_EXPORT="yes"

echo "Call"
echo "-------"
echo "From Inner (export):    $FROM_INNER_EXPORT"
echo "From Inner (no export): $FROM_INNER_NO_EXPORT"

echo "Source"
echo "-------"
source ./
echo "From Inner (export):    $FROM_INNER_EXPORT"
echo "From Inner (no export): $FROM_INNER_NO_EXPORT"

Contents of


echo "From Outer (export):    $FROM_OUTER_EXPORT"
echo "From Outer (no export): $FROM_OUTER_NO_EXPORT"

export FROM_INNER_EXPORT="yes"

Skipping Unreadable Pattern File Error in Mercurial

I had it happened a couple of times now that Mercurial on Windows would give me this error:

skipping unreadable pattern file ‘D:\Documents\Programming\YourProject\’: No such file or directory

This message is strange because the path actually exits.

After some searching I found the solution in the Mercurial bug tracker:

Bug 5325 – “skipping unreadable pattern file” by empty ui.ignore

The title already suggests the solution. There’s an empty ignore= entry in the global mercurial.ini file.


Removing this entry solves the problem.

The file is located here:


Note: It seems this “problem” is caused by SourceTree.

Quick Tip: Start Docker Toolbox Shell via Batch

Are you still stuck with Windows 8.1 or earlier? Then, if you want to use Docker, you have to use the Docker Toolbox.

Docker Toolbox comes with its own “shell” called Docker Quickstart Terminal. It uses Windows’ own command window – which is pretty limited.


As I explored in Command Line Replacement For Windows, there are alternatives.

To be able to interact with Docker Toolbox in these alternative command line tools, you can create a simple batch file that drops you into the same Docker shell as the “Docker Quickstart Terminal”:

@echo off
"%GIT_INSTALL_ROOT%\bin\bash.exe" --login -i "%DOCKER_TOOLBOX_INSTALL_PATH%\"