Posts Tagged ‘mercurial

27
Nov
13

Using ediff with Version Control

This is just a quick note about a nice emacs feature.

If you develop with emacs, you may have heard of/used ediff. This is a very handy diff program which runs inside emacs. We use this with a custom hook at work to work with our version control system. I found out that it works nicely with more standard version control systems as well.

The main way to use ediff is the command

  ediff-buffers

This lets you choose two buffers to diff. To do this for different revisions of a file you can use the command

  ediff-revision

This will ask for which file you want to view revisions of (default, the current buffer), and the two revisions to compare (default latest revision and the current state). The you will have the file you asked for loaded in the two revisions you asked for.

Using mercurial personally I find this more informative than the output from hg diff in complicated cases. I believe this works for any version control system recognised by emacs, e.g. mercurial git and subversion.

08
Apr
13

Comparisons in C++

This is about the comparison operators in C++ and making them easy to implement. There are 6 comparison operators in C++; ==, !=, <, <=, >, and >=. If you want to be able to apply all of these to a class, and you have the right type of order, you only need to implement one function which will determines them all. Incidentally this is called a total order, but I won’t go into what that means here.

The tl;dr of this is look at this repository and use a class from there to easily implement custom comparison operators.

This idea is to map the order onto the real numbers (doubles). So you have a function which takes two instances of a class and returns a double. If this function returns a negative number then the first instance is less than the second, if it returns 0 they are equal and if it returns a positive number then the first instance is greater than the second.

This will be easier to see with an example, so here is a class which is just data storage for an int.

Continue reading ‘Comparisons in C++’

25
Mar
13

C++ Sandbox

This is just a short post to tell you about my C++ sandbox. This is my area for trying out ideas (such as using mutable for caching) or testing C++ 11 features.

This is stored as a mercurial repository on bitbucket here. Please feel free to have a look or add to it if you have something you want to try out. I often use this to test answers to stackoverflow questions. A few of them have generated blog posts here as well.

The rest of this post is the current contents of the repository with a brief description of what I was trying to do with each file.

Arrays

In this file I was playing around with C style arrays, slightly old and a bit dated to be honest.

Comparisons

This may be a future post here. It defines a templatised base class for comparable class (using the curiously recurring template pattern . It means that if you inherit from this class and define one compare function, you get all 6 comparison operators.

ConstPointer

Testing out what you can do with const pointers. With a const Class* const you can only call const methods, with a Class* const you can call anything (this keeps the pointer const not what it points at) and with a Class* you can call anything.

Constness

Found an unusual bug and tested it here. If you have a reference (or a pointer) as a member of your class, in a const function you can call non-const functions on the member. This is because it doesn’t change the reference (or pointer) which the class keeps, what it does is change is the object it references.

CopyReference

Trying to copy a reference without using the copy constructor or assignment operator.

DivideByZero

I wondered what compiler errors/warnings you get from dividing by zero?
It turns out (in g++) that it will not give errors at all. It will warn you if you divide by a literal zero or a const zero, but not a variable with value zero.

ExclusiveOr

This was for checking the results of using xor on integers.

Lambda

This was testing the newly added lambda functions, but changed to using standard template library algorithms.

MustOverride

This was playing around with pure virtual functions with implementations. So you can call them using Base::function() in the derived class.

Mutable

Testing using a mutable member for data caching. This was the basis for the blog post mutable and data caching.

Operators

Testing calling operator() on pointer. Here is the calling code.

  Foo* f = new Foo();
  (*f)();
  (*f).operator()();
  f->operator()();

  delete f;

A common mistake doing this is to leave off the final brackets so.

  (*f).operator();
  f->operator();

But these will return a function pointer to operator(), they won’t call it.

Polygons

Archetypal polygon example for inheritance. A base polygon, a derived rectangle and triangle. Not great as you use a width and height for the polygon but not all polygons have it. Also I prepended all the classes with the letter C and allocated the instances as pointers to the base class.

SelfDeleting

Trying to make an executable to delete itself, unsurprisingly this doesn’t work.

SpareMemory

Something picked up from a dirty programming trick called the programming antihero (if you haven’t read that article I can’t recommend it enough!). In this there was a game programmer who reserved 2Mb of memory to lose later when memory was tight. I was testing to see how this worked by reserving some memory and checking memory usage. It is had to see the exact memory usage from this sort of thing due to differences in types of memory (i.e. private bytes/working set…) but it certainly took some memory.

StructClass

Testing out the differences between structs and classes in C++. I found that apart from private/public as default there is no difference! This includes if you declare class T in a template, you can still use a struct.

UserDefinedLiterals and UserDefinedLiteralsBinary

Trying to use C++ 11 user defined literals to be able to use syntax like this:

  Binary two = 10_b;

I was following a good guide from here.

VariadicPointerOffset

Quite a complicated file. I was trying to use variadic templates to access private methods of a class. Hopefully not of any practical use (it breaks encapsulation and is extremely fragile) but an interesting idea to try,

Windows

Unimplemented test for SetEvent() in windows.h.

get_env

Made to test getenv from cstdlib for getting environmental variables.

hashes

Using C preprocessor magic to join together variable names.

number_of_coins

Given an amount in cents (American currency as it was taken from an answer on stackoverflow) find the number of ways you can make it from quarters, dimes, nickles and pennies. This was taken from this answer.

stack_overflow_virtual

I didn’t think the reply to this question would compile so I tried it out. I realised you can override private virtual functions but not call them.

upper

My thoughts about the answer to this stackoverflow question being wrong, and my idea for a better way.

As always thanks for reading.

17
Sep
12

I’m Back, and This Time There’s Versioning!

It has been a while! Sorry about that but I’ve been on holiday and moving etc, but now I’m back and a lot has changed since I last posted. I have started using distributed version control with git and mercurial via GitHub and BitBucket. This means that you don’t have to just look at my code on Box with it’s limited syntax editing and no versioning. Now you can see exactly what I’ve been up to! Continue reading ‘I’m Back, and This Time There’s Versioning!’