Our Craft

replace_all for C++

danielmeyer — Fri, 06 Nov 2009 16:26:16 +0000

I’m always surprised by some of the basic functionality C++’s standard library lacks. This week’s: a function to replace all occurrences within string x of string a with string b. Of course it can be done, it’s just unwieldy given only the standard library facilities.

That’s why I’m so thankful for the Boost libraries. Looks like <boost/algorithm/string/replace.hpp>’s replace_all() or replace_all_copy() would do the trick!

Is it an assigment or a copy construct?

danielmeyer — Tue, 27 Oct 2009 14:17:53 +0000

Suppose we have the following code:

myclass a;
myclass b = a;

Does the second line invoke b’s default constructor and then the assignment operator? Wouldn’t it be more efficient to rewrite it like this:

myclass b(a);

Actually, though, the first and second versions are equivalent: both result in a single copy constructor call; neither one uses the assignment operator.

Let’s try an example to demonstrate this.

#include 

class myclass
{
private:
    std::string innards;
public:
    myclass& operator=(const myclass& c);
    myclass(const myclass& c);
    myclass();
    ~myclass();
};

myclass& myclass::operator=(const myclass& c)
{
    std::cout << "myclass::operator=\n";
    if(&c != this)
    {
        innards = c.innards;
    }
    return *this;
}

myclass::myclass(const myclass& c)
 : innards(c.innards)
{
    std::cout << "myclass::myclass(const myclass& c)\n";
}

myclass::myclass()
{
    std::cout << "myclass::myclass()\n";
}

myclass::~myclass()
{
    std::cout << "myclass::~myclass()\n";
}

int main(void)
{
    myclass a;
    myclass b = a;
    return 0;
}

The output is:

myclass::myclass()
myclass::myclass(const myclass& c)
myclass::~myclass()
myclass::~myclass()

A default construct, a copy construct, and two destructs.

4040404

danielmeyer — Sat, 24 Oct 2009 17:53:02 +0000

I thought this was funny: the other day I browsed to a nonexistent URL at a website, and because that page was not found it redirected me to 404.html …which also was not found. So I ended up getting a 404 Not Found error on 404.html. Cool!

Simple GUI with Python and TKinter

danielmeyer — Thu, 22 Oct 2009 12:39:14 +0000

It would be nice to have a way to build quick-n-dirty GUIs so I can GUI-enable tools when that makes sense… but I’ve always just had a mental block about “going to that length”.

Yesterday though, I was flipping through Python in a Nutshell and I saw a GUI example there (p. 329) that was so simple I typed it in and tried it out :

import sys, Tkinter
Tkinter.Label(text="Welcome!").pack()
Tkinter.Button(text="Exit", command=sys.exit).pack()
Tkinter.mainloop()

That yields a simple little dialog:

…that can be resized:

I need to try some more of this!

Defining your C++ template in a .cpp file

danielmeyer — Wed, 21 Oct 2009 21:01:24 +0000

Normally all your C++ template classes are implemented in their header files, and when we tried to move the definition of a template class out to a .cpp file, the compiler complained. I thought I remembered reading in Stroustrup’s The C++ Programming Language of a keyword that could help with this very issue, last time I was doing C++ programming. A little digging turned up the name of the keyword: export (TC++PL §9.2.3). However, it’s currently not a good option (also see the beginning of the discussion).

So, back into the header it goes. :)

playcd

danielmeyer — Wed, 21 Oct 2009 14:18:01 +0000

Here is a simple CD player/pauser written in Python:

# Playcd.py - Play or pause an audio CD
# Author: Daniel S. Meyer
# Version 0.1
#
# Usage: python playcd.py
# (On Windows you can create a shortcut whose target is for example
# C:\installs\python\python.exe c:\bin\playcd.py , and place it on
# your Windows desktop.  Then you can assign a shortcut key (say,
# Ctrl+Alt+P) to that shortcut.  Now you can play and pause your
# CD by pressing the key combination.)
#
# TODOs:
# - When saving the state, also save the list
#   of tracks (or at least the number of tracks) so
#   we can detect if the CD is changed and not "resume"
#   in the middle of a different CD.
# - Add options for next/previous tracks
# - Detect if a track is an audio track before attempting to play it
# - Support playing a device other than the first one
# 

import pygame,os

def get_state_file_name():
    return os.environ['TEMP']+'/playcd-state.tmp'

pygame.cdrom.init()
cd0 = pygame.cdrom.CD(0)
cd0.init()
if cd0.get_busy():
    # Currently playing: pause it
    cd0.pause()

    # Save our state for later resume
    cdstate = open(get_state_file_name(),'w')
    cdstate.write(str(cd0.get_current()[0]) + '\n') # current track
    cdstate.write(str(cd0.get_current()[1])) # position within track
    cdstate.close()
else:
    # Not currently playing: try to resume from saved state
    try:
      cdstate = open(get_state_file_name(), 'r')
      curtrack = int(cdstate.readline())
      pos = float(cdstate.readline())
      cdstate.close()
      os.remove(get_state_file_name())
    except:
      # If anything goes wrong restoring our state,
      # just start playing at the beginning
      curtrack = 0
      pos = 0

    cd0.play(curtrack,pos,None)

    # Work around bug where only first track plays
    cd0.pause()
    cd0.resume()

cd0.quit()
pygame.cdrom.quit()

I set up the shortcut and shortcut key as described in the comments. Now I can play or pause an audio CD by pressing Ctrl+P.

(‘Course, I could have avoided this whole issue by getting a multimedia keyboard that has play and pause buttons on it… but then how would I learn Python? ;)

Codesample CSS style

danielmeyer — Tue, 20 Oct 2009 15:04:48 +0000

When I have source code or want to demonstrate typing at the console, I’d like to show that highlighted in a certain way. I know precious little CSS, but here’s the pinnacle of my current achievement: the code sample box:

int x = 5;
++x;

To accomplish this, I first define the codesample style*:

When I want to make use of this style, I just use a

 tag with a class of codesample:
int x = 5;
++x;

…which yields the look featured at the top.
Next step: Right now I  redefine the codesample style in each internal blog post.  As an alternative, there are a couple of ways to specify an external style sheet…maybe I’ll eventually set that up in SharePoint and have each post that needs it just point to that.
*(Here on WordPress I’ve purchased the custom CSS upgrade and I just put the .codesample part in the custom CSS area of the blog site admin; but for a SharePoint blog post I would do as above, putting the