C++: How to handle exception in CTOR

Yesterday I was reading that thread on Linkedin, and I realized once again that the exception handling in/or around a constructor(CTOR) is still an obscure topic as only one commenter mentioned the correct try-catch form for CTOR that can handle every exception even those coming from the initialization list.
The main problem when an exception occur during a CTOR is what have we to do for cleaning memory allocated on the heap.
To summarize the easiest way to handle exception, I produced the following example.

The output will be (I added some comments:

• object_t CTOR ==> object_t allocation in our initializer list put in a smart_ptr (1)
•  test_t CTOR ==> here we enter in the test_t CTOR Impl
• object_t CTOR ==> we allocated a new object (2)
• … here we have thrown the exception
• object_t DTOR ==> the smart pointer object is on the stack, so the stack unwinding run and the memory allocation (1) is cleaned
• here we handle our dynamic allocation ==> we are now in the CTOR catch section, as we have correctly ordered our data member we can check `if (ptr != nullptr)` and delete it if needed.
• object_t DTOR ==> now the memory allocation (2) is released.
• exception just to test... ==> as we have and should always RETHROW the exception, we catch it!

Conclusion: If we based all our class on that model, there is no reason anymore to fear about memory leaks.

Use Beyond Compare as SVN diff tool under Linux

Today a quick note on that topic as I extended my own configuration with that tricks. BeyondCompare is a well known tool to compare and merge different  version file.

I used it a lot combined with ToirtoiseSVN, but as I worked more and more under linux using Remote Workstation, I would also use it. But I mainly work on remote workstation using a putty prompt. I will not describe here how to setup bcompare on a linux workstation, but it’s the 1st thing todo.

Next you have to follow my previous post to redirect the all X application to your local (I mean Windows 7 in my case) X11 server.

Now when you run “svn diff” on a file, you expect to use bcompare. To achieve that goal, follow me step by step.

cd ~

touch .subversion/bcompare_svn.sh

nano .subversion/bcompare_svn.sh

COPY/PASTE the folllowing in that script:

#!/bin/bash

/usr/bin/bcompare $6 $7 &

exit 0

Set execution permission

chmod 755 .subversion/bcompare_svn.sh

nano .subversion/config

Add that line just after “# diff-cmd = diff_program (diff, gdiff, etc.)”

diff-cmd=/home/YOURFOLDER/.subversion/bcompare_svn.sh

And done !

Now at each svn diff  command you start, BeyondCompare will start and display on the own PC.

STL String to/from wide string

Yesterday I spend sometimes to help one of my teammate with a piece of code he wrote to convert a std::string  into a std::wstring. At the beginning it seems to be a complex issue for him and he used a complex C based code managing wchar buffer convert char to wchar with mbstowcs.

But finally as often the STL provide everything we need to convert one string type into the other and it’s quite simple.

I quickly use Ideone to demonstrate how to deal with:

As you see we can just use the iterator (old-style or new style like below)

wstring ws(s.begin(), s.end()); or wstring ws(begin(s), end(s));

Redirect X from remote linux machine on your windows host.

When you use a remote linux machine from your own Windows Desktop machine using a putty connection, you can easily redirect X application on your desktop.

To do that, there is several steps:

1) You need to have a cygwin setup on your windows host, and follow the following instruction to setup all package required to run a X server.

2) Setup your putty connection to allow X11 forwarding.

To check if everything is OK, restart a connection and from linux prompt, check the value of “DISPLAY” (echo $DISPLAY). It should return something similar to:

localhost:13.0

3) To start your windows X11 server, create a shortcut file in your cygwin folder.

the target will be: C:\ cygwin\bin\run.exe /usr/bin/bash.exe -l -c /bin/startxwin.exe

And for its name, “StartX” is a good choice!

Double-click on that shortcut and check that you now have the icon in your Windows systray.

4) To run that server every time you start your window session, save a copy of that shortcut in:

%USERPROFILE%\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup

5) Final check, start xclock from your Linux prompt, the xclock windows will appear on your Windows Desktop !

Text processing: Word List Challenge.

I read this morning a question on Stackoverflow where user tried to solve that text processing challenge with a really complex code.

With only 3 line of code we can do that in C++. Using STL and optionally a BOOST header.

It works in 3 steps:

1) transform to lower case,

2) split using multiple delimiters (boost::any_of),

3) sort the resulting vector.

….

#include <iostream>
#include <string>
#include <vector>
#include <iterator>
#include <algorithm>
using namespace std;

#include "boost/algorithm/string.hpp"

const char* testStr = "This is. a sample piece of, text to illustrate \n this problem.";

int main(int argc, char* argv[])
{
    string inputText(testStr);
    cout << inputText << endl << "*******************" << endl;
    vector<string> strs;
    boost::split(strs,boost::algorithm::to_lower_copy(inputText),boost::is_any_of("\t ,\n,,,."),boost::token_compress_on);
    cout << "list : " << strs.size() << endl;
    cout << "without sort\n", copy(begin(strs), end(strs), ostream_iterator<string>(cout, "\n"));
    std::sort(strs.begin(), strs.end());
    cout << "with sort\n", copy(begin(strs), end(strs), ostream_iterator<string>(cout, "\n"));
    

    return 0;
}

Override a default STL operator >>

In that question, user need to override the default STL operator >> for complex number. First keep in mind that the STL provide a template for complex number define in <complex> and that this type come with its own syntax for I/O.

By default the syntax is (real,imag).

As the user want to use: real+imagi, we have to change the default behavior.

In the following you will see how I wrote a new global function to override the default operator and how I use a Regular Expression to extract the real and imaginary number.

template<class _Ty> inline istream& operator>>(istream& _Istr, complex<_Ty>& _Right)
{    
    // extract a complex<_Ty> with syntax REAL+IMAGi
    long double _Real = 0;
    long double _Imag = 0;
    string s; _Istr >> s;
    string regexPattern = string(R"r(([-+]?\d+\.?\d*|[-+]?\d*\.?\d+)\s*\+\s*([-+]?\d+\.?\d*|[-+]?\d*\.?\d+)i)r");
    regex reg(regexPattern);
    smatch sm;    // same as std::match_results<const char*> cm;
    regex_match (s,sm,reg);
  
    _Real = std::atof(sm[1].str().c_str());
    _Imag = std::atof(sm[2].str().c_str());
    
    _Right = std::complex<_Ty>((_Ty)(_Real), (_Ty)(_Imag));
 
    return (_Istr);
}

There is place for improvement in that version:

• no match


• match are not number


• etc…

But it’s a good example of how to start!

FFMPEG from Compressed to RAW video.

I f consult some of my previous post, you may noticed that I’m an ffmpeg fan.

I often used ffmpeg command line to pipe raw video, in my own soft or some soft I develop in my professional environment. But I recently discover a good to share trick about ffmpeg.

Imagine a video file of 20min at 24fps, you have extract those information using the ffmpeg –i command. But when you extract the raw video frame in a file, you discover that the output file is bigger than expected (like a Tardis….).

Example of simple decoding as YUV 420p:

ffmpeg –I input.mp4 –c:v raw_video –pix_fmt yuv420p – >> output.yuv

the reason can be that if the input file hasn’t PTS corresponding to a Constant Frame-Rate (CFR), ffmpeg duplicate or drop some frames.

hopefully ffmpeg is explicit and full of useful information to troubleshoot those situation, for example in the following ffmpeg output latest line we see that it “duplicate” 60 frame.

frame=  119 fps=0.0 q=0.0 Lsize=  160650kB time=00:00:02.00 bitrate=657102.5kbits/s dup=60 drop=0

To prevent that the parameter to use is “-vsync”.

---

‘-vsync parameter’

Video sync method. For compatibility reasons old values can be specified as numbers. Newly added values will have to be specified as strings always.

‘0, passthrough’

Each frame is passed with its timestamp from the demuxer to the muxer.

‘1, cfr’

Frames will be duplicated and dropped to achieve exactly the requested constant frame rate.

‘2, vfr’

Frames are passed through with their timestamp or dropped so as to prevent 2 frames from having the same timestamp.

‘drop’

As passthrough but destroys all timestamps, making the muxer generate fresh timestamps based on frame-rate.

‘-1, auto’

Chooses between 1 and 2 depending on muxer capabilities. This is the default method.

---

So to only have the encoded frame in our output the correct line is:

ffmpeg –I input.mp4 –vsync 0 –c:v raw_video –pix_fmt yuv420p – >> output.yuv