I would use the native library for each OS i.e. libstdc++ on GNU/Linux and libc++ on Mac OS X.


libc++ is not 100% complete on GNU/Linux, and there&#39;s no real advantage to using it when libstdc++ is more complete.  Also, if you want to link to any other libraries written in C++ they will almost certainly have been built with libstdc++ so you&#39;ll need to link with that too to use them.

More info [here][1] about the completeness of libc++ on various platforms.


  [1]: https://libcxx.llvm.org

Major Linux distributions do not provide LLVM libc++, because:

 1. Unlike Apple and FreeBSD, the GPL+3 is not an issue, so no need to
    implement another stack here. 
 2. Linux components have been developed around GNU libstd++ for ages.  Some of them
    do not build on anything else.
 3. While libc++ is strong in new features, it has some problems with legacy code. 
    
If eventually libc++ became part of distributions, it will be as an optional component. linking against it will probably require extra options.  

Like Jonathan said, you should use whatever tool is included by default.  Clang is safe in Linux to use since is configured as a GCC replacement, so in that aspect you don&#39;t have to worry about 2 compilers.  Also since you are targeting two platforms, you should take a look to cmake.

I am trying to get the index of an element in a vector of `strings`, to use it as an index in another vector of `int` type, is this possible ?

Example: 

    vector &lt;string&gt; Names;
    vector &lt;int&gt; Numbers;

     ... 
    // condition to check whether the name exists or not
    if((find(Names.begin(), Names.end(), old_name_)) != Names.end())  
		{   // if yes
	        cout &lt;&lt;&quot;Enter the new name.&quot;&lt;&lt; endl;
			cin &gt;&gt; name;
			replace(Names.begin(), Names.end(), old_name_, name);
		}

Now I want to get the position of `old_name` in the `Names` vector, to use it in accessing certain element in `Numbers` vector. So that I can say:

    Numbers[position] = 3 ; // or whatever value assigned here.

I tried using:

    vector &lt;string&gt; :: const_iterator pos;
    pos = (find(Names.begin(), Names.end(), old_name_))
    Numbers[pos] = 3;

but obviously this doesn&#39;t work since `pos` is of type string !

How to get position of a certain element in strings vector, to use it as an index in ints vector?

Say I have an array. I wish to pass the array to a function. The function, however, expects two arguments. Is there a way to on the fly convert the array into 2 arguments?
For example:

    a = [0,1,2,3,4]
    b = [2,3]
    a.slice(b)
Would yield an error in Ruby. I need to input `a.slice(b[0],b[1])` I am looking for something more elegant, as in `a.slice(foo.bar(b))`
Thanks.





ruby convert array into function arguments

I am developing command line interface executables for both osx and linux using c/c++.  The project will link against opencv.  Should I use libc++ or libstdc++?  

Should I use libc++ or libstdc++?

<p>I am developing command line interface executables for both osx and linux using c/c++.  The project will link against opencv.  Should I use libc++ or libstdc++?</p>


I have a server application written in ASP.NET on Windows that provides a web service.

How can I call the web service in Linux with cURL?


HTTP POST and GET using cURL in Linux

I want to rename all files and directories that contain the word &quot;special&quot; to &quot;regular&quot;. It should maintain case sensitivity so &quot;Special&quot; won&#39;t become &quot;regular&quot;.

How can i do this in bash recursively? 

Rename files and directories recursively under ubuntu /bash

I know some very basic commands in Linux and am trying to write some scripts. I have written a function which evaluates the sum of last 2-digits in a 5-digit number. The function should concatenate this resultant sum in between the last 2-digits and return it. The reason I want to return this value is because I will be using this value in the other function. 

Ex: if I have 12345, then my function will calculate 4+5 and return 495.

    #!/bin/bash
    
    set -x
    echo &quot;enter: &quot;
            read input
    
    function password_formula
    {
            length=${#input}
            last_two=${input:length-2:length}
            first=`echo $last_two| sed -e &#39;s/$.$/\1 /g&#39;|awk &#39;{print $2}&#39;`
            second=`echo $last_two| sed -e &#39;s/$.$/\1 /g&#39;|awk &#39;{print $1}&#39;`
            let sum=$first+$second
            sum_len=${#sum}
            echo $second
            echo $sum
    
            if [ $sum -gt 9 ]
            then
                   sum=${sum:1}
            fi
    
            value=$second$sum$first
            return $value
    }
    result=$(password_formula)
    echo $result

I am trying to echo and see the result but I am getting the output as shown below. 

    -bash-3.2$ ./file2.sh 
    +++ password_formula
    +++ echo &#39;enter: &#39;
    +++ read input
    12385
    +++ length=8
    +++ last_two=85
    ++++ echo 85
    ++++ sed -e &#39;s/$.$/\1 /g&#39;
    ++++ awk &#39;{print $2}&#39;
    +++ first=5
    ++++ echo 85
    ++++ sed -e &#39;s/$.$/\1 /g&#39;
    ++++ awk &#39;{print $1}&#39;
    +++ second=8
    +++ let sum=5+8
    +++ sum_len=2
    +++ echo 5
    +++ echo 8
    +++ echo 13
    +++ &#39;[&#39; 13 -gt 9 &#39;]&#39;
    +++ sum=3
    +++ value=835
    +++ return 835
    ++ result=&#39;enter: 
    5
    8
    13&#39;
    ++ echo enter: 5 8 13
    enter: 5 8 13


I also tried to print the result as:

    password_formula
    RESULT=$?
    echo $RESULT

But that is giving some unknown value: 

    ++ RESULT=67
    ++ echo 67
    67


How can I properly store the correct value and print (to double check) on the screen? 

Thanks in advance.






In bash, how to store a return value in a variable?

The following command attempts to enumerate all `*.txt` files in the current directory and process them one by one:

    for line in &quot;find . -iname &#39;*.txt&#39;&quot;; do 
         echo $line
         ls -l $line; 
    done


Why do I get the following error?:

    ls: invalid option -- &#39;e&#39;
    Try `ls --help&#39; for more information.



How to perform a for-each loop over all the files under a specified path?

I am stuck on a little problem. I have a command which pipes output to awk but I want to capture the output of to an array one by one.

My example:

    myarr=$(ps -u kdride | awk &#39;{ print $1 }&#39;)

But that capture all my output into one giant string separated by commas:

    output: PID 3856 5339 6483 10448 15313 15314 15315 15316 22348 29589 29593 32657 1

I also tried the following:

    IFS=&quot;,&quot;
    myarr=$(ps -u kdride | awk &#39;{ print $1&quot;,&quot; }&#39;)

    But the output is: PID, 3856, 5339, 6483, 10448, 15293, 15294, 15295, 15296, 22348, 29589, 29593, 32657,
    1

I want to be able to capture each individual pid into its own array element. Setting `IFS = &#39;\n&#39;` does not do anything and retains my original output. What change do I need to do to make this work?

Bash capturing output of awk into array

I&#39;ve started to use my Mac to install Python packages in the same way I do with my Windows PC at work; however on my Mac I&#39;ve come across frequent *permission denied* errors while writing to log files or site-packages.

Therefore I thought about running `pip install &lt;package&gt;` under `sudo` but is that a safe/acceptable use of sudo considering I&#39;m just wanting this to be installed under my current user account?

Example traceback from a logfile I/O error:

    Command /usr/bin/python -c &quot;import setuptools;__file__=&#39;/Users/markwalker/build/pycrypto/setup.py&#39;;exec(compile(open(__file__).read().replace(&#39;\r\n&#39;, &#39;\n&#39;), __file__, &#39;exec&#39;))&quot; install --single-version-externally-managed --record /var/folders/tq/hy1fz_4j27v6rstzzw4vymnr0000gp/T/pip-k6f2FU-record/install-record.txt failed with error code 1 in /Users/markwalker/build/pycrypto
    Storing complete log in /Users/markwalker/Library/Logs/pip.log
    Traceback (most recent call last):
      File &quot;/usr/local/bin/pip&quot;, line 8, in &lt;module&gt;
        load_entry_point(&#39;pip==1.1&#39;, &#39;console_scripts&#39;, &#39;pip&#39;)()
      File &quot;/Library/Python/2.7/site-packages/pip-1.1-py2.7.egg/pip/__init__.py&quot;, line 116, in main
        return command.main(args[1:], options)
      File &quot;/Library/Python/2.7/site-packages/pip-1.1-py2.7.egg/pip/basecommand.py&quot;, line 141, in main
        log_fp = open_logfile(log_fn, &#39;w&#39;)
      File &quot;/Library/Python/2.7/site-packages/pip-1.1-py2.7.egg/pip/basecommand.py&quot;, line 168, in open_logfile
        log_fp = open(filename, mode)
    IOError: [Errno 13] Permission denied: &#39;/Users/markwalker/Library/Logs/pip.log&#39;

**Update**
This was likely down to permissions, however the best approach is to use virtual environments for your python projects. Running `sudo pip` should be avoided unless absolutely necessary.

Getting &quot;Permission Denied&quot; when running pip as root on my Mac

I just made a .c file and compiled it with gcc in the terminal on OS X 10.8.2. 

My syntax was gcc -o `&lt;filename&gt; &lt;sourcefile&gt;` and that was fine. Now I see I have an executable and file `&lt;filename&gt;` tells me as such, but I&#39;m not sure how to actually run it beside double clicking the icon, /lame. When I try running the executable by just typing &lt;filename&gt; I get command not found, but I thought it was executable...?

Thank you. 

EDIT: I&quot;m so stupid, it was just open `&lt;filename&gt;`

Running an executable in Mac Terminal

I&#39;m using MacOS X 10.7.5 and I need a newer OpenSSL version due to [handshake failures](https://stackoverflow.com/questions/7363259/osx10-7-ssl-handshake-failed). There are several tutorials on the internet and I tried the following:

    brew install openssl
    brew link openssl --force

Nevertheless, it does not work:

    openssl version
    OpenSSL 0.9.8r 8 Feb 2011

    brew unlink openssl &amp;&amp; brew link openssl --force
    Unlinking /usr/local/Cellar/openssl/1.0.1e... 1139 links removed
    Linking /usr/local/Cellar/openssl/1.0.1e... 1139 symlinks created

The SVN issue is not resolved either. Any ideas? I would rather not try the MacPorts way because it may interfere with Homebrew.

Update OpenSSL on OS X with Homebrew

Using rvm I upgraded my ruby to 1.9.3-p392, also added 2.0.0, anytime I try to use this version when I run my bundle command I get this error.

    Could not load OpenSSL.
    You must recompile Ruby with OpenSSL support or change the sources in your Gemfile from &#39;https&#39; to &#39;http&#39;. Instructions for compiling with OpenSSL using RVM are
    available at rvm.io/packages/openssl.

I have followed several different instructions on how to fix this. I have tried removing the version and installing it with the rvm options

    --with-openssl-dir=$HOME/.rvm/usr

I have installed OpenSSL with rvm, and macports. Both have not helped the problem. I even changed the file location of the ssl cert using this:

    export SSL_CERT_FILE=/Users/DarkLord/.rvm/usr/ssl/cert.pem

No matter what I do I continue to get the error. 
Can someone please help me fix this?


Ruby and &quot;You must recompile Ruby with OpenSSL support or change the sources in your Gemfile&quot;

I&#39;m a postgres novice. 

I installed the postgres.app for mac. I was playing around with the psql commands and I accidentally dropped the postgres database. I don&#39;t know what was in it. 

I&#39;m currently working on a tutorial: http://www.rosslaird.com/blog/building-a-project-with-mezzanine/

And I&#39;m stuck at `sudo -u postgres psql postgres`

ERROR MESSAGE: `psql: FATAL:  role &quot;postgres&quot; does not exist`

$ which psql 
     
    /Applications/Postgres.app/Contents/MacOS/bin/psql


This is what prints out of `psql -l`

	                                List of databases
	    Name    |   Owner    | Encoding | Collate | Ctype |     Access privileges     
	------------+------------+----------+---------+-------+---------------------------
	 user       | user       | UTF8     | en_US   | en_US | 
	 template0  | user       | UTF8     | en_US   | en_US | =c/user                  +
	            |            |          |         |       | user      =CTc/user      
	 template1  | user       | UTF8     | en_US   | en_US | =c/user                  +
	            |            |          |         |       | user      =CTc/user      
	(3 rows)



So what are the steps I should take? Delete an everything related to psql and reinstall everything?

Thanks for the help guys!

psql: FATAL: role &quot;postgres&quot; does not exist

I have an image and I want to extract a region from it. I have coordinates of left upper corner and right lower corner of this region. In gray scale I do it like this:

    I = cv2.imread(&quot;lena.png&quot;)
    I = cv2.cvtColor(I, cv2.COLOR_RGB2GRAY)
    region = I[248:280,245:288]
    tools.show_1_image_pylab(region)

I can&#39;t figure it out how to do it in color. I thought of extracting each channel R, G, B; slicing this region from each of the channels and to merge them back together but there is gotta be a shorter way. 



Extracting a region from an image using slicing in Python, OpenCV

I have 4 processors and am compiling a processor-hungry application. I read that using `make` with the `-j4` switch was recommended for OpenCV; should I instead use `-j8`? What is the advantage of making for multiple processors?

Using make with -j4 or -j8

I have installed OpenCV using the instructions in https://help.ubuntu.com/community/OpenCV

    $ sudo su
    $ sudo apt-get install build-essential
    $ sudo apt-get install libavformat-dev
    $ sudo apt-get install ffmpeg
    $ sudo apt-get install libcv2.3 libcvaux2.3 libhighgui2.3 python-opencv opencv-doc libcv-dev libcvaux-dev libhighgui-dev

now when i execute &quot;pkg-config --cflags --libs opencv&quot; i get this error:

    Package opencv was not found in the pkg-config search path.
    Perhaps you should add the directory containing `opencv.pc&#39;
    to the PKG_CONFIG_PATH environment variable
    No package &#39;opencv&#39; found

how can I resolve this problem?

**-------UPDATE-------**

OK, I figured out how to solve the problem...

I made a file named &quot;opencv.pc&quot; and copied it to &quot;/usr/local/lib/pkgconfig&quot;
Then i added these two lines to &quot;.bashrc&quot;:

    PKG_CONFIG_PATH=$PKG_CONFIG_PATH:/usr/local/lib/pkgconfig
    export PKG_CONFIG_PATH

that&#39;s it! everything is OK now.

the contents of the file are:

    prefix=/usr
    exec_prefix=${prefix}
    includedir=${prefix}/include
    libdir=${exec_prefix}/lib
    
    Name: opencv
    Description: The opencv library
    Version: 2.x.x
    Cflags: -I${includedir}/opencv -I${includedir}/opencv2
    Libs: -L${libdir} -lopencv_calib3d -lopencv_imgproc -lopencv_contrib -lopencv_legacy -lopencv_core -lopencv_ml -lopencv_features2d -lopencv_objdetect -lopencv_flann -lopencv_video -lopencv_highgui

**UPDATE - 2014**



it seems that the ubuntu community has completed the documentation on installing openCV, all you have to do now is to download the installation script from https://github.com/jayrambhia/Install-OpenCV/blob/master/Ubuntu/opencv_latest.sh and execute it.

Package opencv was not found in the pkg-config search path

How can I crop images, like I&#39;ve done before in PIL, using OpenCV. 

Working example on PIL

    im = Image.open(&#39;0.png&#39;).convert(&#39;L&#39;)
    im = im.crop((1, 1, 98, 33))
    im.save(&#39;_0.png&#39;)

But how I can do it on OpenCV? 

This is what I tried:

    im = cv.imread(&#39;0.png&#39;, cv.CV_LOAD_IMAGE_GRAYSCALE)
    (thresh, im_bw) = cv.threshold(im, 128, 255, cv.THRESH_OTSU)
    im = cv.getRectSubPix(im_bw, (98, 33), (1, 1))
    cv.imshow(&#39;Img&#39;, im)
    cv.waitKey(0)

But it doesn&#39;t work.

I think I incorrectly used `getRectSubPix`. If this is the case, please explain how I can correctly use this function.

How to crop an image in OpenCV using Python

I&#39;m new to opencv, here is a question, what is the python function which act the same as cv::clone() in cpp?
I just try to get a rect by 

        rectImg = img[10:20, 10:20]

but when I draw a line on it ,I find the line appear both on img and the rectImage,so , how can I get this done?

Clone an image in cv2 python

When is it necessary to use use the flag `-stdlib=libstdc++` for the compiler and linker when compiling with gcc?

Does the compiler automatically use libstdc++?  

I am using gcc4.8.2 on Ubuntu 13.10 and I would like to use the c++11 standard. I already pass `-std=c++11` to the compiler.

When is it necessary to use the flag -stdlib=libstdc++?

[libstdc++][1]&#39;s implementation of pair has the following oddity

    template&lt;typename, typename&gt; class __pair_base
      {
        template&lt;typename T, typename U&gt; friend struct pair;
        __pair_base() = default;
        ~__pair_base() = default;
        __pair_base(const __pair_base&amp;) = default;
        __pair_base&amp; operator=(const __pair_base&amp;) = delete;
      };
    
    template&lt;typename T, typename U&gt;
      struct pair
      : private __pair_base&lt;T, U&gt;
    { /* never uses __pair_base */ };

`__pair_base` is never used, neither can it, considering it&#39;s empty. This is particularly confusing since `std::pair` is [required][2] to be conditionally structural

&gt; `pair&lt;T, U&gt;` is a structural type if `T` and `U` are both structural types.

And having private bases makes it non-structural.

[1]: https://github.com/gcc-mirror/gcc/blob/16e2427f50c208dfe07d07f18009969502c25dc8/libstdc%2B%2B-v3/include/bits/stl_pair.h#L189
[2]: https://timsong-cpp.github.io/cppwp/pairs#pair-4

What is the purpose of having an empty pair base class?

[This answer](https://stackoverflow.com/a/10319672/1805388) gives a nice high-level overview of short string optimization (SSO). However, I would like to know in more detail how it works in practice, specifically in the libc++ implementation:

 - How short does the string have to be in order to qualify for SSO?
   Does this depend on the target architecture?
   
 - How does the implementation distinguish between short and long
   strings when accessing the string data? Is it as simple as `m_size &lt;= 16` or is it a flag that is part of some other member variable? (I
   imagine that `m_size` or part of it might also be used to store
   string data).

I asked this question specifically for libc++ because I know that it uses SSO, this is even mentioned on the [libc++ home page](http://libcxx.llvm.org/).

Here are some observations after looking at [the source](http://llvm.org/svn/llvm-project/libcxx/trunk/include/string):

libc++ can be compiled with two slightly different memory layouts for the string class, this is governed by the `_LIBCPP_ALTERNATE_STRING_LAYOUT` flag. Both of the layouts also distinguish between little-endian and big-endian machines which leaves us with a total of 4 different variants. I will assume the &quot;normal&quot; layout and little-endian in what follows.


Assuming further that `size_type` is 4 bytes and that `value_type` is 1 byte, this is what the first 4 bytes of a string would look like in memory:

    // short string: (s)ize and 3 bytes of char (d)ata
    sssssss0;dddddddd;dddddddd;dddddddd
           ^- is_long = 0

    // long string: (c)apacity
    ccccccc1;cccccccc;cccccccc;cccccccc
           ^- is_long = 1

Since the size of the short string is in the upper 7 bits, it needs to be shifted when accessing it:

    size_type __get_short_size() const {
        return __r_.first().__s.__size_ &gt;&gt; 1;
    }

Similarly, the getter and setter for the capacity of a long string uses `__long_mask` to work around the `is_long` bit.

I am still looking for an answer to my first question, i.e. what value would `__min_cap`, the capacity of short strings, take for different architectures?

**Other standard library implementations**

[This answer](https://stackoverflow.com/a/28003328/1805388) gives a nice overview of `std::string` memory layouts in other standard library implementations.

What are the mechanics of short string optimization in libc++?

Section 23.3.7 Class `vector&lt;bool&gt;` [vector.bool], paragraph 1 states:

    template &lt;class Allocator&gt; class vector&lt;bool, Allocator&gt; {
    public:
        // types:
        typedef bool              const_reference;
        ...

However this program fails to compile when using libc++:

    #include &lt;vector&gt;
    #include &lt;type_traits&gt;

    int
    main()
    {
        static_assert(std::is_same&lt;std::vector&lt;bool&gt;::const_reference, bool&gt;{}, &quot;?&quot;);
    }

Furthermore I note that the C++ standard has been consistent in this specification all the way back to C++98.  And I further note that libc++ has consistently not followed this specification since the first introduction of libc++.

What is the motivation for this non-conformance?

Content Type	Original Author	Original Content on Stackoverflow
Question	MobileDev	View Question on Stackoverflow
Solution 1 - Linux	Jonathan Wakely	View Answer on Stackoverflow
Solution 2 - Linux	Mario Vazquez	View Answer on Stackoverflow

Should I use libc++ or libstdc++?

Linux Problem Overview

Linux Solutions

Solution 1 - Linux

Solution 2 - Linux

ruby convert array into function arguments

How to get position of a certain element in strings vector, to use it as an index in ints vector?

Attributions