Make has several [predefined variables][1] among which is `CC`. Initially, it is set at `cc` which is a symlink to the installed C compiler:

    $ readlink -f `which cc`
    /usr/bin/gcc-4.6

Also:

    $ readlink -f `which c++`
    /usr/bin/g++-4.6

You can change it if you want.

You can use `make -p -f /dev/null` to get a list of all implicit rules and variables. I cannot show the output right now because I have a non-standard install and the output is not in English.

[1]: http://www.gnu.org/software/make/manual/make.html#Implicit-Variables &quot;GNU Make Manual&quot;

CXX is an implicit variable in GNU make. [There are others too][1].

Not only that, these implicit variables get used in [implicit rules][2].

Here is an extract relating to how CXX is used by an implicit rule:

&gt; Compiling C++ programs  
&gt;     `n.o` is made automatically from `n.cc`, `n.cpp`, or `n.C` with a recipe
&gt; of the form  
&gt; `$(CXX) $(CPPFLAGS) $(CXXFLAGS) -c`.  
&gt; We encourage you to use the suffix ‘.cc’ for C++ source files instead of ‘.C’.


  [1]: http://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html
  [2]: http://www.gnu.org/software/make/manual/html_node/Catalogue-of-Rules.html#Catalogue-of-Rules

&gt; Can anyone explain where the value of `CXX` comes from?

Like other “magic” variabes (`LD`, `RM`, `MAKE`), it’s predefined internally by make.

This is a variable that an user can override and which has for default value `g++` (in the GNU Make version, at least). There is nothing more to it (it isn&#39;t defined in some file or stuff like that).

Source: [The GNU Make Manual][1]


  [1]: http://www.gnu.org/software/make/manual/make.html#Implicit-Variables

I have this project where I need (on iOS) to detect simple geometric shapes inside an image.

![enter image description here][1]




After searching the internet I have concluded that the best tool for this is OpenCV. The thing is that up until two hours ago I had no idea what OpenCV is and I have never even remotely did anything involving image processing. My main experience is JS/HTML,C#,SQL,Objective-C...

Where do I start with this?

I have found this [answer][2] that I was able to digest and by reading already other stuff, I understand that OpenCV should return an Array of shapes with the points/corners, is this true? Also how will it represent a circle or a half circle? 
Also what about the shape orientation?

Do you know of any Demo iOS project that can demonstrate a similar functionality? 


  [1]: http://i.stack.imgur.com/pKJN7.png
  [2]: https://stackoverflow.com/questions/9663841/how-to-extract-simple-geometric-forms-from-contours-in-opencv

How to detect simple geometric shapes using OpenCV

I am using TimeStamp class to convert seconds into Day,Hours,Minutes,Seconds. I used following code

    public static void calculateTime(long seconds) {
    		int day = (int)TimeUnit.SECONDS.toDays(seconds);		
    		long hours = TimeUnit.SECONDS.toHours(seconds) - TimeUnit.SECONDS.toHours(TimeUnit.SECONDS.toDays(seconds));
    		long minute = TimeUnit.SECONDS.toMinutes(seconds) - TimeUnit.SECONDS.toMinutes(TimeUnit.SECONDS.toHours(seconds));
    		long second = TimeUnit.SECONDS.toSeconds(seconds) - TimeUnit.SECONDS.toSeconds(TimeUnit.SECONDS.toMinutes(seconds));
    		
    		System.out.println(&quot;Day &quot; + day + &quot; Hour &quot; + hours + &quot; Minute &quot; + minute + &quot; Seconds &quot; + second);
    		
    	}

But I am not getting right result. 
For example when I called this method as `calculateTime(3600)` it gives me the result as `Day 0 Hour 1 Minute 60 Seconds 3540` instead of `Day 0 Hour 1 Minute 0 Seconds 0`. 


*What is the wrong with my logic? Please help me.* 

Java: convert seconds into day, hour, minute and seconds using TimeUnit

Code Snippet:

    target_test : test.cc 
        $(CXX) $(CPPFLAGS) $(CFLAGS) test.cc

I know that `CXX` is a variable (containing the compiler command to call), but I was wondering where this variable comes from. The variable is not defined in the makefile and is not an environment variable. Can anyone explain where the value of `CXX` comes from?

Where does the value of CXX in a makefile come from?

Code Snippet:
<pre><code class="hljs language-javascript">target_test : test.cc 
 $(CXX) $(CPPFLAGS) $(CFLAGS) test.cc
</code></pre>
I know that <code>CXX</code> is a variable (containing the compiler command to call), but I was wondering where this variable comes from. The variable is not defined in the makefile and is not an environment variable. Can anyone explain where the value of <code>CXX</code> comes from?

&gt; **Possible Duplicate:**  
&gt; [How do I define a template function within a template class outside of the class definition?](https://stackoverflow.com/questions/5108359/how-do-i-define-a-template-function-within-a-template-class-outside-of-the-class)  

&lt;!-- End of automatically inserted text --&gt;

I&#39;m struggling with the syntax for the case where I have a template member function within a template class:

    template &lt;typename T&gt; class Foo
    {
        void Bar(const T * t);
        template &lt;typename T2&gt; void Bar(const T2 * t);
    };
    
    template &lt;typename T&gt; void Foo&lt;T&gt;::Bar(const T * t)
    {
        // ... no problem ...
    }
    
    template &lt;typename T&gt; void Foo&lt;T&gt;::Bar&lt;typename T2&gt;(const T2 * t)
    {
        // ... this is where I&#39;m tearing my hair out ...
    }

The first member function is fine, but the template member function which handles types other than the base type of the template class is where I am having problems. For the above case I get the following errors:

    template_problem.cpp:12: error: parse error in template argument list
    template_problem.cpp:12: error: expected ‘,’ or ‘...’ before ‘*’ token
    template_problem.cpp:12: error: ISO C++ forbids declaration of ‘T2’ with no type
    template_problem.cpp:12: error: template-id ‘Bar&lt;&lt;expression error&gt; &gt;’ in declaration of primary template
    template_problem.cpp:12: error: prototype for ‘void Foo&lt;T&gt;::Bar(int)’ does not match any in class ‘Foo&lt;T&gt;’
    template_problem.cpp:4: error: candidates are: template&lt;class T&gt; template&lt;class T2&gt; void Foo::Bar(const T2*)
    template_problem.cpp:7: error:                 void Foo&lt;T&gt;::Bar(const T*)
    template_problem.cpp:12: error: template definition of non-template ‘void Foo&lt;T&gt;::Bar(int)’

and I&#39;ve also tried every other syntax variation I can think of for the template version of `Bar`.

How to define a template member function of a template class

Code:

    std::vector&lt;int&gt; x{1,2,3,4};
    std::array&lt;int, 4&gt; y{{1,2,3,4}};

Why do I need double curly braces for std::array?


Why is the C++ initializer_list behavior for std::vector and std::array different?

After conducting some experiments on square matrices of different sizes, a pattern came up. Invariably, **transposing a matrix of size `2^n` is slower than transposing one of size `2^n+1`**. For small values of `n`, the difference is not major.

Big differences occur however over a value of 512. (at least for me)

*Disclaimer: I know the function doesn&#39;t actually transpose the matrix because of the double swap of elements, but it makes no difference.*

Follows the code:

    #define SAMPLES 1000
    #define MATSIZE 512

    #include &lt;time.h&gt;
    #include &lt;iostream&gt;
    int mat[MATSIZE][MATSIZE];
 
    void transpose()
    {
       for ( int i = 0 ; i &lt; MATSIZE ; i++ )
       for ( int j = 0 ; j &lt; MATSIZE ; j++ )
       {
           int aux = mat[i][j];
           mat[i][j] = mat[j][i];
           mat[j][i] = aux;
       }
    }
 
    int main()
    {
       //initialize matrix
       for ( int i = 0 ; i &lt; MATSIZE ; i++ )
       for ( int j = 0 ; j &lt; MATSIZE ; j++ )
           mat[i][j] = i+j;

       int t = clock();
       for ( int i = 0 ; i &lt; SAMPLES ; i++ )
           transpose();
       int elapsed = clock() - t;

       std::cout &lt;&lt; &quot;Average for a matrix of &quot; &lt;&lt; MATSIZE &lt;&lt; &quot;: &quot; &lt;&lt; elapsed / SAMPLES;
    }

Changing `MATSIZE` lets us alter the size (duh!). I posted two versions on ideone:

- **size 512** - average **2.46 ms** - http://ideone.com/1PV7m
- **size 513** - average **0.75  ms** - http://ideone.com/NShpo

In my environment (MSVS 2010, full optimizations), the difference is similar :

- **size 512** - average **2.19 ms** 
- **size 513** - average **0.57  ms**

**Why is this happening?**

Why is transposing a matrix of 512x512 much slower than transposing a matrix of 513x513?

How can I output the value of an `enum class` in C++11? In C++03 it&#39;s like this:

    #include &lt;iostream&gt;
    
    using namespace std;
    
    enum A {
      a = 1,
      b = 69,
      c= 666
    };
    
    int main () {
      A a = A::c;
      cout &lt;&lt; a &lt;&lt; endl;
    }

in c++0x this code doesn&#39;t compile

    #include &lt;iostream&gt;
    
    using namespace std;
    
    enum class A {
      a = 1,
      b = 69,
      c= 666
    };
    
    int main () {
      A a = A::c;
      cout &lt;&lt; a &lt;&lt; endl;
    }


    prog.cpp:13:11: error: cannot bind &#39;std::ostream&#39; lvalue to &#39;std::basic_ostream&lt;char&gt;&amp;&amp;&#39;
    /usr/lib/gcc/i686-pc-linux-gnu/4.5.1/../../../../include/c++/4.5.1/ostream:579:5: error:   initializing argument 1 of &#39;std::basic_ostream&lt;_CharT, _Traits&gt;&amp; std::operator&lt;&lt;(std::basic_ostream&lt;_CharT, _Traits&gt;&amp;&amp;, const _Tp&amp;) [with _CharT = char, _Traits = std::char_traits&lt;char&gt;, _Tp = A]&#39;

compiled at [Ideone.com][1]


  [1]: https://ideone.com/

How can I output the value of an enum class in C++11

On my Linux (and OS X) machines, the `iconv()` function has this prototype:

    size_t iconv (iconv_t, char **inbuf...

while on FreeBSD it looks like this:

    size_t iconv (iconv_t, const char **inbuf...

I would like my C++ code to build on both platforms. With C compilers, passing a `char**` for a `const char**` parameter (or vice versa) typically emits a mere warning; however in C++ it&#39;s a fatal error. So if I pass a `char**`, it won&#39;t compile on BSD, and if I pass a `const char**` it won&#39;t compile on Linux / OS X. How can I write code that compiles on both, without resorting to trying to detect the platform?

One (failed) idea I had was to provide a local prototype that overrides any provided by the header:

    void myfunc(void) {
        size_t iconv (iconv_t, char **inbuf);
        iconv(foo, ptr);
    }

This fails because `iconv` needs C linkage, and you cannot put `extern &quot;C&quot;` within a function (why not?)

The best working idea I&#39;ve come up with is to cast the function pointer itself:

    typedef void (*func_t)(iconv_t, const char **);
    ((func_t)(iconv))(foo, ptr);

but this has the potential to mask other, more serious errors.

How can I portably call a C++ function that takes a char** on some platforms and a const char** on others?

I know that if I defined an enum weekday like this:

	enum weekday {
		MON,
		TUE,
		WED,
		THU,
		FRI,
	};
	
Then, MON would internally equal to 0, by default, and TUE to 1, WED to 2...

But if I define it this way:

	enum weekday {
		MON,
		TUE = 0,
		WED,
		THU,
		FRI,
	};

Then both `MON` and `TUE` would get the value of 0.

How would a system differentiate MON and TUE internally? I mean, if I declare something like this:

    enum weekday today = 0;

Then is today `MON` or `TUE`? Or, philosophically speaking, both?

Why can two different enum enumeration-constants have the same integer value?

I have a C lib and want to call function in this library from C# application. I tried creating a C++/CLI wrapper on the C lib by adding the C lib file as linker input and adding the source files as additional dependencies. 

Is there any better way to achieve this as am not sure how to add C output to c# application.

My C Code - 

    __declspec(dllexport) unsigned long ConnectSession(unsigned long handle,
    							unsigned char *	publicKey,
    							unsigned char	publicKeyLen);

My CPP Wrapper - 

    long MyClass::ConnectSessionWrapper(unsigned long handle,
        							unsigned char *	publicKey,
        							unsigned char	publicKeyLen)
    	{
    		return ConnectSession(handle, publicKey, publicKeyLen);
    	}



Is it possible to call a C function from C#.Net

Does an Integer variable in C occupy 2 bytes or 4 bytes? What are the factors that it depends on?

Most of the textbooks say integer variables occupy 2 bytes.
But when I run a program printing the successive addresses of an array of integers it shows the difference of 4.

Is the size of C &quot;int&quot; 2 bytes or 4 bytes?

I was searching for the source code of the C standard libraries. What I mean with it is, for example, how are cos, abs, printf, scanf, fopen, and all the other standard C functions written, I mean to see their source code.

So while searching for this, I came across with GLIBC, but I don&#39;t know what it actually is. It is GNU C Library, and it contains some source codes, but what are they actually, are they the source code of the standard functions or are they something else? And what is it used for?

What is GLIBC? What is it used for?

I know that arrays in C are just pointers to sequentially stored data. But what differences imply the difference in notation [] and *. I mean in ALL possible usage context.
For example:

    char c[] = &quot;test&quot;;

if you provide this instruction in a function body it will allocate the string on a stack while

    char* c = &quot;test&quot;;

will point to a data (readonly) segment.

Can you list all the differences between these two notations in ALL usage contexts to form a clear general view.

C/C++ int[] vs int* (pointers vs. array notation). What is the difference?

I need to get the nth dependency file from a rule, something similar to $n in bash. I need this because I&#39;d like to feed in individual dependency files as options to the build program.

Here&#39;s an example:

    dep.o: dep.src config1.cfg config2.cfg
        parse -cfg1 $2 -cfg2 $3 -o $@ $&lt;

Is it possible?

How to get the second dependency file using Automatic Variables in a Makefile?

I&#39;m new to this. Just wanted to ask how to compile a makefile. I am using MinGW compiler in C language. Do I have to save all my files in MinGW\bin? because right now my files are in a different directory.

Appreciate the help.

How to compile makefile using MinGW?

I want to print some message while doing build process with a `makefile`. The following one can print the message, but it will not execute the script after it.  How can I fix this issues? 



    ifeq (yes, ${TEST})
            CXXFLAGS := ${CXXFLAGS} -DDESKTOP_TEST
    test:
            @echo &#39;************  TEST VERSION ************&#39;
    else
    release:
            @echo &quot;************ RELEASE VERSIOIN **********&quot;
    endif







How can I print message in Makefile?

I know that the `@` prefix suppresses output from a shell command in Makefiles, and also that the `-` prefix will ignore errors from a shell command. Is there a way to combine the two, i.e. a prefix that suppresses output and ignores errors? I don&#39;t think `@-` or `-@` works.

Suppress and ignore output for Makefile?

I have a makefile in a directory of mine which builds scripts with certain environment variables set. What if I want to create another makefile in the same directory with different environment variables set? How should I name the two make files? Does `makefile.1` and `makefile.2` work? How do I call them?


Content Type	Original Author	Original Content on Stackoverflow
Question	Ananke Leda	View Question on Stackoverflow
Solution 1 - C++	Mihai Maruseac	View Answer on Stackoverflow
Solution 2 - C++	ArjunShankar	View Answer on Stackoverflow
Solution 3 - C++	Konrad Rudolph	View Answer on Stackoverflow
Solution 4 - C++	Norswap	View Answer on Stackoverflow

Where does the value of CXX in a makefile come from?

C++ Problem Overview

C++ Solutions

Solution 1 - C++

Solution 2 - C++

Solution 3 - C++

Solution 4 - C++

Java: convert seconds into day, hour, minute and seconds using TimeUnit

How to detect simple geometric shapes using OpenCV

Attributions