Seeing from your G++ version, you need to update it badly. C++11 has only been available since G++ 4.3. The most recent version is 4.7.

In versions pre-G++ 4.7, you&#39;ll have to use `-std=c++0x`, for more recent versions you can use `-std=c++11`.

Quoting from the [gcc website][1]:

&gt; C++11 features are available as part of the &quot;mainline&quot; GCC compiler in the trunk of GCC&#39;s Subversion repository and in GCC 4.3 and later. To enable C++0x support, add the command-line parameter -std=c++0x to your g++ command line. Or, to enable GNU extensions in addition to C++0x extensions, add -std=gnu++0x to your g++ command line. GCC 4.7 and later support -std=c++11 and -std=gnu++11 as well.


So probably you use a version of g++ which doesn&#39;t support `-std=c++11`. Try `-std=c++0x` instead.

Availability of C++11 features is for versions &gt;= 4.3 only.

  [1]: http://gcc.gnu.org/projects/cxx0x.html

you should try this 

    g++-4.4 -std=c++0x or g++-4.7 -std=c++0x

I also got same error, compiling with -D flag fixed it, Try this:

g++ -Dstd=c++11 &lt;file_name&gt;

How to write `functionToGenerateMD5hash` for this code? I already have `fileVideo` and I need to send the corresponding md5 hash to the server by clicking on the button.

    $(&quot;#someButton&quot;).click(function() {
    var fr = new FileReader();      
    fr.onload = function(e) {            
        string md5 = functionToGenerateMD5hash(e.target.result);
        // send md5 here
    };       
    fr.readAsArrayBuffer(fileVideo);

Also, can you use this to generate a MD5 hash in Node.js?

How to generate an MD5 file hash in JavaScript/Node.js?

I have created an Android Application Project and in MainActivity.java &gt; `onCreate()` it is calling `super.onCreate(savedInstanceState)`.

As a beginner, can anyone explain what is the purpose of the above line?

super.onCreate(savedInstanceState);

I&#39;m trying to compile using `g++` and either the `-std=c++11` or `c++0x` flags.

However, I get this error 

    cc1plus: error: unrecognized command line option &quot;-std=c++11&quot;

**g++ --version**

    g++ (GCC) 4.1.2 20080704 (Red Hat 4.1.2-54)
    Copyright (C) 2006 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

cc1plus: error: unrecognized command line option &quot;-std=c++11&quot; with g++

I'm trying to compile using <code>g++</code> and either the <code>-std=c++11</code> or <code>c++0x</code> flags.
However, I get this error
<pre><code class="hljs language-vbnet">cc1plus: error: unrecognized command line option "-std=c++11"
</code></pre>
g++ --version
<pre><code class="hljs language-ini">g++ (GCC) 4.1.2 20080704 (Red Hat 4.1.2-54)
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
</code></pre>

Thanks to C++11 we received the `std::function` family of functor wrappers. Unfortunately, I keep hearing only bad things about these new additions. The most popular is that they are horribly slow. I tested it and they truly suck in comparison with templates.

    #include &lt;iostream&gt;
    #include &lt;functional&gt;
    #include &lt;string&gt;
    #include &lt;chrono&gt;
    
    template &lt;typename F&gt;
    float calc1(F f) { return -1.0f * f(3.3f) + 666.0f; }
    
    float calc2(std::function&lt;float(float)&gt; f) { return -1.0f * f(3.3f) + 666.0f; }
    
    int main() {
    	using namespace std::chrono;
    
    	const auto tp1 = system_clock::now();
    	for (int i = 0; i &lt; 1e8; ++i) {
    		calc1([](float arg){ return arg * 0.5f; });
    	}
    	const auto tp2 = high_resolution_clock::now();
    
    	const auto d = duration_cast&lt;milliseconds&gt;(tp2 - tp1);	
    	std::cout &lt;&lt; d.count() &lt;&lt; std::endl;
    	return 0;
    }

111 ms vs 1241 ms. I assume this is because templates can be nicely inlined, while `function`s cover the internals via virtual calls.

Obviously templates have their issues as I see them:

 - they have to be provided as headers which is not something you might not wish to do when releasing your library as a closed code,
 - they may make the compilation time much longer unless `extern template`-like policy is introduced,
 - there is no (at least known to me) clean way of representing requirements (concepts, anyone?) of a template, bar a comment describing what kind of functor is expected.

Can I thus assume that `function`s can be used as *de facto* standard of passing functors, and in places where high performance is expected templates should be used?

----------

Edit:

My compiler is the Visual Studio 2012 *without* CTP.

std::function vs template

Is there a defined order in which local variables are deallocated in C++ (11) ?
To be more concise: In which order will side effects of the destructors of two local variables in the same scope become visible?

e.g.:

    struct X{
      ~X(){/*do something*/}
    }

    int main(){
       X x1;
       X x2;
       return 0;
    }

Is `x1` or `x2` destroyed first when main returns or is the order undefined in C++11?

C++ local variable destruction order

I am having difficulty with reading an image, extracting features for training, and testing on new images in OpenCV using SVMs. can someone please point me to a great link? I have looked at the [OpenCV Introduction to Support Vector Machines](http://docs.opencv.org/doc/tutorials/ml/introduction_to_svm/introduction_to_svm.html#introductiontosvms). But it doesn&#39;t help with reading in images, and I am not sure how to incorporate it.

---

My goals are to classify pixels in an image. These pixel would belong to a curves. I understand forming the training matrix (for instance, 
image A
1,1 1,2 1,3 1,4 1,5
2,1 2,2 2,3 2,4 2,5
3,1 3,2 3,3 3,4 3,5

I would form my training matrix as a [3][2]={ {1,1} {1,2} {1,3} {1,4} {1,5} {2,1} ..{} }

However, I am a little confuse about the labels. From my understanding, I have to specify which row (image) in the training matrix corresponds, which corresponds to a curve or non-curve. But, how can I label a training matrix row (image) if there are some pixels belonging to the curve and some not belonging to a curve. For example, my training matrix is [3][2]={ {1,1} {1,2} {1,3} {1,4} {1,5} {2,1} ..{} }, pixels {1,1} and {1,4} belong to the curve but the rest does not.

using OpenCV and SVM with images

-2147483648 is the smallest integer for integer type with 32 bits, but it seems that it will overflow in the `if(...)` sentence:

    if (-2147483648 &gt; 0)
        std::cout &lt;&lt; &quot;true&quot;;
    else
        std::cout &lt;&lt; &quot;false&quot;;

This will print `true` in my testing. However, if we cast -2147483648 to integer, the result will be different:

    if (int(-2147483648) &gt; 0)
        std::cout &lt;&lt; &quot;true&quot;;
    else
        std::cout &lt;&lt; &quot;false&quot;;

This will print `false`.

I&#39;m confused. Can anyone give an explanation on this?


----------


Update 02-05-2012:

Thanks for your comments, in my compiler, the size of int is 4 bytes. I&#39;m using VC for some simple testing. I&#39;ve changed the description in my question.

That&#39;s a lot of very good replys in this post, [AndreyT][1] gave a very detailed explanation on how the compiler will behave on such input, and how this minimum integer was implemented. [qPCR4vir][2] on the other hand gave some related &quot;curiosities&quot; and how integers are represented. So impressive!


  [1]: https://stackoverflow.com/a/14695202/2040924
  [2]: https://stackoverflow.com/a/14695400/2040924

(-2147483648&gt; 0) returns true in C++?

I downloaded and extracted Crypto++ in C:\cryptopp. I used Visual Studio Express 2012 to build all the projects inside (as instructed in readme), and everything was built successfully. Then I made a test project in some other folder and added cryptolib as a dependency. After that, I added the include path so I can easily include all the headers. When I tried to compile, I got an error about unresolved symbols.

To remedy that, I added `C:\cryptopp\Win32\Output\Debug\cryptlib.lib` to link additional dependencies. Now I get this error:

    Error	1	error LNK2038: mismatch detected for &#39;RuntimeLibrary&#39;: value &#39;MTd_StaticDebug&#39; doesn&#39;t match value &#39;MDd_DynamicDebug&#39; in program.obj	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\cryptlib.lib(cryptlib.obj)	CryptoTest
    Error	2	error LNK2038: mismatch detected for &#39;RuntimeLibrary&#39;: value &#39;MTd_StaticDebug&#39; doesn&#39;t match value &#39;MDd_DynamicDebug&#39; in program.obj	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\cryptlib.lib(iterhash.obj)	CryptoTest
    Error	3	error LNK2038: mismatch detected for &#39;RuntimeLibrary&#39;: value &#39;MTd_StaticDebug&#39; doesn&#39;t match value &#39;MDd_DynamicDebug&#39; in program.obj	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\cryptlib.lib(sha.obj)	CryptoTest
    Error	4	error LNK2038: mismatch detected for &#39;RuntimeLibrary&#39;: value &#39;MTd_StaticDebug&#39; doesn&#39;t match value &#39;MDd_DynamicDebug&#39; in program.obj	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\cryptlib.lib(pch.obj)	CryptoTest
    Error	5	error LNK2038: mismatch detected for &#39;RuntimeLibrary&#39;: value &#39;MTd_StaticDebug&#39; doesn&#39;t match value &#39;MDd_DynamicDebug&#39; in program.obj	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\cryptlib.lib(misc.obj)	CryptoTest
    Error	6	error LNK2038: mismatch detected for &#39;RuntimeLibrary&#39;: value &#39;MTd_StaticDebug&#39; doesn&#39;t match value &#39;MDd_DynamicDebug&#39; in program.obj	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\cryptlib.lib(queue.obj)	CryptoTest
    Error	7	error LNK2038: mismatch detected for &#39;RuntimeLibrary&#39;: value &#39;MTd_StaticDebug&#39; doesn&#39;t match value &#39;MDd_DynamicDebug&#39; in program.obj	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\cryptlib.lib(algparam.obj)	CryptoTest
    Error	8	error LNK2038: mismatch detected for &#39;RuntimeLibrary&#39;: value &#39;MTd_StaticDebug&#39; doesn&#39;t match value &#39;MDd_DynamicDebug&#39; in program.obj	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\cryptlib.lib(filters.obj)	CryptoTest
    Error	9	error LNK2038: mismatch detected for &#39;RuntimeLibrary&#39;: value &#39;MTd_StaticDebug&#39; doesn&#39;t match value &#39;MDd_DynamicDebug&#39; in program.obj	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\cryptlib.lib(fips140.obj)	CryptoTest
    Error	10	error LNK2038: mismatch detected for &#39;RuntimeLibrary&#39;: value &#39;MTd_StaticDebug&#39; doesn&#39;t match value &#39;MDd_DynamicDebug&#39; in program.obj	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\cryptlib.lib(cpu.obj)	CryptoTest
    Error	11	error LNK2038: mismatch detected for &#39;RuntimeLibrary&#39;: value &#39;MTd_StaticDebug&#39; doesn&#39;t match value &#39;MDd_DynamicDebug&#39; in program.obj	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\cryptlib.lib(mqueue.obj)	CryptoTest


I also get:


    Error	12	error LNK2005: &quot;public: __thiscall std::_Container_base12::_Container_base12(void)&quot; (??0_Container_base12@std@@QAE@XZ) already defined in cryptlib.lib(cryptlib.obj)	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\msvcprtd.lib(MSVCP110D.dll)	CryptoTest
    Error	13	error LNK2005: &quot;public: __thiscall std::_Container_base12::~_Container_base12(void)&quot; (??1_Container_base12@std@@QAE@XZ) already defined in cryptlib.lib(cryptlib.obj)	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\msvcprtd.lib(MSVCP110D.dll)	CryptoTest
    Error	14	error LNK2005: &quot;public: void __thiscall std::_Container_base12::_Orphan_all(void)&quot; (?_Orphan_all@_Container_base12@std@@QAEXXZ) already defined in cryptlib.lib(cryptlib.obj)	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\msvcprtd.lib(MSVCP110D.dll)	CryptoTest
    Error	15	error LNK2005: &quot;public: __thiscall std::locale::id::id(unsigned int)&quot; (??0id@locale@std@@QAE@I@Z) already defined in cryptlib.lib(iterhash.obj)	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\msvcprtd.lib(MSVCP110D.dll)	CryptoTest
    Warning	16	warning LNK4098: defaultlib &#39;LIBCMTD&#39; conflicts with use of other libs; use /NODEFAULTLIB:library	C:\Data\Work\C++ VS\CryptoTest\CryptoTest\LINK	CryptoTest
    Error	17	error LNK1169: one or more multiply defined symbols found	C:\Data\Work\C++ VS\CryptoTest\Debug\CryptoTest.exe	1	1	CryptoTest

The code I tried to compile was simple (I got this from another site):

    #include &lt;iostream&gt;
    #include &lt;string&gt;
    #include &quot;sha.h&quot;
    #include &quot;hex.h&quot;
    using namespace std;
    
    string SHA256(string data) {
        byte const* pbData = (byte*) data.data();
        unsigned int nDataLen = data.size();
        byte abDigest[32];
    
        CryptoPP::SHA256().CalculateDigest(abDigest, pbData, nDataLen);
    
        return string((char*)abDigest);
    }
    
    int main(void) {
    
    	return 0;
    }

Any ideas how to fix this? I really only need SHA-256 right now, nothing else.
I am using Windows 7 64 bit, and I downloaded VS C++ today, so it should be the newest version.

Mismatch Detected for &#39;RuntimeLibrary&#39;

In this case

    struct Foo {};
    Foo meh() {
      return std::move(Foo());
    }

I&#39;m pretty sure that the move is unnecessary, because the newly created `Foo` will be an xvalue.

But what in cases like these?

    struct Foo {};
    Foo meh() {
      Foo foo;
      //do something, but knowing that foo can safely be disposed of
      //but does the compiler necessarily know it?
      //we may have references/pointers to foo. how could the compiler know?
      return std::move(foo); //so here the move is needed, right?
    }

There the move is needed, I suppose?

When should std::move be used on a function return value?

I want to create a `std::set` with a custom comparison function. I could define it as a class with `operator()`, but I wanted to enjoy the ability to define a lambda where it is used, so I decided to define the lambda function in the initialization list of the constructor of the class which has the `std::set` as a member. But I can&#39;t get the type of the lambda. Before I proceed, here&#39;s an example:

    class Foo
    {
    private:
         std::set&lt;int, /*???*/&gt; numbers;
    public:
         Foo () : numbers ([](int x, int y)
                           {
                               return x &lt; y;
                           })
         {
         }
    };

I found two solutions after searching: one, using `std::function`. Just have the set comparison function type be `std::function&lt;bool (int, int)&gt;` and pass the lambda exactly like I did. The second solution is to write a make_set function, like `std::make_pair`.

SOLUTION 1:

    class Foo
    {
    private:
         std::set&lt;int, std::function&lt;bool (int, int)&gt; numbers;
    public:
         Foo () : numbers ([](int x, int y)
                           {
                               return x &lt; y;
                           })
         {
         }
    };

SOLUTION 2:

    template &lt;class Key, class Compare&gt;
    std::set&lt;Key, Compare&gt; make_set (Compare compare)
    {
         return std::set&lt;Key, Compare&gt; (compare);
    }

The question is, do I have a good reason to prefer one solution over the other? I prefer the first one because it makes use of standard features (make_set is not a standard function), but I wonder: does using `std::function` make the code (potentially) slower? I mean, does it lower the chance the compiler inlines the comparison function, or it should be smart enough to behave exactly the same like it would it was a lambda function type and not `std::function` (I know, in this case it can&#39;t be a lambda type, but you know, I&#39;m asking in general) ?

(I use GCC, but I&#39;d like to know what popular compilers do in general)

**SUMMARY, AFTER I GOT LOTS OF GREAT ANSWERS:**

If speed is critical, the best solution is to use an class with `operator()` aka functor. It&#39;s easiest for the compiler to optimize and avoid any indirections.

For easy maintenance and a better general-purpose solution, using C++11 features, use `std::function`. It&#39;s still fast (just a little bit slower than the functor, but it may be negligible) and you can use any function - `std::function`, lambda, any callable object.

There&#39;s also an option to use a function pointer, but if there&#39;s no speed issue I think `std::function` is better (if you use C++11).

There&#39;s an option to define the lambda function somewhere else, but then you gain nothing from the comparison function being a lambda expression, since you could as well make it a class with `operator()` and the location of definition wouldn&#39;t be the set construction anyway.

There are more ideas, such as using delegation. If you want a more thorough explanation of all solutions, read the answers :)

C++11 std::set lambda comparison function

You know, we can wrap or store a lambda function to a `std::function`:


    #include &lt;iostream&gt;
    #include &lt;functional&gt;
    int main()
    {
        std::function&lt;float (float, float)&gt; add = [](float a, float b)
        //            ^^^^^^^^^^^^^^^^^^^^
        {
            return a + b;
        };

        std::cout &lt;&lt; add(1, 2) &lt;&lt; std::endl;
    }


My question is around `std::function`, as you can see it is a template class but it can accept any kind of **function signature**.

For example `float (float, float)` in this form `return_value (first_arg, second_arg)`.

What&#39;s the structure of `std::function` and how does it accept a function signature like `x(y,z)` and how it works with it? Is `float (float, float)` a new valid expression in C++?

How std::function works

I am trying to test a method - and getting an error:

&gt; Cannot create an instance of the variable type &#39;Item&#39; because it does not have the new() constraint

Required information for below:

    public interface IHasRect
    {
        Rectangle Rectangle { get; }
    }

Helper class:

    class Item : IHasRect
    {
        public Item(Point p, int size)
        {
            m_size = size;
            m_rectangle = new Rectangle(p.X, p.Y, m_size, m_size); 
        }
    }

For the function to be tested, I need to instantiate an object...

    public class SomeClass&lt;T&gt; where T : IHasRect

The test:

    public void CountTestHelper&lt;Item&gt;() where Item : IHasRect
    {
        Rectangle rectangle = new Rectangle(0, 0, 100, 100); 
        SomeClass&lt;Item&gt; target = new SomeClass&lt;Item&gt;(rectangle);            
        Point p = new Point(10,10);
        Item i = new Item(p, 10);      // error here        
        ...
    }
    [TestMethod()]
    public void CountTest()
    {
        CountTestHelper&lt;Item&gt;();
    }   

   
I am trying to understand what this error means, or how to fix it, by reading http://msdn.microsoft.com/en-us/library/d5x73970.aspx and http://msdn.microsoft.com/en-us/library/x3y47hd4.aspx - but it doesn&#39;t help.

I don&#39;t understand this error - I have already constrained the &quot;SomeClass&quot; to be of &lt;Item&gt; type. I cannot constrain the entire Test class (the unit test class generated by Visual Studio, which contains all the tests) - I will get a number of other errors otherwise. The Item class doesn&#39;t have any template...

Cannot create an instance of the variable type &#39;Item&#39; because it does not have the new() constraint


I use python 2.7 in a virtual environment and PyCharm 2.7 (new build as of feb 07 2013).

Whenever I open a python file in it that has unambiguous errors (equivalent to compile errors in other languages, e.g. using undeclared variables, calling non-existing functions), it shows red stripes in the gutter of the file.

So, I discover errors randomly as I happened to navigate to a file that contains them. What I would really like is to be able to list all of the python errors in a separate window. The Visual Studio 2005/2008/2010/... IDE has a separate &quot;Errors&quot; view that lists all of them with file names and line numbers, and gives me the ability to click on any one of these errors and navigate directly to the source.

Does PyCharm have anything like this?


Can PyCharm list all of Python errors in a project?

When I try to compile this class with javac, I get a compilation error and Test.class is not created.

    public class Test {
    	public static void main(String[] args) {
    		int x = 1L;  // &lt;- this cannot compile
    	}
    }

But when I create this class in Eclipse, I can see that Test.class appears in target/classes. When I try to run this class from command line with java.exe, I get 

&gt;    Exception in thread &quot;main&quot; java.lang.Error: Unresolved compilation problem: &lt;br/&gt;
&gt;    	Type mismatch: cannot convert from long to int

Does Eclipse use its own special Java compiler to create a broken .class? How does java.exe know about complilation problems in .class?


How can Eclipse create a class with unresolved compilation problems?

I want to wrap a test project containing C++ and OpenMP code with Cython, and build it with distutils via a `setup.py` file. The content of my file looks like this:

    from distutils.core import setup
    from distutils.extension import Extension
    from Cython.Build import cythonize
    from Cython.Distutils import build_ext
    
    
    modules = [Extension(&quot;Interface&quot;,
    					 [&quot;Interface.pyx&quot;, &quot;Parallel.cpp&quot;],
    					 language = &quot;c++&quot;,
    					 extra_compile_args=[&quot;-fopenmp&quot;],
    					 extra_link_args=[&quot;-fopenmp&quot;])]
    
    for e in modules:
    	e.cython_directives = {&quot;embedsignature&quot; : True}
    
    setup(name=&quot;Interface&quot;,
    	 cmdclass={&quot;build_ext&quot;: build_ext},
    	 ext_modules=modules)

The `-fopenmp` flag is used with gcc to compile and link against OpenMP. However, if I just invoke

    cls ~/workspace/CythonOpenMP/src $ python3 setup.py build

this flag is not recognized, because the compiler is clang:

    running build
    running build_ext
    skipping &#39;Interface.cpp&#39; Cython extension (up-to-date)
    building &#39;Interface&#39; extension
    cc -Wno-unused-result -fno-common -dynamic -DNDEBUG -g -O3 -Wall -Wstrict-prototypes -I/usr/local/include -I/usr/local/opt/sqlite/include -I/usr/local/Cellar/python3/3.3.0/Frameworks/Python.framework/Versions/3.3/include/python3.3m -c Interface.cpp -o build/temp.macosx-10.8-x86_64-3.3/Interface.o -fopenmp
    clang: warning: argument unused during compilation: &#39;-fopenmp&#39;
    cc -Wno-unused-result -fno-common -dynamic -DNDEBUG -g -O3 -Wall -Wstrict-prototypes -I/usr/local/include -I/usr/local/opt/sqlite/include -I/usr/local/Cellar/python3/3.3.0/Frameworks/Python.framework/Versions/3.3/include/python3.3m -c Parallel.cpp -o build/temp.macosx-10.8-x86_64-3.3/Parallel.o -fopenmp
    clang: warning: argument unused during compilation: &#39;-fopenmp&#39;
    Parallel.cpp:24:10: warning: unknown pragma ignored [-Wunknown-pragmas]
            #pragma omp parallel for
                    ^
    1 warning generated.
    c++ -bundle -undefined dynamic_lookup -L/usr/local/lib -L/usr/local/opt/sqlite/lib build/temp.macosx-10.8-x86_64-3.3/Interface.o build/temp.macosx-10.8-x86_64-3.3/Parallel.o -o build/lib.macosx-10.8-x86_64-3.3/Interface.so -fopenmp
    ld: library not found for -lgomp
    clang: error: linker command failed with exit code 1 (use -v to see invocation)
    error: command &#39;c++&#39; failed with exit status 1

 I&#39;ve unsucessfully tried to specify gcc:

    cls ~/workspace/CythonOpenMP/src $ python3 setup.py build --compiler=g++-4.7
    running build
    running build_ext
    error: don&#39;t know how to compile C/C++ code on platform &#39;posix&#39; with &#39;g++-4.7&#39; compiler


How can I tell distutils to use gcc?







How to tell distutils to use gcc?

I have the following generic method, but VS gives me a compile error on that. (Operator &#39;??&#39; cannot be applied to operands of type &#39;T&#39; and &#39;T&#39;)

    public static T Method&lt;T&gt;(T model) where T : new()
    {
        var m = model ?? new T();
    }

Does some one have any idea why?

Edit: Is it possible the reason is that T can be a struct in my case, and a struct is an non-nullable type? 

Operator &#39;??&#39; cannot be applied to operands of type &#39;T&#39; and &#39;T&#39;

I&#39;m trying to compile my program and it returns this error :

    usr/bin/ld: cannot find -l&lt;nameOfTheLibrary&gt;

in my makefile I use the command `g++` and link to my library which is a symbolic link to my library located on an other directory.

Is there an option to add to make it work please?

usr/bin/ld: cannot find -l&lt;nameOfTheLibrary&gt;

I use gcc 4.8.1 from http://hpc.sourceforge.net on Mac OSX Mountain Lion. I am trying to compile a C++ program which uses the `to_string` function in `&lt;string&gt;`. I need to use the flag `-std=c++11` every time:

    g++ -std=c++11 -o testcode1 code1.cpp

Is there a way to include this flag by default?

How do I enable C++11 in gcc?

I am trying to use a custom class as key for an `unordered_map`, like the following:

    #include &lt;iostream&gt;
    #include &lt;algorithm&gt;
    #include &lt;unordered_map&gt;

    using namespace std;

    class node;
    class Solution;

    class Node {
	public:
		int a;
		int b; 
		int c;
		Node(){}
		Node(vector&lt;int&gt; v) {
			sort(v.begin(), v.end());
			a = v[0];		
			b = v[1];		
			c = v[2];		
		}

		bool operator==(Node i) {
			if ( i.a==this-&gt;a &amp;&amp; i.b==this-&gt;b &amp;&amp;i.c==this-&gt;c ) {
				return true;
			} else {
				return false;
			}
		}
    };

    int main() {
    	unordered_map&lt;Node, int&gt; m;    

    	vector&lt;int&gt; v;
    	v.push_back(3);
    	v.push_back(8);
    	v.push_back(9);
    	Node n(v);

    	m[n] = 0;

    	return 0;
    }

However, g++ gives me the following error:

    In file included from /usr/include/c++/4.6/string:50:0,
                     from /usr/include/c++/4.6/bits/locale_classes.h:42,
                     from /usr/include/c++/4.6/bits/ios_base.h:43,
                     from /usr/include/c++/4.6/ios:43,
                     from /usr/include/c++/4.6/ostream:40,
                     from /usr/include/c++/4.6/iostream:40,
                     from 3sum.cpp:4:
    /usr/include/c++/4.6/bits/stl_function.h: In member function ‘bool std::equal_to&lt;_Tp&gt;::operator()(const _Tp&amp;, const _Tp&amp;) const [with _Tp = Node]’:
    /usr/include/c++/4.6/bits/hashtable_policy.h:768:48:   instantiated from ‘bool std::__detail::_Hash_code_base&lt;_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false&gt;::_M_compare(const _Key&amp;, std::__detail::_Hash_code_base&lt;_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false&gt;::_Hash_code_type, std::__detail::_Hash_node&lt;_Value, false&gt;*) const [with _Key = Node, _Value = std::pair&lt;const Node, int&gt;, _ExtractKey = std::_Select1st&lt;std::pair&lt;const Node, int&gt; &gt;, _Equal = std::equal_to&lt;Node&gt;, _H1 = std::hash&lt;Node&gt;, _H2 = std::__detail::_Mod_range_hashing, std::__detail::_Hash_code_base&lt;_Key, _Value, _ExtractKey, _Equal, _H1, _H2, std::__detail::_Default_ranged_hash, false&gt;::_Hash_code_type = long unsigned int]’
    /usr/include/c++/4.6/bits/hashtable.h:897:2:   instantiated from ‘std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::_Node* std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::_M_find_node(std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::_Node*, const key_type&amp;, typename std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::_Hash_code_type) const [with _Key = Node, _Value = std::pair&lt;const Node, int&gt;, _Allocator = std::allocator&lt;std::pair&lt;const Node, int&gt; &gt;, _ExtractKey = std::_Select1st&lt;std::pair&lt;const Node, int&gt; &gt;, _Equal = std::equal_to&lt;Node&gt;, _H1 = std::hash&lt;Node&gt;, _H2 = std::__detail::_Mod_range_hashing, _Hash = std::__detail::_Default_ranged_hash, _RehashPolicy = std::__detail::_Prime_rehash_policy, bool __cache_hash_code = false, bool __constant_iterators = false, bool __unique_keys = true, std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::_Node = std::__detail::_Hash_node&lt;std::pair&lt;const Node, int&gt;, false&gt;, std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::key_type = Node, typename std::_Hashtable&lt;_Key, _Value, _Allocator, _ExtractKey, _Equal, _H1, _H2, _Hash, _RehashPolicy, __cache_hash_code, __constant_iterators, __unique_keys&gt;::_Hash_code_type = long unsigned int]’
    /usr/include/c++/4.6/bits/hashtable_policy.h:546:53:   instantiated from ‘std::__detail::_Map_base&lt;_Key, _Pair, std::_Select1st&lt;_Pair&gt;, true, _Hashtable&gt;::mapped_type&amp; std::__detail::_Map_base&lt;_Key, _Pair, std::_Select1st&lt;_Pair&gt;, true, _Hashtable&gt;::operator[](const _Key&amp;) [with _Key = Node, _Pair = std::pair&lt;const Node, int&gt;, _Hashtable = std::_Hashtable&lt;Node, std::pair&lt;const Node, int&gt;, std::allocator&lt;std::pair&lt;const Node, int&gt; &gt;, std::_Select1st&lt;std::pair&lt;const Node, int&gt; &gt;, std::equal_to&lt;Node&gt;, std::hash&lt;Node&gt;, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, false, false, true&gt;, std::__detail::_Map_base&lt;_Key, _Pair, std::_Select1st&lt;_Pair&gt;, true, _Hashtable&gt;::mapped_type = int]’
    3sum.cpp:149:5:   instantiated from here
    /usr/include/c++/4.6/bits/stl_function.h:209:23: error: passing ‘const Node’ as ‘this’ argument of ‘bool Node::operator==(Node)’ discards qualifiers [-fpermissive]
    make: *** [threeSum] Error 1
        
I guess, I need the tell C++ how to hash class `Node`, however, I am not quite sure how to do it. How can I accomplish this tasks?

C++ unordered_map using a custom class type as the key

I guess it&#39;s pretty self explanatory - I can&#39;t seem to use C++11 features, even though I think I have everything set up properly - which likely means that I don&#39;t.

Here&#39;s my code:

	#include &lt;cstdlib&gt;
	#include &lt;iostream&gt;

	class Object {
		private:
			int value;
			
		public:
			Object(int val) {
				value = val;
			}
			
			int get_val() {
				return value;
			}
			
			void set_val(int val) {
				value = val;
			}
	};

	int main() {
		
		Object *obj = new Object(3);
		std::unique_ptr&lt;Object&gt; smart_obj(new Object(5));
		std::cout &lt;&lt; obj-&gt;get_val() &lt;&lt; std::endl;
		return 0;
	}

Here&#39;s my version of g++:

&lt;!-- language: none --&gt;

	ubuntu@ubuntu:~/Desktop$ g++ --version
	g++ (Ubuntu/Linaro 4.7.3-2ubuntu1~12.04) 4.7.3
	Copyright (C) 2012 Free Software Foundation, Inc.
	This is free software; see the source for copying conditions.  There is NO
	warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Here&#39;s how I&#39;m compiling the code:

&lt;!-- language: none --&gt;

	ubuntu@ubuntu:~/Desktop$ g++ main.cpp -o run --std=c++11
	main.cpp: In function ‘int main()’:
	main.cpp:25:2: error: ‘unique_ptr’ is not a member of ‘std’
	main.cpp:25:24: error: expected primary-expression before ‘&gt;’ token
	main.cpp:25:49: error: ‘smart_obj’ was not declared in this scope

Note that I&#39;ve tried both `-std=c++11` and `-std=c++0x` to no avail.

I&#39;m running Ubuntu 12.04 LTS from a flash drive on an Intel x64 machine.


error: ‘unique_ptr’ is not a member of ‘std’

In a piece of code I reviewed lately, which compiled fine with `g++-4.6`, I encountered a strange try to create a `std::shared_ptr` from `std::unique_ptr`:

    std::unique_ptr&lt;Foo&gt; foo...
    std::make_shared&lt;Foo&gt;(std::move(foo));

This seems rather odd to me. This should be `std::shared_ptr&lt;Foo&gt;(std::move(foo));` afaik, though I&#39;m not perfectly familiar with moves (and I know `std::move` is only a cast, nothing get&#39;s moved).

Checking with different compilers on this SSC(NUC*)E

    
    #include &lt;memory&gt;

    int main()
    {
       std::unique_ptr&lt;int&gt; foo(new int);
       std::make_shared&lt;int&gt;(std::move(foo));
    }

Results of compilation:

- g++-4.4.7     gives compilation error
- g++-4.6.4     compiles without any error
- g++-4.7.3     gives **internal compiler error**
- g++-4.8.1     gives compilation error
- clang++-3.2.1 compiles without any error

So the question is: which compiler is right in terms of the standard? Does the standard require this to be an invalid statement, a valid statement or is this simply undefined?


**Addition**

We&#39;ve agreed on that some of these compilers, such as clang++ and g++-4.6.4, permit the conversion while they shouldn&#39;t. However with g++-4.7.3 (which produces an internal compiler error on `std::make_shared&lt;Foo&gt;(std::move(foo));`), correctly rejects `int bar(std::move(foo));` 

Because of this huge difference in behavior, I&#39;m leaving the question as it is, although part of it would be answerable with the reduction to `int bar(std::move(foo));`. 

----------
*) NUC: Not universally compilable

creating a shared_ptr from unique_ptr

Is there more beyond advance takes negative numbers?

What&#39;s the difference between std::advance and std::next?

Why isn&#39;t `std::initializer_list` a core-language built-in?
---------------------------------------------------------------

It seems to me that it&#39;s quite an important feature of C++11 and yet it doesn&#39;t have its own reserved keyword (or something alike).

Instead, `initializer_list` it&#39;s *just* a template class from the standard library that has a special, implicit *mapping* from the new *braced-init-list* `{...}` syntax that&#39;s handled by the compiler.

At first thought, this solution is quite *hacky*.

Is this the way new additions to the C++ language will be now implemented: by **implicit roles** of some template classes and not by the **core** language?

---
**Please consider these examples:**

       widget&lt;int&gt; w = {1,2,3}; //this is how we want to use a class

why was a new class chosen:

       widget( std::initializer_list&lt;T&gt; init )

instead of using something **similar** to any of these ideas:

       widget( T[] init, int length )  // (1)
       widget( T... init )             // (2)
       widget( std::vector&lt;T&gt; init )   // (3)

1. a classic array, you could probably add `const` here and there
2. three dots already exist in the language (var-args, now variadic templates), why not re-use the syntax (and make it feel *built-in*)
3. just an existing container, could add `const` and `&amp;`

All of them are already a part of the language. I only wrote my 3 first ideas, I am sure that there are **many** other approaches.

Why isn&#39;t std::initializer_list a language built-in?

Can somebody give a simple example which demonstrates the functionality of `std::ref`? I mean an example in which some other constructs (like tuples, or data type templates) are used **only if** it is impossible to explain `std::ref` without them.

I found two questions about `std::ref` [here][1] and [here][2]. But in the first one it goes about a bug in a compiler and in the second one, examples of use of `std::ref` do not contain `std::ref` and they involve tuples and data type templates which make understanding of these examples complex.


  [1]: https://stackoverflow.com/questions/9582312/how-to-use-stdref
  [2]: https://stackoverflow.com/questions/11833070/when-is-the-use-of-stdref-necessary

What would be a &quot;Hello, World!&quot; example for &quot;std::ref&quot;?

I&#39;m trying to declare a `priority_queue of nodes`, using `bool Compare(Node a, Node b)` as the comparator function (which is outside the node class).

What I currently have is:
    
    priority_queue&lt;Node, vector&lt;Node&gt;, Compare&gt; openSet;

For some reason, I&#39;m getting `Error: &quot;Compare&quot; is not a type name`

Changing the declaration to `priority_queue &lt;Node, vector&lt;Node&gt;, bool Compare&gt;`

gives me `Error: expected a &#39;&gt;&#39;`

I&#39;ve also tried:

    priority_queue&lt;Node, vector&lt;Node&gt;, Compare()&gt; openSet;
    priority_queue&lt;Node, vector&lt;Node&gt;, bool Compare()&gt; openSet;
    priority_queue&lt;Node, vector&lt;Node&gt;, Compare&lt;Node, Node&gt;&gt; openSet; 

How should I correctly declare my `priority_queue`?

declaring a priority_queue in c++ with a custom comparator

I am developing a C++ application using CodeBlocks 10.05 on Debian 7.0.0.

For some reason, the following code


    #include &lt;iostream&gt;

    std::vector&lt; int &gt; delaunayDiv(const std::vector&lt; int &lt;T&gt; &gt; &amp; vP, cv::Rect boundRect,
        std::vector&lt;int&gt;&amp; triangles, int&amp; numTriangles, bool lookRight);

returns the following error

    error: &#39;vector&#39; in namespace &#39;std&#39; does not name a type


Content Type	Original Author	Original Content on Stackoverflow
Question	user1382306	View Question on Stackoverflow
Solution 1 - C++	antonijn	View Answer on Stackoverflow
Solution 2 - C++	stefan	View Answer on Stackoverflow
Solution 3 - C++	papo	View Answer on Stackoverflow
Solution 4 - C++	pankaj kushwaha	View Answer on Stackoverflow

cc1plus: error: unrecognized command line option "-std=c++11" with g++

C++ Problem Overview

C++ Solutions

Solution 1 - C++

Solution 2 - C++

Solution 3 - C++

Solution 4 - C++

super.onCreate(savedInstanceState);

How to generate an MD5 file hash in JavaScript/Node.js?

Attributions