As you have found out yourself, the difference between the two options is whether GNU extensions that violates/extend the C++ standard are enabled or not. The GNU C++ extensions are described [here][1]. You can also use most of the GNU C extensions (described [here][2]) in your C++ programs.  It would be also useful to read about the `-Wpedantic` GCC option [here][3].

Note that some extensions can still be in effect when using `-std=c++11`, as long as they do not contradict the standard. For instance, when using the MinGW compiler, I need the extensions for a working `Boost.Lexical_Cast`. But, as long as you don&#39;t use any of them, you are better off sticking to the standard without extensions for maximum portability. This might come in handy if you find yourself forced to change compiler.

  [1]: http://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Extensions.html
  [2]: https://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html
  [3]: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html

I have a new meteor project. I&#39;m guessing the `.meteor` dir has a combination of configuration files (needed) and temporary files (not needed). 

So what&#39;s in your `.gitignore`?

What should I put in a meteor .gitignore file?

I want to do pretty much the same like in [this ticket at djangoproject.com][1], but with some additonal formatting. From this query

    &gt;&gt;&gt; MyModel.objects.values(&#39;cryptic_value_name&#39;)
    [{&#39;cryptic_value_name&#39;: 1}, {&#39;cryptic_value_name&#39;: 2}]

I want to get something like that:

    &gt;&gt;&gt; MyModel.objects.values(renamed_value=&#39;cryptic_value_name&#39;)
    [{&#39;renamed_value&#39;: 1}, {&#39;renamed_value&#39;: 2}]

Is there another, more builtin way or do I have to do this manually?

  [1]: https://code.djangoproject.com/ticket/12222

How to rename items in values() in Django?

What are the differences between `-std=c++11` and `-std=gnu++11` as compilation parameter for gcc and clang? Same question with `c99` and `gnu99`? I know about C++ and C standards, it&#39;s the differences in the parameters that interest me.

I&#39;ve read somewhere that it has to do with some extensions but it is not clear to me which ones and how to choose between one or the other for a new project.

What are the differences between -std=c++11 and -std=gnu++11?

<p>What are the differences between <code>-std=c++11</code> and <code>-std=gnu++11</code> as compilation parameter for gcc and clang? Same question with <code>c99</code> and <code>gnu99</code>? I know about C++ and C standards, it's the differences in the parameters that interest me.</p>
<p>I've read somewhere that it has to do with some extensions but it is not clear to me which ones and how to choose between one or the other for a new project.</p>


In what cases should we include **cassert**?



In what cases we need to include &lt;cassert&gt;?

I have three files . The contents of main.cpp are


    #include&lt;iostream&gt;
    #include&lt;QString&gt;
    
    #include &quot;util.h&quot;
    
    int main()
    {
        using Util::convert2QString;
    
        using namespace std;
        int n =22;
        QString tmp = convert2QString&lt;int&gt;(n);
    
        return 0;
    }



util.h

    namespace Util
    {
        template&lt;class T&gt;
        QString convert2QString(T type , int digits=0);
    }


util.cpp


    namespace Util
    {
        template&lt;class T&gt;
            QString convert2QString(T type, int digits=0)
            {
                using std::string;
                
                string temp = (boost::format(&quot;%1%&quot;) % type).str();
    
                return QString::fromStdString(temp);
            }
    }

When I try to compile these files with following command I get undefined reference error

    vickey@tb:~/work/trash/template$ g++ main.cpp  util.cpp -lQtGui -lQtCore  -I. -I/usr/local/Trolltech/Qt-4.8.0/include/QtCore -I/usr/local/Trolltech/Qt-4.8.0/include/QtGui -I/usr/local/Trolltech/Qt-4.8.0/include
    /tmp/cca9oU6Q.o: In function `main&#39;:
    main.cpp:(.text+0x22): undefined reference to `QString Util::convert2QString&lt;int&gt;(int, int)&#39;
    collect2: ld returned 1 exit status

Is there something wrong with the template declaration or implementation ? why M I getting these linking errors :?


undefined reference to template function

Let&#39;s say I have a class with a method that returns a `shared_ptr`.

What are the possible benefits and drawbacks of returning it by reference or by value?

Two possible clues:

 - **Early object destruction.** If I return the `shared_ptr` by (const) reference, the reference counter is not incremented, so I incur the risk of having the object deleted when it goes out of scope in another context (e.g. another thread). Is this correct? What if the environment is single-threaded, can this situation happen as well?
 - **Cost.** Pass-by-value is certainly not free. Is it worth avoiding it whenever possible?

Thanks everybody.

How to return smart pointers (shared_ptr), by reference or by value?

I&#39;ve got the following code:

    #include &lt;stdio.h&gt;
    int main(int argc, char **argv) {
        int i = 0;
        (i+=10)+=10;
        printf(&quot;i = %d\n&quot;, i);
        return 0;
    }

If I try to compile it as a C source using gcc I get an error:

    error: lvalue required as left operand of assignment

But if I compile it as a C++ source using g++ I get no error and when i run the executable:

    i = 20

Why the different behavior?

What&#39;s the result of += in C and C++?

`counter` is an `int`

    void SentryManager::add(std::string name,std::shared_ptr&lt;Sentry&gt;){
    	name = name + std::to_string(counter);
    }

What would be the best way to stop this error? When I was being lazy I just made the int `long long` (or something), but I&#39;m sure there is a better way of solving this.

Error message:

    sentrymanager.cpp(8): error C2668: &#39;std::to_string&#39; : ambiguous call to overloaded function

I am using Visual C++ 2010 Express.

std::to_string - more than instance of overloaded function matches the argument list

How can you list all functions in a program with GDB?


Ask GDB to list all functions in a program

Does the typical `malloc` (for x86-64 platform and Linux OS) naively lock a mutex at the beginning and release it when done, or does it lock a mutex in a more clever way at a finer level, so that lock contention is reduced? If it indeed does it the second way, how does it do it?

How does malloc work in a multithreaded environment?

If have some code where I would like to use C++11x extensions as much as possible, but have a fallback if this is not supported. Currently the OSX version of GCC and the VisualC compiler has little to no support for C++11x, so I use:
    
    #if (defined(__APPLE__) || (defined(_WIN32)))
       ...fallback code without C++11x ...
    #else
       ... code using C++11x ...
    #endif

And this works, but is not really the right thing to do, especially since the gcc compiler in MacPorts DOES support c++11x.

Is there a `#define C11X_SUPPORTED` type macro? Perhaps something only GCC has?

Is there a preprocessor directive for detecting C++11x support?

&gt; **TL/DR**: Where can I find more information about building a GCC 4.7.0 cross-compiling toolchain for ARM (gnueabi) platform (intended to run on a Raspberry Pi device)?

I have just got a brand new Raspberry Pi and I am very eager to start programming for it. I&#39;ve managed to install the GCC toolchain (I am using the Arch Linux system image) and compiled some basic programs, all working fine.

I&#39;ve also tried to compile the Boost libraries because I often use them in my projects and everything seemed to work fine by following the instructions (`./bootstrap.sh` + `./b2`) except for the fact that the compilation was painfully slow. I left it on for a few hours but it barely got past the first few source files. After I left it running for the night, I discovered that the build process aborted due to RAM shortage.

So, my guess is that Rasp Pi is simply underpowered for compiling something of such size as Boost. So, cross-compilation comes to my mind. However, even though there is a lot of information about ARM cross compilation available online, I find it confusing. Where does one even start? 

I have a recent GCC version (4.7.0) available on my Raspberry Pi, so I would ideally like to cross-compile with the same version. Where can I get the GCC 4.7.0 toolchain for ARM? (I will be compiling on x86 CentOS 6.2)

**Edit:**

I deallocated unneeded GPU memory and set up a 4GB swap partition on a USB drive, while build files are on a NFS share. Boost is now compiling much much faster, so it is manageable. I would still like to know how can I set up a GCC 4.7 toolchain for cross compilation on my x86 PC though, since I intend to do a lot of compiling and I would like it to be as fast as possible.

**Edit 2:**

Since GCC 4.7.0 is relatively new, there does not seem to be a pre-built cross-compiler (i386-&gt;ARM). I will probably have to build one myself, which seems an non-trivial task (I&#39;ve tried and failed). Does anyone know of a tutorial to follow for building a GCC cross-compiler, hopefully for one of the recent versions?  

I&#39;ve tried with [this great shell script][1] (which worked great for building a same-arch compiler) and I&#39;ve successfully built `binutils` and GCC&#39;s prerequisites, but then GCC build kept failing with many cryptic errors. I am really lost here, so I would greatly appreciate your help.

GCC on Raspberry Pi was configured with

    --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib 
    --mandir=/usr/share/man --infodir=/usr/share/info 
    --with-bugurl=https://bugs.archlinux.org/ 
    --enable-languages=c,c++,fortran,lto,objc,obj-c++ --enable-shared 
    --enable-threads=posix --with-system-zlib --enable-__cxa_atexit 
    --disable-libunwind-exceptions --enable-clocale=gnu 
    --disable-libstdcxx-pch --enable-libstdcxx-time 
    --enable-gnu-unique-object --enable-linker-build-id --with-ppl 
    --enable-cloog-backend=isl --enable-lto --enable-gold 
    --enable-ld=default --enable-plugin --with-plugin-ld=ld.gold 
    --with-linker-hash-style=gnu --disable-multilib --disable-libssp 
    --disable-build-with-cxx --disable-build-poststage1-with-cxx 
    --enable-checking=release --host=arm-unknown-linux-gnueabi 
    --build=arm-unknown-linux-gnueabi 

**Edit 3:**

I managed to build a 4.7 GCC toolchain for ARM (yay!) using [this shell script][2] as suggested by user dwelch in the comments. I also built `newlib` and `libstdc++` using [this article][3] as a guide. The toolchain works fine, but hen I run the executable on my Raspberry Pi, it fails with `Illegal instruction`. What could be the cause of that? 


  [1]: http://joelinoff.com/blog/?p=658
  [2]: http://github.com/dwelch67/raspberrypi
  [3]: http://wiki.osdev.org/OS_Specific_Toolchain

Cross-compilation for Raspberry Pi in GCC. Where to start?

Consider the following scenario:

- Shared Library libA.so ,with no dependencies.
- Shared Library libB.so, with libA.so as its dependency.

I want to compile a binary file that links with the libB.
Should I link the binary with libB only or with libA either?

Is there any way to link only with the direct dependencies, letting the resolution of unresolved symbols from the dependencies for runtime?

I&#39;m worried about the fact that the library libB implementation may change in the future, introducing other dependencies (libC, libD, libE for instance). Am I going to have problems with that?

In other words:

- libA files: a.cpp a.h
- libB files: b.cpp b.h
- main program files: main.cpp

Of course, b.cpp includes a.h and main.cpp includes b.h.

Compilation commands:

    g++ -fPIC a.cpp -c
    g++ -shared -o libA.so a.o
    
    g++ -fPIC b.cpp -c -I.
    g++ -shared -o libB.so b.o -L. -lA

Which of the bellow options should I use?

    g++ main.cpp -o main -I. -L. -lB

or

    g++ main.cpp -o main -I. -L. -lB -lA

I couldn&#39;t use the first option. The linker complains about the unresolved symbols from the library libA. But it sound a little strange to me.

Thanks very much.


**-- Updated comments:**


When I link the binary, the linker will try to resolve all symbols from the main and the libB. However, libB has undefined symbols from the libA. That&#39;s why the linker complains about that.

That&#39;s why I need to link with the libA too.
However I found a way to ignore unresolved symbols from shared libraries.
Looks like I should use the following command line to do that:

    g++ main.cpp -o main -I. -L. -lB -Wl,-unresolved-symbols=ignore-in-shared-libs

Looks like it is still possible to use the `-rpath` option.
However I need to understand it a little better.

Does anyone knows any possible pitfalls when using the `-Wl,-unresolved-symbols=ignore-in-shared-libs` option?


**-- Updated comments 2:**


`-rpath` should not be used for this purpose. It is useful to force a library to be found in a given directory. The `-unresolved-symbol` approach looks much better.


Thanks again.

Linking with dynamic library with dependencies

I am trying to construct a `std::thread` with a member function that takes no arguments and returns `void`. I can&#39;t figure out any syntax that works - the compiler complains no matter what. What is the correct way to implement `spawn()` so that it returns a `std::thread` that executes `test()`?

    #include &lt;thread&gt;
    class blub {
      void test() {
      }
    public:
      std::thread spawn() {
        return { test };
      }
    };

Start thread with member function

There&#39;s a well known image (cheat sheet) called &quot;C++ Container choice&quot;. It&#39;s a flow chart to choose the best container for the wanted usage.

Does anybody know if there&#39;s already a C++11 version of it?

This is the previous one:
![eC++ Container choice][1]


  [1]: http://i.stack.imgur.com/16vVM.png

How can I efficiently select a Standard Library container in C++11?

Is there a semantic difference between `auto const` and `const auto`, or do they mean the same thing?


Is &#39;auto const&#39; and &#39;const auto&#39; the same?

Consider the following program which inserts a range of elements into a vector:

    vector&lt;string&gt; v1;
    vector&lt;string&gt; v2;
    
    v1.push_back(&quot;one&quot;);
    v1.push_back(&quot;two&quot;);
    v1.push_back(&quot;three&quot;);

    v2.push_back(&quot;four&quot;);
    v2.push_back(&quot;five&quot;);
    v2.push_back(&quot;six&quot;);
    
    v1.insert(v1.end(), v2.begin(), v2.end());

This efficiently copies the range, allocating enough space in the target vector for the entire range so that a maximum of one resize will be required.  Now consider the following program which attempts to move a range into a vector:

    vector&lt;string&gt; v1;
    vector&lt;string&gt; v2;
    
    v1.push_back(&quot;one&quot;);
    v1.push_back(&quot;two&quot;);
    v1.push_back(&quot;three&quot;);
    
    v2.push_back(&quot;four&quot;);
    v2.push_back(&quot;five&quot;);
    v2.push_back(&quot;six&quot;);
    
    for_each ( v2.begin(), v2.end(), [&amp;v1]( string &amp; s )
    {
    	v1.emplace_back(std::move(s));
    });

This performs a successful move but doesn&#39;t enjoy the benefits that insert() has with regard to preallocating space in the target vector, so the vector could be resized several times during the operation.  

So my question is, is there an insert equivalent which can move a range into a vector?

Is there a standard way of moving a range into a vector?

As [this question](https://stackoverflow.com/questions/199966/how-do-you-use-gcc-to-generate-assembly-code-in-intel-syntax) shows, with g++, I can do `g++ -S -masm=intel test.cpp`.
Also, with clang, I can do `clang++ -S test.cpp`, but `-masm=intel` is not supported by clang (`warning argument unused during compilation: -masm=intel`). How do I get intel syntax with clang?

How to generate assembly code with clang in Intel syntax?

I am working in C++ under Mac OS X (10.8.2) and I recently came up with the need of using C++11 features, which are available through the clang++ compiler using the libc++ stdlib.
However, I also need to use some legacy library compiled and linked against libstdc++ (coming from MacPorts). 

In doing so, I got linking errors, since the headers of the legacy libraries using, e.g., `std::string`, required to be resolved against the `std::__1::basic_string` (i.e., the libc++ implementation of `std::string`) instead of the `std::basic_string` implementation.

Is there a way to mix the two libraries in development (e.g. by using some preprocessors flags?)


Using libstdc++ compiled libraries with clang++ -stdlib=libc++

I&#39;ve got some code in which I need to forward-declare the a template class (or at least, forward-declaring would make things a lot easier for me...). I&#39;ve written a simplified version of the problem I&#39;m having so I can display it here:

    template&lt;bool&gt;
    class MyTemplateClass;
    
    int main( int argc, char* argv[] )
    {
    	MyTemplateClass&lt;false&gt; myTemp;  // error here
    	myTemp.GetTheValue();
    	return 0;
    }
    
    template&lt;bool bShouldMult&gt;
    class MyTemplateClass
    {
    	int m_myint;
    	float m_myfloat;
    
    public:
    	MyTemplateClass() : m_myint(5), m_myfloat(3.0f) {}
    	float GetTheValue()
    	{
    		return m_myint * (bShouldMult ? m_myfloat : 1.0f);
    	}	
    
    };

The error I&#39;m getting at the commented line is:

    Error - implicit instantiation of undefined template &#39;MyTemplateClass&lt;false&gt;&#39;

What other detail do I need to include in a forward declaration of MyTemplateClass? Since the error isn&#39;t coming from the next line down I&#39;m assuming it isn&#39;t due to the fact that the method is undefined. The compiler I&#39;m using is LLVM/CLang, and I&#39;m compiling on Mac.

&quot;Implicit instantiation of undefined template&quot; when forward declaring template class

In my C++11 code I get the clang warning &quot;Declaration requires an exit-time destructor&quot; in the following case:

    static const std::map&lt;int, const someStruct&gt; mymap = {
        {1, {
            &quot;A&quot;,
            &quot;B&quot;,
            &quot;C&quot;
        }},
        {2, {
            &quot;D&quot;,
            &quot;E&quot;,
            &quot;F&quot;
        }}
    };

As far as I understand Google an &quot;exit-time destructor&quot; is required to destroy main() and statics in a deterministic way to prevent crashes on exit due to &quot;already released variables&quot;. Is that right? Can someone explain it better?

Plus: What can I do about it (I don&#39;t want to disable the warning)? The code above is used within context of one thread only.

Looks like this is the way Chromium deals with these cases; would that be the right way for my case as well?

    #define CR_DEFINE_STATIC_LOCAL(type, name, arguments) \
      static type&amp; name = *new type arguments

(Source: https://chromium.googlesource.com/chromium/src/+/32352ad08ee673a4d43e8593ce988b224f6482d3/base/basictypes.h)

How to deal with &quot;exit-time destructor&quot; warning in clang?

Currently I am interested in ARM in general and specifically iphone/android targets. But I just want to know more about clang, since it feels to play important role in the years to come.

I tried

    clang -cc1 --help|grep -i list
    clang -cc1 --help|grep arch|grep -v search
    clang -cc1 --help|grep target

     -triple &lt;value&gt;         Specify target triple (e.g. i686-apple-darwin9)

I know clang has -triplet parameter, but how can I list all possible values for it?
I found that clang is very different to gcc in respect to cross compiling, in GCC world you should have separate binary for everything, like PLATFORM_make or PLATFORM_ld (i*86-pc-cygwin i*86-*-linux-gnu etc. http://git.savannah.gnu.org/cgit/libtool.git/tree/doc/PLATFORMS)

in clang world, it&#39;s only one binary (as I read on some forums). But how do I get the list of supported targets? And if my target it not supported on my distro(linux/windows/macos/whatever) how can I get the one that supports more platform?

if I SVN latest clang like this:

    svn co http://llvm.org/svn/llvm-project/cfe/trunk clang

will I get most of platforms?
It looks like Clang was not built with cross compiling in mind right away, but since it&#39;s llvm based it should be very cross-friendly in theory? thank you!


Content Type	Original Author	Original Content on Stackoverflow
Question	Klaim	View Question on Stackoverflow
Solution 1 - C++	daramarak	View Answer on Stackoverflow

What are the differences between -std=c++11 and -std=gnu++11?

C++ Problem Overview

C++ Solutions

Solution 1 - C++

How to rename items in values() in Django?

What should I put in a meteor .gitignore file?

Attributions