Sure

    namespace mynamespace
    {
       using namespace std;
       using namespace glm;
    }

You can use `using` to achieve this:

    namespace mynamespace {
        using namespace std;
        using namespace glm;
    }


I have written a kafka consumer and producer that worked fine until today.
This morning, when I started zookeeper and kafka, my consumer was not able to read messages, and I found this in the zookeeper logs:

    INFO Got user-level KeeperException when processing sessionid:0x151c41e62e10000 
         type:create cxid:0x2a zxid:0x1e txntype:-1 reqpath:n/a 
         Error Path:/brokers/ids 
         Error:KeeperErrorCode = NodeExists for /brokers/ids   
         (org.apache.zookeeper.server.PrepRequestProcessor)



Zookeeper &amp; Kafka error KeeperErrorCode=NodeExists

How is it possible?

    let exists = NSFileManager.defaultManager().fileExistsAtPath(path.absoluteString)
    print(&quot;exists: \(exists)&quot;) //false

This is `path.absoluteString`

    //file:///Users/kuna/Library/Developer/CoreSimulator/Devices/92BD140D-5C14-43C4-80D6-904BB9594ED6/data/Containers/Data/Application/5B818832-BB19-4047-A7F8-1487F36868D6/Documents/wishlists/68/147/128/IMG_0006.PNG

And you can see it is there where it should be:

[![enter image description here][1]][1]

What is going on?

  [1]: http://i.stack.imgur.com/rBIOk.png

NSFileManager.defaultManager().fileExistsAtPath returns false instead of true

Is it possible to have a namespace that has all the declarations of more than one other namespace? Like this:

    namespace std {...};
    namespace glm {...};
    
    namespace mynamespace = std; //mynamespace is an alias for std
    namespace mynamespace += glm; //mynamespace will hold glm functions as well.

Multiple aliases for a namespace?

Is it possible to have a namespace that has all the declarations of more than one other namespace? Like this:
<pre><code class="hljs language-arduino">namespace std {...};
namespace glm {...};

namespace mynamespace = std; //mynamespace is an alias for std
namespace mynamespace += glm; //mynamespace will hold glm functions as well.
</code></pre>

Is there a legitimate use of `void*` in C++? Or was this introduced because C had it?

Just to recap my thoughts:

**Input**: If we want to allow multiple input types we can overload functions and methods, alternatively we can define a common base class, or template (thanks for mentioning this in the answers). In both cases the code get&#39;s more descriptive and less error prone (provided the base class is implemented in a sane way).

**Output**: I can&#39;t think of any situation where I would prefer to receive  `void*` as opposed to something derived from a known base class.

Just to make it clear what I mean: I&#39;m not specifically asking if there is a use-case for `void*`, but if there is a case where `void*` is the best or only available choice. Which has been perfectly answered by several people below.

Is there a legitimate use for void*?

In our project, we use quite a lot of &quot;usings&quot; to explicitly state what is variable supposed to represent. It is mainly used for `std::string` identificators like `PortalId` or `CakeId`. Now what we currently can do is

    using PortalId = std::string;
    using CakeId   = std::string;
    
    PortalId portal_id(&quot;2&quot;);
    CakeId cake_id(&quot;is a lie&quot;);
    
    portal_id = cake_id; // OK

which we don&#39;t like. We would like to have type check during compile time to prevent us from mixing apples and oranges while preserving most of the yum yum methods from the original object.

So the question is - can this be done in C++ such that the usage would be close to what follows, assignments would fail and we could still use it with, say, maps and other containers?

    SAFE_TYPEDEF(std::string, PortalId);
    SAFE_TYPEDEF(std::string, CakeId);
    
    int main()
    {
        PortalId portal_id(&quot;2&quot;);
        CakeId cake_id(&quot;is a lie&quot;);
        std::map&lt;CakeId, PortalId&gt; p_to_cake; // OK
        
        p_to_cake[cake_id]   = portal_id; // OK
        p_to_cake[portal_id] = cake_id;   // COMPILER ERROR
    
        portal_id = cake_id;        // COMPILER ERROR
        portal_id = &quot;1.0&quot;;          // COMPILER ERROR
        portal_id = PortalId(&quot;42&quot;); // OK
        return 0;

    }

We have already tried macros in combination with templates but didn&#39;t quite get what we needed. And to add - we CAN use c++17.

**EDIT:** The code we came up with was 

    #define SAFE_TYPEDEF(Base, name) \
    class name : public Base { \
    public: \
        template &lt;class... Args&gt; \
        explicit name (Args... args) : Base(args...) {} \
        const Base&amp; raw() const { return *this; } \
    };

which is ugly &lt;strike&gt;and doesn&#39;t work. And by it doesn&#39;t work I mean that compiler was ok with `portal_id = cake_id;`&lt;/strike&gt;.

**EDIT2:** Added `explicit` keyword, with which our code actually works nicely for our example. Not sure though whether this is the right way to go and whether it covers all unfortunate situations.

Strongly typed using and typedef

An often used statement like `(void)x;` allows to suppress warnings about unused variable `x`. But if I try compiling the following, I get some results I don&#39;t quite understand:

    int main()
    {
        int x;
        (short)x;
        (void)x;
        (int)x;
    }

Compiling this with g++, I get the following warnings:

    $ g++ test.cpp -Wall -Wextra -o test
    test.cpp: In function ‘int main()’:
    test.cpp:4:13: warning: statement has no effect [-Wunused-value]
         (short)x;
                 ^
    test.cpp:6:11: warning: statement has no effect [-Wunused-value]
         (int)x;
               ^

So I conclude that casting to `void` is very different from casting to any other types, be the target type the same as `decltype(x)` or something different. My guess at possible explanations is:

* It is just a convention that `(void)x;` but not the other casts will suppress warnings. All the statements equally don&#39;t have any effect.
* This difference is somehow related to the fact that `void x;` isn&#39;t a valid statement while `short x;` is.

Which of these if any is more correct? If none, then how can the difference in compiler warnings be explained?

What does casting to `void` really do?

Given the following code:

    struct Window{
        void show();
        //stuff
    }w1, w2, w3;

    struct Widget{
        void show();
        //stuff
    }w4, w5, w6;

    struct Toolbar{
        void show();
        //stuff
    }t1, t2, t3;

I want to `show` a bunch of items:

    for (auto &amp;obj : {w3, w4, w5, t1})
        obj.show();

However this does not compile since the `std::initializer_list&lt;T&gt;` in the `for`-loop cannot deduce `T` and in fact there is not really a `T` that would fit. I don&#39;t want to create a type erasure type because of the amount of code required and the unnecessary runtime overhead. How do I correctly write my loop so that the type of `obj` is deduced for every item in the conceptual list separately?

Iterating over different types

Consider:

    #include&lt;iostream&gt;
    using namespace std;

    class Base
    {
        public:
            virtual void show() { cout&lt;&lt;&quot; In Base \n&quot;; }
    };

    class Derived: public Base
    {
        public:
           void show() { cout&lt;&lt;&quot;In Derived \n&quot;; }
    };

    int main(void)
    {
        Base *bp = new Derived;
        bp-&gt;show();  // RUN-TIME POLYMORPHISM
        return 0;
    }

Why does this code cause run-time polymorphism and why can not it be solved at compile time?


Content Type	Original Author	Original Content on Stackoverflow
Question	Pilpel	View Question on Stackoverflow
Solution 1 - C++	ForEveR	View Answer on Stackoverflow
Solution 2 - C++	alain	View Answer on Stackoverflow

Multiple aliases for a namespace?

C++ Problem Overview

C++ Solutions

Solution 1 - C++

Solution 2 - C++

NSFileManager.defaultManager().fileExistsAtPath returns false instead of true

Zookeeper & Kafka error KeeperErrorCode=NodeExists

Attributions