The purpose of this limitation? Just a guess, but:

 - you may use the template class/template member function only within the enclosing function. Therefore you already know all used types within the function and hence can directly specify the used types  (for several types, of course, the template variant would have saved some typing).
 - although it might not seem so, it is work for all compiler creators and space for bugs and so it must be worth the effort.

Fun Fact: Try to use a local class within a function as a return type for a (c++0x)-lambda function declared in the function: MSVC 2010: internal compiler error ^^.

It is possible to ignore some parts of code from a php file when it&#39;s analyzed by `PHP_CodeSniffer`?

Ignore code snippets in PHP_CodeSniffer

I&#39;m building an audio player for a non-native sound format. The application&#39;s hierarchy is based on the iPod.app. It has a number of `UITableView`&#39;s and one `UIView` (`TrackView`) that uses an `MPVolumeView` to allow the user to change the volume onscreen. Until the first time `TrackView` becomes visible changing the volume using the hardware buttons displays the system volume overlay as expected (and desired). When `TrackView` is visible these overlays don&#39;t appear since the `MPVolumeView` updates when changing the volume with the hardware buttons (also desired).

Here&#39;s the problem: once you back out of `TrackView` the system volume overlay does not appear when using the hardware volume buttons. I have tried programmatically allocing, creating and adding the `MPVolumeView` in `TrackViewController viewWillAppear:` and then removing, releasing and nil-ing the same `MPVolumeView` in `TrackViewController viewWillDisappear:`.

This does not happen in the iPod.app. Once backing out of the view that contains an `MPVolumeView` the system volume overlays display when using the hardware volume buttons.

What am I missing?

---

**Update 2:** This appears to be a bug in MPVolumeView that was introduced sometime after iOS 3.2 and fixed in 4.2.

---

**Update:** I made a simple reduction from the default window-based application project that exhibits the same behavior. Once an MPVolumeView becomes visible the system volume overlays are never seen in the application again.

VolumeAppDelegate.h:


	#import &lt;UIKit/UIKit.h&gt;
	#import &lt;MediaPlayer/MediaPlayer.h&gt;

	@interface VolumeAppDelegate : NSObject &lt;UIApplicationDelegate&gt; {
	    UIWindow *window;
		IBOutlet UIView *volumeView;
		IBOutlet MPVolumeView *mpVolumeView;
	}

	@property (nonatomic, retain) IBOutlet UIWindow *window;

	-(IBAction)toggleVolumeView:(id)sender;

	@end

VolumeAppDelegate.m:

	#import &quot;VolumeAppDelegate.h&quot;

	@implementation VolumeAppDelegate

	@synthesize window;

	-(IBAction)toggleVolumeView:(id)sender{
	
		if (mpVolumeView == nil){
			mpVolumeView = [[MPVolumeView alloc] initWithFrame:volumeView.bounds];
			[volumeView addSubview:mpVolumeView];
		}
		else{
			[mpVolumeView removeFromSuperview];
			[mpVolumeView release];
			mpVolumeView = nil;
		}
	}

	- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    
    
	    [self.window makeKeyAndVisible];
		mpVolumeView = nil;
	    return YES;
	}

	- (void)dealloc {
	    [window release];
	    [super dealloc];
	}

	@end

You&#39;ll need to add the MediaPlayer framework to your project in Xcode and open up MainWindow.xib in Interface builder to add the UIView and UIButton IBOutlets and hookup the IBAction to the UIButton.

How do I turn the system volume overlay back on after using MPVolumeView?

Given the following code:

    void f()
    {
        class A
        {
            template &lt;typename T&gt;
            void g() {}
        };
    }

g++ 4.4 (and also  `g++-4.6 -std=gnu++0x`) complains: &quot;invalid declaration of member template in local class&quot;.

Apparently local classes are not allowed to have template members. What is the purpose of this limitation? Will it be removed in C++0x?

Note: If I make the local class itself a template, rather than giving it a template member:

    void f()
    {
        template &lt;typename T&gt;
        class A
        {
            void g() {}
        };
    }

I get &quot;error: a template declaration cannot appear at block scope&quot;.

Member template in local class

Given the following code:
<pre><code class="hljs language-arduino">void f()
{
 class A
 {
 template &#x3C;typename T>
 void g() {}
 };
}
</code></pre>
g++ 4.4 (and also <code>g++-4.6 -std=gnu++0x</code>) complains: "invalid declaration of member template in local class".
Apparently local classes are not allowed to have template members. What is the purpose of this limitation? Will it be removed in C++0x?
Note: If I make the local class itself a template, rather than giving it a template member:
<pre><code class="hljs language-arduino">void f()
{
 template &#x3C;typename T>
 class A
 {
 void g() {}
 };
}
</code></pre>
I get "error: a template declaration cannot appear at block scope".

What&#39;s the [`&lt;iosfwd&gt;` header][1] used for? Why is it necessary?

Any example?

  [1]: https://en.cppreference.com/w/cpp/header/iosfwd

What is the &lt;iosfwd&gt; header?

I&#39;m a bit confused regarding the difference between `push_back` and `emplace_back`.

    void emplace_back(Type&amp;&amp; _Val);
    void push_back(const Type&amp; _Val);
    void push_back(Type&amp;&amp; _Val);

As there is a `push_back` overload taking a rvalue reference I don&#39;t quite see what the purpose of `emplace_back` becomes?

push_back vs emplace_back

I know it is a common issue, but looking for references and other material I don&#39;t find a clear answer to this question.

Consider the following code:

    #include &lt;string&gt;
    
    // ...
    // in a method
    std::string a = &quot;Hello &quot;;
    std::string b = &quot;World&quot;;
    std::string c = a + b;

The compiler tells me it cannot find an overloaded operator for `char[dim]`.

Does it mean that in the string there is not a + operator?

But in several examples there is a situation like this one. If this is not the correct way to concat more strings, what is the best way?

Concatenating strings doesn&#39;t work as expected

`unique_ptr&lt;T&gt;` does not allow copy construction, instead it supports move semantics. Yet, I can return a `unique_ptr&lt;T&gt;` from a function and assign the returned value to a variable.

    #include &lt;iostream&gt;
    #include &lt;memory&gt;
     
    using namespace std;
     
    unique_ptr&lt;int&gt; foo()
    {
      unique_ptr&lt;int&gt; p( new int(10) );
     
      return p;                   // 1
      //return move( p );         // 2
    }
     
    int main()
    {
      unique_ptr&lt;int&gt; p = foo();
     
      cout &lt;&lt; *p &lt;&lt; endl;
      return 0;
    }

The code above compiles and works as intended. So how is it that line `1` doesn&#39;t invoke the copy constructor and result in compiler errors? If I had to use line `2` instead it&#39;d make sense (using line `2` works as well, but we&#39;re not required to do so).

I know C++0x allows this exception to `unique_ptr` since the return value is a temporary object that will be destroyed as soon as the function exits, thus guaranteeing the uniqueness of the returned pointer. I&#39;m curious about how this is implemented, is it special cased in the compiler or is there some other clause in the language specification that this exploits?

Returning unique_ptr from functions

I mean something like:

    int main() 
    {
      void a() 
      {
          // code
      }
      a();

      return 0;
    }
 




Can we have functions inside functions in C++?

I am creating an email engine in mvc3 and I am trying to use razor views as email templates.
I heard this is possible but I have not yet found any information about it. 

Razor views as email templates

Is it possible to have a template class, which inherit from QObject (and has Q_OBJECT macro in it&#39;s declaration)?

I would like to create something like adapter for slots, which would do something, but the slot can take arbitrary number of arguments (number of arguments depends on the template argument).

I just tried doing it, and got linker errors. I guess gmake or moc is not getting called on this template class. Is there a way to do this? Maybe by explicitly instantiating templates?

QT : Templated Q_OBJECT class

When I use a specialized template in different object files, I get a &quot;multiple definition&quot; error when linking. The only solution I found involves using the &quot;inline&quot; function, but it just seems like some workaround. How do I solve that without using the &quot;inline&quot; keyword? If that&#39;s not possible, why?

Here is the example code:

    paulo@aeris:~/teste/cpp/redef$ cat hello.h 
    #ifndef TEMPLATE_H
    #define TEMPLATE_H
    
    #include &lt;iostream&gt;
    
    template &lt;class T&gt;
    class Hello
    {
    public:
        void print_hello(T var);
    };
    
    template &lt;class T&gt;
    void Hello&lt;T&gt;::print_hello(T var)
    {
        std::cout &lt;&lt; &quot;Hello generic function &quot; &lt;&lt; var &lt;&lt; &quot;\n&quot;;
    }

    template &lt;&gt; //inline
    void Hello&lt;int&gt;::print_hello(int var)
    {
        std::cout &lt;&lt; &quot;Hello specialized function &quot; &lt;&lt; var &lt;&lt; &quot;\n&quot;;
    }

    #endif

---

    paulo@aeris:~/teste/cpp/redef$ cat other.h 
    #include &lt;iostream&gt;
    
    void other_func();

---

    paulo@aeris:~/teste/cpp/redef$ cat other.c 
    #include &quot;other.h&quot;
    
    #include &quot;hello.h&quot;
    
    void other_func()
    {
        Hello&lt;char&gt; hc;
        Hello&lt;int&gt; hi;
    
        hc.print_hello(&#39;a&#39;);
        hi.print_hello(1);
    }

---


    paulo@aeris:~/teste/cpp/redef$ cat main.c 
    #include &quot;hello.h&quot;
    
    #include &quot;other.h&quot;
    
    int main()
    {
        Hello&lt;char&gt; hc;
        Hello&lt;int&gt; hi;
    
        hc.print_hello(&#39;a&#39;);
        hi.print_hello(1);
    
        other_func();
    
        return 0;
    }

---

    paulo@aeris:~/teste/cpp/redef$ cat Makefile
    all:
        g++ -c other.c -o other.o -Wall -Wextra
        g++ main.c other.o -o main -Wall -Wextra

---

Finally:


    paulo@aeris:~/teste/cpp/redef$ make
    g++ -c other.c -o other.o -Wall -Wextra
    g++ main.c other.o -o main -Wall -Wextra
    other.o: In function `Hello&lt;int&gt;::print_hello(int)&#39;:
    other.c:(.text+0x0): multiple definition of `Hello&lt;int&gt;::print_hello(int)&#39;
    /tmp/cc0dZS9l.o:main.c:(.text+0x0): first defined here
    collect2: ld returned 1 exit status
    make: ** [all] Erro 1


If I uncomment the &quot;inline&quot; inside hello.h, the code will compile and run, but that just seems like some kind of &quot;workaround&quot; to me: what if the specialized function is big and used many times? Will I get a big binary? Is there any other way to do this? If yes, how? If not, why?

I tried to look for answers, but all I got was &quot;use inline&quot; without any further explanation.

Thanks

multiple definition of template specialization when using different objects

While playing with templates in c++ I encountered a problem converting typename T to string. For example:

    template &lt;typename T&gt;
    class Matrix {
       public:
            Matrix() {
               //my_type = string type of T. i.e. if T is char. I want my_type to be &quot;char&quot;.
            }
       string my_type;
    }

How do I convert T to a string that says what T is.

Note: I&#39;m just playing around so please do not worry about when one might need such a thing.

How to convert typename T to string in c++

While setting up my project and working to keep apps non-dependent, I&#39;ve hit a snag. I&#39;d like all of the templates from the different apps to have a consistent header and footer. Here&#39;s what I&#39;m trying:

    myproject/
             base/
                 templates/
                          header.html
                          footer.html
             app1/
                 templates/
                          my_app1_page.html -&gt; want to include &#39;header.html&#39;
                                               and &#39;footer.html&#39; from base app
Pretend there are many more apps that want to do this as well. Is this possible and/or the right way to do it?

django include template from another app

C++0x is going to make the following code and similar code ill-formed, because it requires a so-called *narrowing conversion* of a `double` to a `int`. 

    int a[] = { 1.0 };

I&#39;m wondering whether this kind of initialization is used much in real world code. How many code will be broken by this change? Is it much effort to fix this in your code, if your code is affected at all?

---

For reference, see 8.5.4/6 of n3225

&gt;A narrowing conversion is an implicit conversion
&gt;
- from a floating-point type to an integer type, or
- from long double to double or float, or from double to float, except where the source is a constant expression and the actual value after conversion is within the range of values that can be represented (even if it cannot be represented exactly), or
- from an integer type or unscoped enumeration type to a ﬂoating-point type, except where the source is a constant expression and the actual value after conversion will fit into the target type and will produce the original value when converted back to the original type, or
- from an integer type or unscoped enumeration type to an integer type that cannot represent all the values of the original type, except where the source is a constant expression and the actual value after conversion will fit into the target type and will produce the original value when converted back to the original type.


Narrowing conversions in C++0x. Is it just me, or does this sound like a breaking change?

I am considering the use of some C++11 features (like `auto` for instance) in some cross-platform projects (Windows+Mac). On Windows, Visual Studio supports parts of the upcoming C++11 standard that would allow me to simplify parts of the code base so naturally I would be interested in starting to use these features.

But as far as I am aware, the current XCode version (3.2.4 + GCC 4.2) does not support any C++11 features at all. Can I upgrade the GCC version or the CLang version somehow? Or should I just bite my tongue and wait for Apple to package a new version sometime in the future?


  

Can I use C++11 with Xcode?

I was experimenting with C++0x variadic templates when I stumbled upon this issue:

    template &lt; typename ...Args &gt;
    struct identities
    {
        typedef Args type; //compile error: &quot;parameter packs not expanded with &#39;...&#39;
    };

    //The following code just shows an example of potential use, but has no relation
    //with what I am actually trying to achieve.
    template &lt; typename T &gt;
    struct convert_in_tuple
    {
        typedef std::tuple&lt; typename T::type... &gt; type;
    };

    typedef convert_in_tuple&lt; identities&lt; int, float &gt; &gt;::type int_float_tuple;

GCC 4.5.0 gives me an error when I try to typedef the template parameters pack.

Basically, I would like to &quot;store&quot; the parameters pack in a typedef, without unpacking it. Is it possible? If not, is there some reason why this is not allowed?

Is it possible to &quot;store&quot; a template parameter pack without expanding it?

I have the following Java code that uses a *local class*.

&lt;!-- language: lang-java --&gt;

    import java.util.Arrays;

    public class X&lt;T&gt; {
        void m() {
            class Z {}

            for (Object o : Arrays.asList(1, 2, 3))
                if (o instanceof Z) {}
        }
    }

It does not compile with the following error message:

&lt;!-- language: lang-none --&gt;

    X.java:8: error: illegal generic type for instanceof
                if (o instanceof Z) {}
                                 ^
    1 error

I understand that the local class `Z` inherits the generic type signature of `X&lt;T&gt;`, being an inner class. The same kind of compilation error appears in this example, where `Z` is not local, but still inner:

&lt;!-- language: lang-java --&gt;

    import java.util.Arrays;

    public class X&lt;T&gt; {
        class Z {}

        void m() {
            for (Object o : Arrays.asList(1, 2, 3))
                if (o instanceof Z) {} // Compilation error
        }
    }

It can be worked around either by making `Z` non-inner / static:

&lt;!-- language: lang-java --&gt;

    import java.util.Arrays;

    public class X&lt;T&gt; {
        static class Z {}

        void m() {
            for (Object o : Arrays.asList(1, 2, 3))
                if (o instanceof Z) {} // Compiles now
        }
    }

Or by [qualifying `X.Z`][1]:

&lt;!-- language: lang-java --&gt;

    import java.util.Arrays;

    public class X&lt;T&gt; {
        class Z {}

        void m() {
            for (Object o : Arrays.asList(1, 2, 3)) {
                if (o instanceof X.Z) {}    // Compiles now
                if (o instanceof X&lt;?&gt;.Z) {} // Also
            }
        }
    }

But how can I qualify a local class, or work around this limitation, without changing the local class itself?

  [1]: https://stackoverflow.com/a/1895696/521799




Content Type	Original Author	Original Content on Stackoverflow
Question	HighCommander4	View Question on Stackoverflow
Solution 1 - C++	eci	View Answer on Stackoverflow

Member template in local class

C++ Problem Overview

C++ Solutions

Solution 1 - C++

How do I turn the system volume overlay back on after using MPVolumeView?

Ignore code snippets in PHP_CodeSniffer

Attributions