Yes. You also need to implement the destructor:

    class A {
    public:
        virtual ~A() = 0;
    };

    inline A::~A() { }

should suffice.

And since this got a down vote, I should clarify: If you derive anything from A and then try to delete or destroy it, `A`&#39;s destructor will eventually be called. Since it is pure and doesn&#39;t have an implementation, undefined behavior will ensue. On one popular platform, that will invoke the purecall handler and crash.

Edit: fixing the declaration to be more conformant, compiled with http://www.comeaucomputing.com/tryitout/

Private destructors: they will give you an error when you create an object of a derived class -- not otherwise. A diagnostic may appear though.

&gt; **12.4 Destructors**

&gt; 6 A destructor can be declared virtual (10.3) or pure virtual (10.4); if any objects of that class or any derived class are created in the program, the destructor shall be defined.

A class with a pure virtual destructor is an abstract class.
Note well:

&gt; **10.4 Abstract classes**
&gt;
&gt; 2 A pure virtual function need be defined only if called with, or as if with (12.4), the qualified-id syntax (5.1).

&gt; [*Note*:a function declaration cannot provide both a pure-specifier and a definition —end
note ]

Taken straight from the draft:

    struct C {
       virtual void f() = 0 { }; // ill-formed
    };



I know that the following is case sensitive:

    if (StringA == StringB) {

So is there an operator which will compare two strings in an insensitive manner?

Is there a C# case insensitive equals operator?

I have the following array, which I would like to reindex so the keys are reversed (ideally starting at 1):

Current array (**edit:** the array actually looks like this):

    Array (
    
    [2] =&gt; Object
        (
            [title] =&gt; Section
            [linked] =&gt; 1
        )

    [1] =&gt; Object
        (
            [title] =&gt; Sub-Section
            [linked] =&gt; 1
        )

    [0] =&gt; Object
        (
            [title] =&gt; Sub-Sub-Section
            [linked] =&gt; 
        )

    )

How it should be:

    Array (

    [1] =&gt; Object
        (
            [title] =&gt; Section
            [linked] =&gt; 1
        )

    [2] =&gt; Object
        (
            [title] =&gt; Sub-Section
            [linked] =&gt; 1
        )

    [3] =&gt; Object
        (
            [title] =&gt; Sub-Sub-Section
            [linked] =&gt; 
        )

    )


How do you reindex an array in PHP but with indexes starting from 1?

Is it wrong to write:

    class A {
    public:
        virtual ~A() = 0;
    };

for an abstract base class?

At least that compiles in MSVC... Will it crash at run time?



Pure virtual destructor in C++

Is it wrong to write:
<pre><code class="hljs language-arduino">class A {
public:
 virtual ~A() = 0;
};
</code></pre>
for an abstract base class?
At least that compiles in MSVC... Will it crash at run time?

I&#39;d like to have access to the `$HOME` environment variable in a C++ program that I&#39;m writing. If I were writing code in C, I&#39;d just use the `getenv()` function, but I was wondering if there was a better way to do it. Here&#39;s the code that I have so far:


    std::string get_env_var( std::string const &amp; key ) {                                 
        char * val;                                                                        
        val = getenv( key.c_str() );                                                       
        std::string retval = &quot;&quot;;                                                           
        if (val != NULL) {                                                                 
            retval = val;                                                                    
        }                                                                                  
        return retval;                                                                        
    }           

Should I use `getenv()` to access environment variables in C++? Are there any problems that I&#39;m likely to run into that I can avoid with a little bit of knowledge?

Accessing Environment Variables In C++

What is the use of having destructor as private?

Does it matter which way I declare the `main` function in a C++ (or C) program? 

Difference between void main and int main in C/C++?

Qt documentation states that signals and slots can be `direct`, `queued` and `auto`. 

It also stated that if object that owns slot &#39;lives&#39; in a thread different from object that owns signal, emitting such signal will be like posting message - signal emit will return instantly and slot method will be called in target thread&#39;s event loop. 

Unfortunately, documentation do not specify that &#39;lives&#39; stands for and no examples is available. I have tried the following code:

**`main.h:`**

    class CThread1 : public QThread
    {
    Q_OBJECT
    public:
        void run( void )
        {
            msleep( 200 );
            std::cout &lt;&lt; &quot;thread 1 started&quot; &lt;&lt; std::endl;
            MySignal();
            exec();
        }
    signals:
        void MySignal( void );
    };
    
    class CThread2 : public QThread
    {
    Q_OBJECT
    public:
        void run( void )
        {
            std::cout &lt;&lt; &quot;thread 2 started&quot; &lt;&lt; std::endl;
            exec();
        }
    public slots:
        void MySlot( void )
        {
            std::cout &lt;&lt; &quot;slot called&quot; &lt;&lt; std::endl;
        }
    };


**`main.cpp:`**

    int main(int argc, char *argv[])
    {
        QCoreApplication a(argc, argv);
        CThread1 oThread1;
        CThread2 oThread2;
        QObject::connect( &amp; oThread1, SIGNAL( MySignal() ),
            &amp; oThread2, SLOT( MySlot() ) );
        oThread1.start();
        oThread2.start();
        oThread1.wait();
        oThread2.wait();
        return a.exec();
    }

Output is:

    thread 2 started
    thread 1 started

`MySlot()` is never called :(. What I&#39;m doing wrong?

How to emit cross-thread signal in Qt?

I have a vector of myObjects in global scope.
I have a method which uses a `std::vector&lt;myObject&gt;::const_iterator` to traverse the vector, and doing some comparisons to find a specific element.
Once I have found the required element, I want to be able to return a pointer to it (the vector exists in global scope).

If I return `&amp;iterator`, am I returning the address of the iterator or the address of what the iterator is pointing to?

Do I need to cast the `const_iterator` back to a myObject, then return the address of that?

Returning a pointer to a vector element in c++

I&#39;m writing a module and want to have a unified exception hierarchy for the exceptions that it can raise (e.g. inheriting from a `FooError` abstract class for all the `foo` module&#39;s specific exceptions). This allows users of the module to catch those particular exceptions and handle them distinctly, if needed. But many of the exceptions raised from the module are raised because of some other exception; e.g. failing at some task because of an OSError on a file.

What I need is to **“wrap” the exception caught such that it has a different type and message**, so that information is available further up the propagation hierarchy by whatever catches the exception. But I don&#39;t want to lose the existing type, message, and stack trace; that&#39;s all useful information for someone trying to debug the problem. A top-level exception handler is no good, since I&#39;m trying to decorate the exception before it makes its way further up the propagation stack, and the top-level handler is too late.

This is partly solved by deriving my module `foo`&#39;s specific exception types from the existing type (e.g. `class FooPermissionError(OSError, FooError)`), but that doesn&#39;t make it any easier to wrap the existing exception instance in a new type, nor modify the message.

Python&#39;s [PEP 3134][1] “Exception Chaining and Embedded Tracebacks” discusses a change accepted in Python 3.0 for “chaining” exception objects, to indicate that a new exception was raised during the handling of an existing exception.

What I&#39;m trying to do is related: I need it also working in earlier Python versions, and I need it not for chaining, but only for polymorphism. What is the right way to do this?


  [1]: http://www.python.org/dev/peps/pep-3134/

Re-raise exception with a different type and message, preserving existing information

&gt; **Possible Duplicate:**  
&gt; [C++ overload resolution](https://stackoverflow.com/questions/72010/c-overload-resolution)  

&lt;!-- End of automatically inserted text --&gt;

I ran into a problem where after my class overrode a function of its base class, all of the overloaded versions of the functions were then hidden.  Is this by design or am I just doing something wrong?

Ex.

    class foo
    {
      public:
        foo(void);
        ~foo(void);
        virtual void a(int);
        virtual void a(double);
    };

    class bar : public foo 
    {
      public:
        bar(void);
        ~bar(void);
        void a(int);
    };

the following would then give a compile error saying there is no a(double) function in bar.

    main() 
    {
      double i = 0.0;
      bar b;
      b.a(i);
    }

Overriding a Base&#39;s Overloaded Function in C++

Being new to ASP.NET MVC, I&#39;ve been wondering about the signature of Controller methods. In all the examples I&#39;ve seen, they always seem to return ActionResult, even if they actually return a ViewResult instance or similar.

Here&#39;s a commonly seen example:

    public ActionResult Index()
    {
        return this.View();
    }

In such a case, wouldn&#39;t it make more sense to declare the method as `public ViewResult Index()`, and get stronger type support?

Experimentation indicates that this works, so it seems possible.

I do realize that there may be situations where the polymorphism is desired (e.g. if you want to redirect only in certain situations, but show a view in other situations), but if the method *always* returns a view, I&#39;d find a ViewResult more desirable.

In terms of future compatibility, ActionResult obviously provides a more robust signature, but if one controls the entire code base, it&#39;s always possible to change a method&#39;s signature to a more general return type if that should become necessary in the future.

Are the any other considerations that I&#39;m not aware of, or should I just go ahead and declare my controller methods with specific return types?

Must ASP.NET MVC Controller Methods Return ActionResult?

What is polymorphism, what is it for, and how is it used?

I&#39;m working on a multithreaded application, and I want to debug it using GDB.

Problem is, one of my threads keeps dying with the message:

    pure virtual method called
    terminate called without an active exception
    Abort

I know the cause of that message, but I have no idea where in my thread it occurs. A backtrace would really be helpful.

When I run my app in GDB, it pauses every time a thread is suspended or resumed. I want my app to continue running normally until one of the threads dies with that exception, at which point everything should halt so that I can get a backtrace.

Run an Application in GDB Until an Exception Occurs

What is the order in which the destructors and the constructors are called in C++? Using the examples of some Base classes and Derived Classes

What is the order in which the destructors and the constructors are called in C++

When overriding a class in C++ (with a virtual destructor) I am implementing the destructor again as virtual on the inheriting class, but do I need to call the base destructor?

If so I imagine it&#39;s something like this...

    MyChildClass::~MyChildClass() // virtual in header
    {
        // Call to base destructor...
        this-&gt;MyBaseClass::~MyBaseClass();
    
        // Some destructing specific to MyChildClass
    }

Am I right?

Do I need to explicitly call the base virtual destructor?

I have an `class A` which uses a heap memory allocation for one of its fields. Class A  is instantiated and stored as a pointer field in another class (`class B`.

When I&#39;m done with an object of class B, I call `delete`, which I assume calls the destructor... But does this call the destructor of class A as well?

### Edit:
From the answers, I take that (please edit if incorrect):

1. `delete` of an instance of B calls B::~B();
2. which calls `A::~A();`
3. `A::~A` *should* explicitly `delete` all heap-allocated member variables of the A object;
4. Finally the memory block storing said instance of class B is returned to the heap - when *new* was used, it first allocated a block of memory on heap, then invoked constructors to initialize it, now after all destructors have been invoked to finalize the object the block where the object resided is returned to the heap.

Does delete on a pointer to a subclass call the base class destructor?

    class Package:
        def __init__(self):
            self.files = []

        # ...

        def __del__(self):
            for file in self.files:
                os.unlink(file)

`__del__(self)` above fails with an AttributeError exception.  I understand [Python doesn&#39;t guarantee](http://docs.python.org/reference/datamodel.html#customization) the existence of &quot;global variables&quot; (member data in this context?) when `__del__()` is invoked.  If that is the case and this is the reason for the exception, how do I make sure the object destructs properly?

How do I correctly clean up a Python object?

Whilst compiling with avr-gcc I have encountered linker errors such as the following:

    undefined reference to `__cxa_pure_virtual&#39;

I&#39;ve found [this document][1] which states:

&gt; The `__cxa_pure_virtual` function is an error handler that is invoked when a pure virtual function is called.
&gt;
&gt; If you are writing a C++ application that has pure virtual functions you must supply your own `__cxa_pure_virtual` error handler function. For example:
&gt;
&gt; `extern &quot;C&quot; void __cxa_pure_virtual() { while (1); }`

Defining this function as suggested fixes the errors but I&#39;d like to know:

* what the purpose of this function is,
* why I should need to define it myself and
* why it is acceptable to code it as an infinite loop?

  [1]: http://ccgi.rowley.co.uk/support/faq.php?do=article&amp;articleid=127

What is the purpose of __cxa_pure_virtual?

I understand the need for a virtual destructor. But why do we need a ***pure*** virtual destructor? In one of the C++ articles, the author has mentioned that we use pure virtual destructor when we want to make a class abstract. 

But we can make a class abstract by making any of the member functions as pure virtual. 

So my questions are 
 
1. When do we really make a destructor pure virtual? Can anybody give a good real time example? 

2. When we are creating abstract classes is it a good practice to make the destructor also pure virtual? If yes..then why?




Why do we need a pure virtual destructor in C++?

Nearly the final step but still some strange erros....

    bash-3.2$ make
    g++ -Wall -c -g Myworld.cc
    g++ -Wall -g solvePlanningProblem.o Position.o AStarNode.o PRM.o PRMNode.o World.o SingleCircleWorld.o Myworld.o RECTANGLE.o CIRCLE.o -o solvePlanningProblem
    Undefined symbols:
      &quot;vtable for Obstacle&quot;, referenced from:
          Obstacle::Obstacle()in Myworld.o
      &quot;typeinfo for Obstacle&quot;, referenced from:
          typeinfo for RECTANGLEin RECTANGLE.o
          typeinfo for CIRCLEin CIRCLE.o
    ld: symbol(s) not found
    collect2: ld returned 1 exit status
    make: *** [solvePlanningProblem] Error 1

What&#39;s the meaning of vtable and typeinfo?


Undefined symbols &quot;vtable for ...&quot; and &quot;typeinfo for...&quot;?

My basic understanding is that there is no implementation for a pure virtual function, however, I was told there might be implementation for pure virtual function. 

    class A {
    public:
        virtual void f() = 0;
    };
    
    void A::f() {
        cout&lt;&lt;&quot;Test&quot;&lt;&lt;endl;
    }

Is code above OK?

What&#39;s the purpose to make it a pure virtual function with an implementation?




Pure virtual function with implementation

We always declare a pure virtual function as:

    virtual void fun () = 0 ;

I.e., it is always assigned to 0.

What I understand is that this is to initialize the vtable entry for this function to NULL and any other value here results in a compile time error. Is this understanding correct or not?


Content Type	Original Author	Original Content on Stackoverflow
Question	Ivan Krechetov	View Question on Stackoverflow
Solution 1 - C++	MSN	View Answer on Stackoverflow
Solution 2 - C++	dirkgently	View Answer on Stackoverflow

Pure virtual destructor in C++

C++ Problem Overview

C++ Solutions

Solution 1 - C++

Solution 2 - C++

How do you reindex an array in PHP but with indexes starting from 1?

Is there a C# case insensitive equals operator?

Attributions