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  • How to increase virtual hard drive space

    - by Chris
    I have a Microsoft Virutal PC hard drive (.vhd format) that's maxed out it's 16 gig hard drive space. What would be the best way to increase this diskspace? Booting into the machine (windows xp professional) and using the disk management snap in, I can see that the virtual drive has approximately 40 more unused gigs of space. Trying to use diskpart, I find out that Windows XP can't extend the boot partition. So I'm at an empass, any suggestions on how to increase the partion or to increase the actual virtual hard drive would be great. Note: The virtual hard drive is running on Windows 7 using XP mode.

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  • linux ftp server with virtual users

    - by kjertil
    i know there are already similar questions for this matter but the answers doesn't really make much sense to anyone who is not really technically comfortable in Linux. I've already tried articles like these for example: http://howto.gumph.org/content/setup-virtual-users-and-directories-in-vsftpd/ with the result of accidently breaking the whole system. The problem is that, while there are several technical possibilities to set up virtual users with a FTP server, it is not as easy as managing for instance a Filezilla server on Windows. I've seen some Web based GUI's but most of them seems to be out of date. The different flavours of Linux and the large amount of different popular FTP servers also seems to make the matter more complicated. I guess my question is, is there a way, to set up virtual FTP users on Linux without the hastle of having to manually edit PAM, MYSQL and config files?

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  • Duplicate Name Exists error from virtual PC with NAT network configuration

    - by Phred
    Whenever I change the network configuration to NAT for a virtual machine running under Virtual PC 2007, I get "A duplicate name exists on the network." error. There is no other machine on my network with this name and running the VM in any other network configuration doesn't cause this error. This seems to be a common problem with Virtual PC 2007 based on a google search but no-one seems to have a solution to it. So far, I've discovered that turning off NETBIOS over TCP causes the problem to go away but I need to join this VM to a domain and you can't do that if NETBIOS over TCP is turned off.

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  • Cannot get Virtual PC to install on my machine

    - by Brandon
    I'm not sure what I'm doing wrong. I ran the tool to check that hardware virtualization is enabled, and it is (i7-2600). I ran the proper version of the Virtual PC download (Windows 7 Professional x64) and I can see the Windows Virtual PC (KB958559) update in my installed updates list. The .msu file runs fine without errors, but even after multiple reboots I can't actually find a trace of Virtual PC anywhere. There is nothing in program files, the start menu, or system32. Am I missing a step?

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  • Override number of parameters of pure virtual functions

    - by Jir
    I have implemented the following interface: template <typename T> class Variable { public: Variable (T v) : m_value (v) {} virtual void Callback () = 0; private: T m_value; }; A proper derived class would be defined like this: class Derived : public Variable<int> { public: Derived (int v) : Variable<int> (v) {} void Callback () {} }; However, I would like to derive classes where Callback accepts different parameters (eg: void Callback (int a, int b)). Is there a way to do it?

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  • How to use more than 3 virtual disks in Linux using CentOS and XenServer

    - by 010110110101
    I've attached 5 virtual disks to a Virtual Machine in Citrix XenServer. The VM has the xs-tools installed. Initially it said that it couldn't add so many disks. After I installed the xs-tools, it let me add all the disks. But /dev doesn't show all the disks. It shows these: /dev/xvda /dev/xvdb /dev/xvdc /dev/cdrom Perhaps it is bound by the limits of an IDE bus? (3 disks + CD-ROM) If so, how does one change the VM to use SCSI? Edit: According to the documentation: 2.6.3. VM Block Devices In the PV Linux case, block devices are passed through as PV devices. XenServer does not attempt to emulate SCSI or IDE, but instead provides a more suitable interface in the virtual environment in the form of xvd* devices. It is also possible to get an sd* device using the same mechanism, where the PV driver inside the VM takes over the SCSI device namespace. This is not desirable so it is best to use xvd* where possible for PV guests (this is the default for Debian and RHEL). For Windows or other fully virtualized guests, XenServer emulates an IDE bus in the form of an hd* device. When using Windows, installing the Citrix Tools for Virtual Machines installs a special PV driver that works in a similar way to Linux, except in the fully virtualized environment. Still, with 5 virtual disks attached, I don't see the other xvd devices. Edit #2: (attached requested info) Host Machine: XenServer 6.1 Linux version 2.6.32.43-0.4.1.xs1.6.10.777.170770xen (geeko@buildhost) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-51)) #1 SMP Wed Apr 17 05:52:03 EDT 2013 Guest Machine: CentOS release 6.4 (Final) Linux version 2.6.32-358.6.2.el6.x86_64 ([email protected]) (gcc version 4.4.7 20120313 (Red Hat 4.4.7-3) (GCC) ) #1 SMP Thu May 16 20:59:36 UTC 2013 Output of 'fdisk -l' on Guest Machine: Note, the disk beyond the first 3 attached are not displaying -- there should be 4 100GB disks. (There are a total of 5 disks displayed in XenCenter -- 16GB, 100GB, 100GB, 100GB, 100GB) Disk /dev/xvdb: 107.4 GB, 107374182400 bytes 255 heads, 63 sectors/track, 13054 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0xfb6c95b9 Device Boot Start End Blocks Id System /dev/xvdb1 1 13054 104856223+ 83 Linux Disk /dev/xvda: 17.2 GB, 17179869184 bytes 255 heads, 63 sectors/track, 2088 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x000e5f41 Device Boot Start End Blocks Id System /dev/xvda1 * 1 64 512000 83 Linux Partition 1 does not end on cylinder boundary. /dev/xvda2 64 2089 16264192 8e Linux LVM Disk /dev/xvdc: 107.4 GB, 107374182400 bytes 255 heads, 63 sectors/track, 13054 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0xed249ced Device Boot Start End Blocks Id System /dev/xvdc1 1 13054 104856223+ 83 Linux Disk /dev/mapper/vg_blue-lv_root: 14.6 GB, 14571012096 bytes 255 heads, 63 sectors/track, 1771 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00000000 Disk /dev/mapper/vg_blue-lv_swap: 2080 MB, 2080374784 bytes 255 heads, 63 sectors/track, 252 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x00000000 I see that the Linux versions say SMP. The Guest VM doesn't say "xen" in the name. However, I have already run yum install kernel-xen. Could be a clue?

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  • Virtual PC on Windows 7 - Hardware-assisted virtualization is disabled

    - by DLux
    I am running a Lenovo Thinkpad T61 with an Intel Core 2 Duo T7300 processor. When I run Virtual PC in Windows 7, I get the following error: Unable to start Windows Virtual PC because hardware-assisted virtualization is disabled. When running the Hardware-Assisted Virtualization Detection Tool from Microsoft says: Hardware-assited virtualization is not enabled on this computer. Now, in the BIOS, I do have virtualization enabled and according to Intel this processor supports Intel-VT. What am I missing here?

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  • IIS6 Virtual Directory 500 Error on Remote Share

    - by David
    We have our servers at the server farm in a domain. Let's call it LIVE. Our developer computers live in a completely separate corporate domain, miles and miles away. Let's call it CORP. We have a large central storage unit (unix) that houses images and other media needed by many webservers in the server farm. The IIS application pools run as (let's say) LIVE\MediaUser and use those credentials to connect to a central storage share as a virtual directory, retrieve the images, and serve them as if they were local on each server. The problem is in development. On my development machine. I log in as CORP\MyName. My IIS 6 application pool runs as Network Service. I can't run it as a user from the LIVE domain because my machine isn't (and can not be) joined to that domain. I try to create a virtual directory, point it to the same network directory, click Connect As, uncheck the "Always use the authenticated user's credentials when validating access to the network directory" checkbox so that I can enter the login info, enter the credentails for LIVE\MediaUser, click OK, verify the password, etc. This doesn't work. I get "HTTP Error 500 - Internal server error" from IIS. The IIS log file reports sc-status = 500, sc-substatus = 16, and sc-win32-status = 1326. The documentation says this means "UNC authorization credentials are incorrect" and the Win32 status means "Logon failure: unknown user name or bad password." This would be all and good if it were anywhere close to accurate. I double- and trouble-checked it. Tried multiple known good logins. The IIS manager allows me to view the file tree in its window, it's only the browser that kicks me out. I even tried going to the virtual directory's Directory Security tab, and under Authentication and Access Control, I tried using the same LIVE domain username for the anonymous access credential. No luck. I'm not trying to run any ASP, ASP.NET, or other dynamic anything out of the virtual directory. I just want IIS to be able to load static images, css, and js files. If anyone has some bright ideas I would be most appreciative!

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  • virtual appliance resources

    - by user11457
    I'm fairly new to appliances but have become a big fan of virtualization. Are there any other resources where I can find and download virtual appliances outside of vmware's website? Or do I just need to scour the web looking them? I'm sure there is a central site that lists all the virtual appliances available, if there isn't I'm going to write one.

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  • How to activate Virtual Desktop on Fortigate 100A ?

    - by Deniz
    We did recently update the firmware on our Fortigate 100A box and after the upgrade we tried to use the "Virtual Desktop" feature. (This isn't a new firmware feature) We can't find a way to activate or use it. Does anyone have any experience on "Virtual Desktop" of Fortigate devices ?

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  • Can GPU capabilities impact virtual machine performance?

    - by Dave White
    While this many not seem like a programming question directly, it impacts my development activities and so it seems like it belongs here. It seems that more and more developers are turning to virtual environments for development activities on their computers, SharePoint development being a prime example. Also, as a trainer, I have virtual training environments for all of the classes that I teach. I recently purchased a new Dell E6510 to travel around with. It has the i7 620M (Dual core, HyperThreaded cpu running at 2.66GHz) and 8 GB of memory. Reading the spec sheet, it sounded like it would be a great laptop to carry around and run virtual machines on. Getting the laptop though, I've been pretty disappointed with the user experience of developing in a virtual machine. Giving the Virtual Machine 4 GB of memory, it was slow and I could type complete sentences and watch the VM "catchup". My company has training laptops that we provide for our classes. They are Dell Precision M6400 Intel Core 2 Duo P8700 running at 2.54Ghz with 8 GB of memory and the experience on this laptops is night and day compared to the E6510. They are crisp and you barely aware that you are running in a virtual environment. Since the E6510 should be faster in all categories than the M6400, I couldn't understand why the new laptop was slower, so I did a component by component comparison and the only place where the E6510 is less performant than the M6400 is the graphics department. The M6400 is running a nVidia FX 2700m GPU and the E6510 is running a nVidia 3100M GPU. Looking at benchmarks of the two GPUs suggest that the FX 2700M is twice as fast as the 3100M. http://www.notebookcheck.net/Mobile-Graphics-Cards-Benchmark-List.844.0.html 3100M = 111th (E6510) FX 2700m = 47th (Precision M6400) Radeon HD 5870 = 8th (Alienware) The host OS is Windows 7 64bit as is the guest OS, running in Virtual Box 3.1.8 with Guest Additions installed on the guest. The IDE being used in the virtual environment is VS 2010 Premium. So after that long setup, my question is: Is the GPU significantly impacting the virtual machine's performance or are there other factors that I'm not looking at that I can use to boost the vm's performance? Do we now have to consider GPU performance when purchasing laptops where we expect to use virtualized development environments? Thanks in advance. Cheers, Dave

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  • IIS6 Virtual Directory 500 Error on Remote Share

    - by David Boike
    We have our servers at the server farm in a domain. Let's call it LIVE. Our developer computers live in a completely separate corporate domain, miles and miles away. Let's call it CORP. We have a large central storage unit (unix) that houses images and other media needed by many webservers in the server farm. The IIS application pools run as (let's say) LIVE\MediaUser and use those credentials to connect to a central storage share as a virtual directory, retrieve the images, and serve them as if they were local on each server. The problem is in development. On my development machine. I log in as CORP\MyName. My IIS 6 application pool runs as Network Service. I can't run it as a user from the LIVE domain because my machine isn't (and can not be) joined to that domain. I try to create a virtual directory, point it to the same network directory, click Connect As, uncheck the "Always use the authenticated user's credentials when validating access to the network directory" checkbox so that I can enter the login info, enter the credentails for LIVE\MediaUser, click OK, verify the password, etc. This doesn't work. I get "HTTP Error 500 - Internal server error" from IIS. The IIS log file reports sc-status = 500, sc-substatus = 16, and sc-win32-status = 1326. The documentation says this means "UNC authorization credentials are incorrect" and the Win32 status means "Logon failure: unknown user name or bad password." This would be all and good if it were anywhere close to accurate. I double- and trouble-checked it. Tried multiple known good logins. The IIS manager allows me to view the file tree in its window, it's only the browser that kicks me out. I even tried going to the virtual directory's Directory Security tab, and under Authentication and Access Control, I tried using the same LIVE domain username for the anonymous access credential. No luck. I'm not trying to run any ASP, ASP.NET, or other dynamic anything out of the virtual directory. I just want IIS to be able to load static images, css, and js files. If anyone has some bright ideas I would be most appreciative!

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  • Virtual Function Implementation

    - by Gokul
    Hi, I have kept hearing this statement. Switch..Case is Evil for code maintenance, but it provides better performance(since compiler can inline stuffs etc..). Virtual functions are very good for code maintenance, but they incur a performance penalty of two pointer indirections. Say i have a base class with 2 subclasses(X and Y) and one virtual function, so there will be two virtual tables. The object has a pointer, based on which it will choose a virtual table. So for the compiler, it is more like switch( object's function ptr ) { case 0x....: X->call(); break; case 0x....: Y->call(); }; So why should virtual function cost more, if it can get implemented this way, as the compiler can do the same in-lining and other stuff here. Or explain me, why is it decided not to implement the virtual function execution in this way? Thanks, Gokul.

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  • Two dimensional virtual desktop space in Ubuntu

    - by Herms
    Is there any way to create a 2-dimensional virtual desktop space in Ubuntu? The only control I'm seeing is the number of virtual desktops, but they seem to only go in a line. I'm used to having a 2-dimensional space (so I can go up/down/left/right instead of just left/right), and I'd really like to have that in ubuntu as well.

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  • Resource Pool sharing in Hyper-V Virtual Machines

    - by user67905
    I understand that we can install Hyper-V on one server and run a number of Virtual Machines on it, upto the limit of resources of that server. I want to know if it is possible to install Hyper-V lumped on two or more servers, so that the Virtual Machines can use the underlying resources pool of both the servers? And also if that same is possible for an “n” number of servers, instead of just 2 servers.

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  • In a Virtual Machine, is the Virtual Processor using RAM or part of the Processor?

    - by Jason H
    I am running a 15" MacBook Pro (2.66GHz) and 4GB of RAM. I am considering downgrading to a 13 MacBook Pro (2.4GHz) with 8GB of RAM. Most of what I do at work is through Windows and I need to run it virtually. So my real question is when running a virtual machine will the virtual processor be utilizing RAM or part of the hosts processor? My assumption is that it will utilize the allocated RAM but I have seen zero documentation to support that.

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  • Virtual session on windows xp

    - by dotnet-practitioner
    What is the easiest way to install , setup, and run virtual session on my fresh install on my windows xp computer? I want to be able to browse , install a new software in a new virtual session instead of machine itself. What is available out there? What kind of software it would take and are there any free solutions out there? Easiest solution would be very helpful for me.

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  • What's the relationship between meta-circular interpreters, virtual machines and increased performance?

    - by Gomi
    I've read about meta-circular interpreters on the web (including SICP) and I've looked into the code of some implementations (such as PyPy and Narcissus). I've read quite a bit about two languages which made great use of metacircular evaluation, Lisp and Smalltalk. As far as I understood Lisp was the first self-hosting compiler and Smalltalk had the first "true" JIT implementation. One thing I've not fully understood is how can those interpreters/compilers achieve so good performance or, in other words, why is PyPy faster than CPython? Is it because of reflection? And also, my Smalltalk research led me to believe that there's a relationship between JIT, virtual machines and reflection. Virtual Machines such as the JVM and CLR allow a great deal of type introspection and I believe they make great use it in Just-in-Time (and AOT, I suppose?) compilation. But as far as I know, Virtual Machines are kind of like CPUs, in that they have a basic instruction set. Are Virtual Machines efficient because they include type and reference information, which would allow language-agnostic reflection? I ask this because many both interpreted and compiled languages are now using bytecode as a target (LLVM, Parrot, YARV, CPython) and traditional VMs like JVM and CLR have gained incredible boosts in performance. I've been told that it's about JIT, but as far as I know JIT is nothing new since Smalltalk and Sun's own Self have been doing it before Java. I don't remember VMs performing particularly well in the past, there weren't many non-academic ones outside of JVM and .NET and their performance was definitely not as good as it is now (I wish I could source this claim but I speak from personal experience). Then all of a sudden, in the late 2000s something changed and a lot of VMs started to pop up even for established languages, and with very good performance. Was something discovered about the JIT implementation that allowed pretty much every modern VM to skyrocket in performance? A paper or a book maybe?

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  • C++ virtual functions.Problem with vtable

    - by adivasile
    I'm doing a little project in C++ and I've come into some problems regarding virtual functions. I have a base class with some virtual functions: #ifndef COLLISIONSHAPE_H_ #define COLLISIONSHAPE_H_ namespace domino { class CollisionShape : public DominoItem { public: // CONSTRUCTOR //------------------------------------------------- // SETTERS //------------------------------------------------- // GETTERS //------------------------------------------------- virtual void GetRadius() = 0; virtual void GetPosition() = 0; virtual void GetGrowth(CollisionShape* other) = 0; virtual void GetSceneNode(); // OTHER //------------------------------------------------- virtual bool overlaps(CollisionShape* shape) = 0; }; } #endif /* COLLISIONSHAPE_H_ */ and a SphereShape class which extends CollisionShape and implements the methods above /* SphereShape.h */ #ifndef SPHERESHAPE_H_ #define SPHERESHAPE_H_ #include "CollisionShape.h" namespace domino { class SphereShape : public CollisionShape { public: // CONSTRUCTOR //------------------------------------------------- SphereShape(); SphereShape(CollisionShape* shape1, CollisionShape* shape2); // DESTRUCTOR //------------------------------------------------- ~SphereShape(); // SETTERS //------------------------------------------------- void SetPosition(); void SetRadius(); // GETTERS //------------------------------------------------- cl_float GetRadius(); cl_float3 GetPosition(); SceneNode* GetSceneNode(); cl_float GetGrowth(CollisionShape* other); // OTHER //------------------------------------------------- bool overlaps(CollisionShape* shape); }; } #endif /* SPHERESHAPE_H_ */ and the .cpp file: /*SphereShape.cpp*/ #include "SphereShape.h" #define max(a,b) (a>b?a:b) namespace domino { // CONSTRUCTOR //------------------------------------------------- SphereShape::SphereShape(CollisionShape* shape1, CollisionShape* shape2) { } // DESTRUCTOR //------------------------------------------------- SphereShape::~SphereShape() { } // SETTERS //------------------------------------------------- void SphereShape::SetPosition() { } void SphereShape::SetRadius() { } // GETTERS //------------------------------------------------- void SphereShape::GetRadius() { } void SphereShape::GetPosition() { } void SphereShape::GetSceneNode() { } void SphereShape::GetGrowth(CollisionShape* other) { } // OTHER //------------------------------------------------- bool SphereShape::overlaps(CollisionShape* shape) { return true; } } These classes, along some other get compiled into a shared library. Building libdomino.so g++ -m32 -lpthread -ldl -L/usr/X11R6/lib -lglut -lGLU -lGL -shared -lSDKUtil -lglut -lGLEW -lOpenCL -L/home/adrian/AMD-APP-SDK-v2.4-lnx32/lib/x86 -L/home/adrian/AMD-APP-SDK-v2.4-lnx32/TempSDKUtil/lib/x86 -L"/home/adrian/AMD-APP-SDK-v2.4-lnx32/lib/x86" -lSDKUtil -lglut -lGLEW -lOpenCL -o build/debug/x86/libdomino.so build/debug/x86//Material.o build/debug/x86//Body.o build/debug/x86//SphereShape.o build/debug/x86//World.o build/debug/x86//Engine.o build/debug/x86//BVHNode.o When I compile the code that uses this library I get the following error: ../../../lib/x86//libdomino.so: undefined reference to `vtable for domino::CollisionShape' ../../../lib/x86//libdomino.so: undefined reference to `typeinfo for domino::CollisionShape' Command used to compile the demo that uses the library: g++ -o build/debug/x86/startdemo build/debug/x86//CMesh.o build/debug/x86//CSceneNode.o build/debug/x86//OFF.o build/debug/x86//Light.o build/debug/x86//main.o build/debug/x86//Camera.o -m32 -lpthread -ldl -L/usr/X11R6/lib -lglut -lGLU -lGL -lSDKUtil -lglut -lGLEW -ldomino -lSDKUtil -lOpenCL -L/home/adrian/AMD-APP-SDK-v2.4-lnx32/lib/x86 -L/home/adrian/AMD-APP-SDK-v2.4-lnx32/TempSDKUtil/lib/x86 -L../../../lib/x86/ -L"/home/adrian/AMD-APP-SDK-v2.4-lnx32/lib/x86" (the -ldomino flag) And when I run the demo, I manually tell it about the library: LD_LIBRARY_PATH=../../lib/x86/:$AMDAPPSDKROOT/lib/x86:$LD_LIBRARY_PATH bin/x86/startdemo After reading a bit about virtual functions and virtual tables I understood that virtual tables are handled by the compiler and I shouldn't worry about it, so I'm a little bit confused on how to handle this issue. I'm using gcc version 4.6.0 20110530 (Red Hat 4.6.0-9) (GCC) Later edit: I'm really sorry, but I wrote the code by hand directly here. I have defined the return types in the code. I apologize to the 2 people that answered below. I have to mention that I am a beginner at using more complex project layouts in C++.By this I mean more complex makefiles, shared libraries, stuff like that.

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  • Visitor and templated virtual methods

    - by Thomas Matthews
    In a typical implementation of the Visitor pattern, the class must account for all variations (descendants) of the base class. There are many instances where the same method content in the visitor is applied to the different methods. A templated virtual method would be ideal in this case, but for now, this is not allowed. So, can templated methods be used to resolve virtual methods of the parent class? Given (the foundation): struct Visitor_Base; // Forward declaration. struct Base { virtual accept_visitor(Visitor_Base& visitor) = 0; }; // More forward declarations struct Base_Int; struct Base_Long; struct Base_Short; struct Base_UInt; struct Base_ULong; struct Base_UShort; struct Visitor_Base { virtual void operator()(Base_Int& b) = 0; virtual void operator()(Base_Long& b) = 0; virtual void operator()(Base_Short& b) = 0; virtual void operator()(Base_UInt& b) = 0; virtual void operator()(Base_ULong& b) = 0; virtual void operator()(Base_UShort& b) = 0; }; struct Base_Int : public Base { void accept_visitor(Visitor_Base& visitor) { visitor(*this); } }; struct Base_Long : public Base { void accept_visitor(Visitor_Base& visitor) { visitor(*this); } }; struct Base_Short : public Base { void accept_visitor(Visitor_Base& visitor) { visitor(*this); } }; struct Base_UInt : public Base { void accept_visitor(Visitor_Base& visitor) { visitor(*this); } }; struct Base_ULong : public Base { void accept_visitor(Visitor_Base& visitor) { visitor(*this); } }; struct Base_UShort : public Base { void accept_visitor(Visitor_Base& visitor) { visitor(*this); } }; Now that the foundation is laid, here is where the kicker comes in (templated methods): struct Visitor_Cout : public Visitor { template <class Receiver> void operator() (Receiver& r) { std::cout << "Visitor_Cout method not implemented.\n"; } }; Intentionally, Visitor_Cout does not contain the keyword virtual in the method declaration. All the other attributes of the method signatures match the parent declaration (or perhaps specification). In the big picture, this design allows developers to implement common visitation functionality that differs only by the type of the target object (the object receiving the visit). The implementation above is my suggestion for alerts when the derived visitor implementation hasn't implement an optional method. Is this legal by the C++ specification? (I don't trust when some says that it works with compiler XXX. This is a question against the general language.)

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  • Why does virtual assignment behave differently than other virtual functions of the same signature?

    - by David Rodríguez - dribeas
    While playing with implementing a virtual assignment operator I have ended with a funny behavior. It is not a compiler glitch, since g++ 4.1, 4.3 and VS 2005 share the same behavior. Basically, the virtual operator= behaves differently than any other virtual function with respect to the code that is actually being executed. struct Base { virtual Base& f( Base const & ) { std::cout << "Base::f(Base const &)" << std::endl; return *this; } virtual Base& operator=( Base const & ) { std::cout << "Base::operator=(Base const &)" << std::endl; return *this; } }; struct Derived : public Base { virtual Base& f( Base const & ) { std::cout << "Derived::f(Base const &)" << std::endl; return *this; } virtual Base& operator=( Base const & ) { std::cout << "Derived::operator=( Base const & )" << std::endl; return *this; } }; int main() { Derived a, b; a.f( b ); // [0] outputs: Derived::f(Base const &) (expected result) a = b; // [1] outputs: Base::operator=(Base const &) Base & ba = a; Base & bb = b; ba = bb; // [2] outputs: Derived::operator=(Base const &) Derived & da = a; Derived & db = b; da = db; // [3] outputs: Base::operator=(Base const &) ba = da; // [4] outputs: Derived::operator=(Base const &) da = ba; // [5] outputs: Derived::operator=(Base const &) } The effect is that the virtual operator= has a different behavior than any other virtual function with the same signature ([0] compared to [1]), by calling the Base version of the operator when called through real Derived objects ([1]) or Derived references ([3]) while it does perform as a regular virtual function when called through Base references ([2]), or when either the lvalue or rvalue are Base references and the other a Derived reference ([4],[5]). Is there any sensible explanation to this odd behavior?

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