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  • What are the reasons for casting a void pointer?

    - by Maulrus
    I'm learning C++ from scratch, and as such I don't have an expert understanding of C. In C++, you can't cast a void pointer to whatever, and I understand the reasons behind that. However, I know that in C, you can. What are the possible reasons for this? It just seems like it's be a huge hole in type safety, which (to me) seems like a bad thing.

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  • C++ and C#, "Casting" Functions to Delegates, What's the scoop?

    - by Jacob G
    In C# 2.0, I can do the following: public class MyClass { delegate void MyDelegate(int myParam); public MyClass(OtherObject obj) { //THIS IS THE IMPORTANT PART obj.SomeCollection.Add((MyDelegate)MyFunction); } private void MyFunction(int myParam); { //... } } Trying to implement the same thing in C++, it appears I have to do: MyDelegate del = gcnew MyDelegate(this, MyFunction); obj-SomeCollection-Add(del); Obviously I can create a new instance of the delegate in C# as well instead of what's going on up there. Is there some kind of magic going on in the C# world that doesn't exist in C++ that allows that cast to work? Some kind of magic anonymous delegate? Thanks.

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  • Why does casting a NaN to a long yeild a valid result?

    - by brainimus
    In the sample code below I am dividing by zero which when I step through it with the debugger the (dividend / divisor) yeilds an Infinity or NaN (if the divisor is zero). When I cast this result to a long I get a valid result, usually something like -9223372036854775808. Why is this cast valid? Why doesn't it stop executing (throw an exception for example) rather than assign an arbitrary value? double divisor = 0; double dividend = 7; long result = (long)(dividend / divisor);

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  • Casting as Interface? Awesome.

    - by cam
    I was just experimenting around with one of my programs, trying to make it looks prettier and I found something interesting. I have an interface that 4 classes inherit from. I found that if I pass the Class as an object to a method like so: ClassTest classtest = new ClassTest(); DoOperation(classtest); private void DoOperation(object o) { ((InterfaceClass)o).DoThis(); } So I can pass any type of class that inherits from InterfaceClass, and it will preform the proper interface operation? This is the coolest thing I've ever found from OOP (something I've never really studied) I really thought interfaces were created for the sole purpose of organization for developers. Are there more uses for interfaces than this?

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  • Why does casting a NaN to a long yield a valid result?

    - by brainimus
    In the sample code below I am dividing by zero which when I step through it with the debugger the (dividend / divisor) yields an Infinity or NaN (if the divisor is zero). When I cast this result to a long I get a valid result, usually something like -9223372036854775808. Why is this cast valid? Why doesn't it stop executing (throw an exception for example) rather than assign an arbitrary value? double divisor = 0; double dividend = 7; long result = (long)(dividend / divisor);

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  • Why does casting to double using "String * 1" fail? Will CDbl(String) work on all systems?

    - by Jamie Bull
    I have an application which contains the line below to assign a parsed XML value to a variant array. V(2) = latNode.Text * 1 This works fine on my system (Windows 7, Excel 2010) but doesn't work on some other system or systems - and I've not been able to get a response from the user who reported the problem. I've switched out the offending line for: V(2) = CDbl(latNode.Text) This still works on my system, but then I had no problem in the first place. The question is on what systems does the first approach fail and why, and will the second method always work? I'm sure I've used the "Stying * 1" trick elsewhere before and would like to know how concerned I should be about tracking down other occurrences. Thanks.

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  • C++/CLI and C#, "Casting" Functions to Delegates, What's the scoop?

    - by Jacob G
    In C# 2.0, I can do the following: public class MyClass { delegate void MyDelegate(int myParam); public MyClass(OtherObject obj) { //THIS IS THE IMPORTANT PART obj.SomeCollection.Add((MyDelegate)MyFunction); } private void MyFunction(int myParam); { //... } } Trying to implement the same thing in C++/CLI, it appears I have to do: MyDelegate del = gcnew MyDelegate(this, MyFunction); obj-SomeCollection-Add(del); Obviously I can create a new instance of the delegate in C# as well instead of what's going on up there. Is there some kind of magic going on in the C# world that doesn't exist in C++/CLI that allows that cast to work? Some kind of magic anonymous delegate? Thanks.

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  • Casting/dereferencing member variable pointer from void*, is this safe?

    - by Damien
    Hi all, I had a problem while hacking a bigger project so I made a simpel test case. If I'm not omitting something, my test code works fine, but maybe it works accidentally so I wanted to show it to you and ask if there are any pitfalls in this approach. I have an OutObj which has a member variable (pointer) InObj. InObj has a member function. I send the address of this member variable object (InObj) to a callback function as void*. The type of this object never changes so inside the callback I recast to its original type and call the aFunc member function in it. In this exampel it works as expected, but in the project I'm working on it doesn't. So I might be omitting something or maybe there is a pitfall here and this works accidentally. Any comments? Thanks a lot in advance. (The problem I have in my original code is that InObj.data is garbage). #include <stdio.h> class InObj { public: int data; InObj(int argData); void aFunc() { printf("Inside aFunc! data is: %d\n", data); }; }; InObj::InObj(int argData) { data = argData; } class OutObj { public: InObj* objPtr; OutObj(int data); ~OutObj(); }; OutObj::OutObj(int data) { objPtr = new InObj(data); } OutObj::~OutObj() { delete objPtr; } void callback(void* context) { ((InObj*)context)->aFunc(); } int main () { OutObj a(42); callback((void*)a.objPtr); }

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  • Calling child constructor by casting (ChildClass)parentObject; to track revisions

    - by FreshCode
    To track revisions of a Page class, I have a PageRevision class which inherits from Page and adds a revision ID (Guid RevisionID;). If possible, how should I cast an existing Page object to a PageRevision and ensure that the PageRevision constructor is called to create a new revision ID? I could could have a PageRevision(Page page) constructor which generates the Guid and copies all the Page attributes, but I want to automate it, especially if a Page class has many attributes (and I later add one, and forget to modify the copy constructor). Desired use Page page = new Page(123, "Page Title", "Page Body"); // where 123 is page ID PageRevision revision = (PageRevision)page; // now revision.RevisionID should be a new Guid. Page, PageRevision classes: public class Page { public int ID { get; set; } public string Title { get; set; } public string Body { get; set; } } public class PageRevision : Page { public Guid RevisionID { get; set; } public PageRevision() { this.RevisionID = Guid.NewGuid(); } }

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  • sql: can i do a max(count(*)) ?

    - by every_answer_gets_a_point
    here's my code: select yr,count(*) from movie join casting on casting.movieid=movie.id join actor on casting.actorid = actor.id where actor.name = 'John Travolta' group by yr here's the question Which were the busiest years for 'John Travolta'. Show the number of movies he made for each year. here's the table structure movie(id, title, yr, score, votes, director) actor(id, name) casting(movieid, actorid, ord) this is the output i am getting: yr count(*) 1976 1 1977 1 1978 1 1981 1 1994 1 etcetc i need to get the rows for which count(*) is max how do i do this?

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  • sql - HAVING at least 10

    - by every_answer_gets_a_point
    i would like to display results for values that are only 10 and over select name, count(*) from actor join casting on casting.actorid = actor.id where casting.ord = 1 group by name order by 2 desc that will return this: name count(*) Sean Connery 19 Harrison Ford 19 Robert De Niro 18 Sylvester Stallone 18 etc but i want to return values of count(*) that are only above 10 how do i do this? with having?

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  • Google Chromecast cast tab from Chrome browser sucks

    - by Ken Hortsch
    Originally posted on: http://geekswithblogs.net/BlueProbe/archive/2013/07/28/153537.aspxOk so I know it’s in beta.  And I should have known when I watched the Nexus 7 and Chromecast press event and the demo showed the browser casting a slide show.  But, when I bought one of these little $35 beauties and tried casting a soccer game from ESPN it was pathetic with a 2 FPS rate.  Netflix and YouTube are awesome.  We’ll see what we get out of beta.

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  • What are the safety benefits of a type system?

    - by vandros526
    In Javascript: The Good Parts by Douglas Crockford, he mentions in his inheritance chapter, "The other benefit of classical inheritance is that it includes the specification of a system of types. This mostly frees the programmer from having to write explicit casting operations, which is a very good thing because when casting, the safety benefits of a type system are lost." So first of all, what actually is safety? protection against data corruption, or hackers, or system malfunctions, etc? What are the safety benefits of a type system? What makes a type system different that allows it to provide these safety benefits?

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  • Avoiding DNS timeouts when a dns server fails

    - by user65124
    Hi there. We have a small datacenter with about a hundred hosts pointing to 3 internal dns servers (bind 9). Our problem comes when one of the internal dns servers becomes unavailable. At that point all the clients that point to that server start performing very slowly. The problem seems to be that the stock linux resolver doesn't really have the concept of "failing over" to a different dns server. You can adjust the timeout and number of retries it uses, (and set rotate so it will work through the list), but no matter what settings one uses our services perform much more slowly if a primary dns server becomes unavailable. At the moment this is one of the largest sources of service disruptions for us. My ideal answer would be something like "RTFM: tweak /etc/resolv.conf like this...", but if that's an option I haven't seen it. I was wondering how other folks handled this issue? I can see 3 possible types of solutions: Use linux-ha/Pacemaker and failover ips (so the dns IP VIPs are "always" available). Alas, we don't have a good fencing infrastructure, and without fencing pacemaker doesn't work very well (in my experience Pacemaker lowers availability without fencing). Run a local dns server on each node, and have resolv.conf point to localhost. This would work, but it would give us a lot more services to monitor and manage. Run a local cache on each node. Folks seem to consider nscd "broken", but dnrd seems to have the right feature set: it marks dns servers as up or down, and won't use 'down' dns servers. Any-casting seems to work only at the ip routing level, and depends on route updates for server failure. Multi-casting seemed like it would be a perfect answer, but bind does not support broadcasting or multi-casting, and the docs I could find seem to suggest that multicast dns is more aimed at service discovery and auto-configuration rather than regular dns resolving. Am I missing an obvious solution?

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  • Avoiding DNS timeouts when a dns server fails

    - by Neil Katin
    We have a small datacenter with about a hundred hosts pointing to 3 internal dns servers (bind 9). Our problem comes when one of the internal dns servers becomes unavailable. At that point all the clients that point to that server start performing very slowly. The problem seems to be that the stock linux resolver doesn't really have the concept of "failing over" to a different dns server. You can adjust the timeout and number of retries it uses, (and set rotate so it will work through the list), but no matter what settings one uses our services perform much more slowly if a primary dns server becomes unavailable. At the moment this is one of the largest sources of service disruptions for us. My ideal answer would be something like "RTFM: tweak /etc/resolv.conf like this...", but if that's an option I haven't seen it. I was wondering how other folks handled this issue? I can see 3 possible types of solutions: Use linux-ha/Pacemaker and failover ips (so the dns IP VIPs are "always" available). Alas, we don't have a good fencing infrastructure, and without fencing pacemaker doesn't work very well (in my experience Pacemaker lowers availability without fencing). Run a local dns server on each node, and have resolv.conf point to localhost. This would work, but it would give us a lot more services to monitor and manage. Run a local cache on each node. Folks seem to consider nscd "broken", but dnrd seems to have the right feature set: it marks dns servers as up or down, and won't use 'down' dns servers. Any-casting seems to work only at the ip routing level, and depends on route updates for server failure. Multi-casting seemed like it would be a perfect answer, but bind does not support broadcasting or multi-casting, and the docs I could find seem to suggest that multicast dns is more aimed at service discovery and auto-configuration rather than regular dns resolving. Am I missing an obvious solution?

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  • Empty interface to combine multiple interfaces

    - by user1109519
    Suppose you have two interfaces: interface Readable { public void read(); } interface Writable { public void write(); } In some cases the implementing objects can only support one of these but in a lot of cases the implementations will support both interfaces. The people who use the interfaces will have to do something like: // can't write to it without explicit casting Readable myObject = new MyObject(); // can't read from it without explicit casting Writable myObject = new MyObject(); // tight coupling to actual implementation MyObject myObject = new MyObject(); None of these options is terribly convenient, even more so when considering that you want this as a method parameter. One solution would be to declare a wrapping interface: interface TheWholeShabam extends Readable, Writable {} But this has one specific problem: all implementations that support both Readable and Writable have to implement TheWholeShabam if they want to be compatible with people using the interface. Even though it offers nothing apart from the guaranteed presence of both interfaces. Is there a clean solution to this problem or should I go for the wrapper interface? UPDATE It is in fact often necessary to have an object that is both readable and writable so simply seperating the concerns in the arguments is not always a clean solution. UPDATE2 (extracted as answer so it's easier to comment on) UPDATE3 Please beware that the primary usecase for this is not streams (although they too must be supported). Streams make a very specific distinction between input and output and there is a clear separation of responsibilities. Rather, think of something like a bytebuffer where you need one object you can write to and read from, one object that has a very specific state attached to it. These objects exist because they are very useful for some things like asynchronous I/O, encodings,...

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  • ctypes and PySide

    - by Timothy Baldridge
    I'm building an app with PySide, there's some image manipulation that needs to be done and using Python code for this is way too slow. Therefore I hacked out a .dll file that will do it for me. The function definition is as follows: extern "C" { QRectF get_image_slant(QImage *img, float slantangle, float offset) { Now I can load this function in via ctypes. But I can't seem to get ctypes to accept a QImage. I tried calling it like this: ext.get_image_slant(QImage(), 0, 0) And the reply I get is: File "<stdin>", line 1, in <module> ctypes.ArgumentError: argument 1: <type 'exceptions.TypeError'>: Don't know how to convert parameter 1 I tired casting the QImage to a c_void_p and it doesn't like that either. From what I can tell QImage() in python should map exactly to a QImage * in C, but Python doesn't seem to understand that.. Is there any way to force the casting?

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  • Storing extended IIdentity in HttpContext.Current.User (IPrinciple)

    - by UpTheCreek
    I have created an ExtendedId class which extends GenericIdentity. (This stores Id as well as name) In a httpmodule I stored this extended id in Current.User like so: HttpContext.Current.User = new GenericPrincipal(myExtendedId, roles); Problem is, later, how do I get at my ExtendedId type again? If I try this: ExtendedId eId = (ExtendedId)HttpContext.Current.User.Identity; I get a casting error. I have a feeling I'm doing something stupid here with casting, but I'm a bit foggy this morning.

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  • 'Set = new HashSet' or 'HashSet = new Hashset'?

    - by Pureferret
    I'm intialising a HashSet like so in my program: Set<String> namesFilter = new HashSet<String>(); Is this functionally any different if I initilise like so? HashSet<String> namesFilter = new HashSet<String>(); I've read this about the collections interface, and I understand interfaces (well, except their use here). I've read this excerpt from Effective Java, and I've read this SO question, but I feel none the wiser. Is there a best practice in Java, and if so, why? My intuition is that it makes casting to a different type of Set easier in my first example. But then again, you'd only be casting to something that was a collection, and you could convert it by re-constructing it.

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