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• Does C++ require a destructor call for each placement new?

  - by Josh Haberman

  I understand that placement new calls are usually matched with explicit calls to the destructor. My question is: if I have no need for a destructor (no code to put there, and no member variables that have destructors) can I safely skip the explicit destructor call? Here is my use case: I want to write C++ bindings for a C API. In the C API many objects are accessible only by pointer. Instead of creating a wrapper object that contains a single pointer (which is wasteful and semantically confusing). I want to use placement new to construct an object at the address of the C object. The C++ object will do nothing in its constructor or destructor, and its methods will do nothing but delegate to the C methods. The C++ object will contain no virtual methods. I have two parts to this question. Is there any reason why this idea will not work in practice on any production compiler? Does this technically violate the C++ language spec?

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• How to avoid "incomplete implementation" warning in partial base class

  - by garph0

  I have created a protocol that my classes need to implement, and then factored out some common functionality into a base class, so I did this: @protocol MyProtocol - (void) foo; - (void) bar; @end @interface Base <MyProtocol> @end @interface Derived_1 : Base @end @interface Derived_2 : Base @end @implementation Base - (void) foo{ //something foo } @end @implementation Derived_1 - (void) bar{ //something bar 1 } @end @implementation Derived_2 - (void) bar{ //something bar 2 } @end In this way in my code I use a generic id<MyProtocol>. The code works (as long as Base is not used directly) but the compiler chokes at the end of the implementation of Base with a warning: Incomplete implementation of class Base Is there a way to avoid this warning or, even better, a more proper way to obtain this partially implemented abstract base class behavior in Objc?

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• What is the Effect of Declaring 'extern "C"' in the Header to a C++ Shared Library?

  - by Adam

  Based on this question I understand the purpose of the construct in linking C libraries with C++ code. Now suppose the following: I have a '.so' shared library compiled with a C++ compiler. The header has a 'typedef stuct' and a number of function declarations. If the header includes the extern "C" declaration... #ifdef __cplusplus extern "C" { #endif // typedef struct ...; // function decls #ifdef __cplusplus } #endif ... what is the effect? Specifically I'm wondering if there are any detrimental side effects of that declaration since the shared library is compiled as C++, not C. Is there any reason to have the extern "C" declaration in this case?

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• C Struct : typedef Doubt !

  - by Mahesh

  In the given code snippet, I expected the error symbol Record not found. But it compiled and ran fine on Visual Studio 2010 Compiler. I ran it as a C program from Visual Studio 2010 Command Prompt in the manner - cl Record.c Record Now the doubt is, doesn't typedef check for symbols ? Does it work more like a forward declaration ? #include "stdio.h" #include "conio.h" typedef struct Record R; struct Record { int a; }; int main() { R obj = {10}; getch(); return 0; }

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• Can I use a static var to "cache" the result? C++

  - by flyout

  I am using a function that returns a char*, and right now I am getting the compiler warning "returning address of local variable or temporary", so I guess I will have to use a static var for the return, my question is can I make something like if(var already set) return var else do function and return var? This is my function: char * GetUID() { TCHAR buf[20]; StringCchPrintf(buf, 20*sizeof(char), TEXT("%s"), someFunction()); return buf; } And this is what I want to do: char * GetUID() { static TCHAR buf[20]; if(strlen(buf)!=0) return buf; StringCchPrintf(buf, 20*sizeof(char), TEXT("%s"), someFunction()); return buf; } Is this a well use of static vars? And should I use ZeroMemory(&buf, 20*sizeof(char))? I removed it because if I use it above the if(strlen...) my TCHAR length is never 0, should I use it below?

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• Help Me: Loading Qt dialogs from python Scripts

  - by krishnanunni

  Hello, im a novice into developing an application using backend as Python (2.5) and Qt(3) as front end GUI designer. I have 5 diffrent dialogs to implement the scripts. i just know to load the window (main window) from qt import * from dialogselectkernelfile import * from formcopyextract import * import sys if __name__ == "__main__": app = QApplication(sys.argv) f = DialogSelectKernelFile() f.show() app.setMainWidget(f) app.exec_loop() main dialog opens on running. i have a set of back,Next,Cancel buttons pusing on each should open the next or previous dialogs. i use the pyuic compiler to source translation.how can i do this from python. please reply i`m running out of time.i dont know how to load another dialog from a signal of push button in another dialog. Help me pls Thanks a Lot

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• c++ when to put method out side the class

  - by user63898

  i saw that some times in c++ applications using only namespace declarations with header and source file like this : #ifndef _UT_ #define _UT_ #include <string> #include <windows.h> namespace UT { void setRootPath(char* program_path, char* file_path); char * ConvertStringToCharP(std::string str); }; #endif //and then in UT.cpp #include "UT.h" namespace UT { char * ConvertStringToCharP(std::string str) { char * writable = new char[str.size() + 1]; std::copy(str.begin(), str.end(), writable); writable[str.size()] = '\0'; return writable; } void setRootPath(char* program_path, char* file_path) { //... } } is it better then defining classic class with static methods? or just simple class ? dose this method has something better for the compiler linker ? the methods in this namespace are called allot of times .

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• C/C++: Scanning a TIFF file using LIBTIFF

  - by Matt07

  My problem is to scan a tiff image in C and get all the pixel value (let's say for saving them in a txt file) in C/C++. I scanned the web and i found a library named "TIFFLIB" that should do what i was looking for. I downloaded it using the ubuntu package manager, but the gcc doesn't recognize the library. How do i link the library to the compiler? Have I installed it correctly? Is there any better/easier way to do that?

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• C++0x implementation guesstimates?

  - by dsimcha

  The C++0x standard is on its way to being complete. Until now, I've dabbled in C++, but avoided learning it thoroughly because it seems like it's missing a lot of modern features that I've been spoiled by in other languages. However, I'd be very interested in C++0x, which addresses a lot of my complaints. Any guesstimates, after the standard is ratified, as to how long it will take for major compiler vendors to provide reasonably complete, production-quality implementations? Will it happen soon enough to reverse the decline in C++'s popularity, or is it too little, too late? Do you believe that C++0x will become "the C++" within a few years, or do you believe that most people will stick to the earlier standard in practice and C++0x will be somewhat of a bastard stepchild, kind of like C99?

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• C++ assignment - stylish or performance?

  - by joejax

  Having been writing Java code for many years, I was amazed when I see this C++ statement: int a,b; int c = (a=1, b=a+2, b*3); My question is: Is this a choice of coding style, or it has real benefit? (looking for a practicle use case) I think the compiler will see it the same as following: int a=1, b=a+2; int c = b*3; (What's the offical name for this? I assume it's a standard C/C++ syntax.)

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• Why can I derived from a templated/generic class based on that type in C# / C++

  - by stusmith

  Title probably doesn't make a lot of sense, so I'll start with some code: class Foo : public std::vector<Foo> { }; ... Foo f; f.push_back( Foo() ); Why is this allowed by the compiler? My brain is melting at this stage, so can anyone explain whether there are any reasons you would want to do this? Unfortunately I've just seen a similar pattern in some production C# code and wondered why anyone would use this pattern.

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• Prevent Erroneous Property Assignment

  - by Gordon

  Porting android applications to iphone applications always gives me the following pattern that I accidentally create: - (void) myFunc:(id)prop { self.property = property; } Which instead should be: - (void) myFunc:(id)prop { self.property = prop; } This always causes my program to quietly break because property gets reset to its existing value rather than being set to the new value, 'prop'. I cannot name the parameter 'prop' to 'property' since the compile complains that the parameter masks the instance variables visibility. Is there a good way to avoid this situation? There are no compiler warnings. Is there a way to make xcode prevent this? I cannot see very many situations where you would set a property to the value of its underlying instance variable (maybe to trigger a KVO binding?), but I don't see myself doing that in majority of cases. I understand the above code is synthetic and should be done with @synthesize, but I am just using it as a simplified example to illustrate my point.

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• doubt in Exceptions

  - by Ajay Singh

  class MyException extends Exception { MyException() {} MyException(String msg) { super(msg);} } public class NewException { static void f() throws MyException { System.out.println("throwing exception from f()"); throw new ClassCastException(); } static void g() throws MyException { System.out.println("throwing exception from g()"); throw new MyException("parametrized "); } public static void main(String ...strings ) { try { f(); } catch(MyException e) { e.printStackTrace(System.out); } try { g(); } catch(MyException e) { e.printStackTrace(System.out); } } } In the function f() iam specifying that "MyException " exception will be thrown and actually iam throwing some other exception which has no relation with MyException but still the compiler does not report any complain.Why is it so??

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• How to generate a specific CPPDEFINE such as -DOEM="FOO BAR" using Scons

  - by Vineet

  My intention is to end up with a compiler command line including -DOEM="FOO BAR" I have the following in my SConstruct file: opts = Options( 'overrides.py', ARGUMENTS ) opts.Add( 'OEM_NAME', 'Any string can be used here', 'UNDEFINED' ) . . . if (env.Dictionary('OEM_NAME') != 'UNDEFINED'): OEM_DEFINE = 'OEM=' + str(env.Dictionary('OEM_NAME')) env.Append( CPPDEFINES=[ OEM_DEFINE ] ) Then I put the following in the "overrides.py" file: OEM_NAME = "FOO BAR" I seem to end up with "-DOEM=FOO BAR" in the command line that gets generated. Can someone point me in the right direction? Thanks.

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• Design pattern: Polymorphism for list of objects

  - by ziang

  Suppose I have a class A, and A1, A2 inherits from A. There are 2 functions: List<A1> getListA1(){...} List<A2> getListA2(){...} Now I want to do something similar to both A1 and A2 in another function public void process(List<A>){...} If I want to pass the instance of either ListA1 or ListA2, of course the types doesn't match because the compiler doesn't allow the coercion from List< A1 to List< A. I can't do something like this: List<A1> listA1 = getListA1(); List<A> newList = (List<A>)listA1; //this is not allowed. So what is the best approach to the process()? Is there any way to do it in a universal way rather than write the similar code to both List and List?

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• Strange exception phenomenon in Windows 7

  - by Level 2

  I spot some interesting articles about exception handle in CodeProject http://www.codeproject.com/KB/cpp/seexception.aspx After reading, I decided to do some experiment. The first time I try to execute the following code char *p; p[0] = 0; The program died without question. But After several times when I executed the same problem binary code, it magically did fine. Even the following code is doing well. Any clue or explanation? char *p p[1000] = 'd'; cout<<p[1000]<<endl; My O/S is Windows 7 64bit and compiler is VS2008 rc1.

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• Eclipse RCP: How to troubleshoot plugin dependencies & classpath problems?

  - by paul

  I am working on an RCP project based on eclipse. It has been working fine but recently I thought I'd upgrade it to use a new eclipse version (3.2 - 3.5). After a bit of trouble, it seemed to be working fine. Then I did something. Don't know what. The end result is that I'm getting a classpath error when one of my plugins (A) tries to access a class in one of the dependent plugins (B) (also one of mine). As far as I can see, Plugin A has Plugin B in its' dependency list and the compiler shows no errors. To test, I created a new Plugin C with one class and accessed the class from Plugin A. That works fine. Does anyone have any hints for troubleshooting such issues? A checklist of settings to check? I've been stuggling with this for hours and getting nowhere! Particularly frustrating as it was working until I changed something! Thanks

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• C++ template overloading - wrong function called

  - by DeadMG

  template<typename T> T* Push(T* ptr); template<typename T> T* Push(T& ref); template<typename T, typename T1> T* Push(T1&& ref); I have int i = 0; Push<int>(i); But the compiler calls it ambiguous. How is that ambiguous? The second function is clearly the preferred match since it's more specialized. Especially since the T1&& won't bind to an lvalue unless I explicitly forward/move it. Sorry - i is an int. Otherwise, the question would make no sense, and I thought people would infer it since it's normally the loop iterator.

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• Precision problems of real numbers in Fortran

  - by saladoil

  I've been trying to use Fortran for my research project, with the GNU Fortran compiler (gfortran), latest version, but I've been encountering some problems in the way it processes real numbers. If you have for example the code: program test implicit none real :: y = 23.234, z z = y * 100000 write(*,*) y, z end program You'll get as output: 23.23999 2323400.0. I find this really strange. Can someone tell me what's exactly happening here? Looking at z I can see that y does retain its precision, so for calculations that shouldn't be a problem I suppose. But why is the output of y not exactly the same as the value that I've specified, and what can I do to make it exactly the same?

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• Why "constructor-way" of declaring variable in "for-loop" allowed but in "if-statement" not allowed?

  - by PiotrNycz

  Consider this simple example: /*1*/ int main() { /*2*/ for (int i(7); i;){break;} /*3*/ if (int i(7)) {} /*4*/ } Why line-2 compiles just fine, whilst line-3 gives the error? This is little strange to me why if-statement is in this aspect treated worse than for-loop? If this is compiler specific - I tested with gcc-4.5.1: prog.cpp: In function 'int main()': prog.cpp:3:7: error: expected primary-expression before 'int' prog.cpp:3:7: error: expected ')' before 'int' I was inspired by this question [UPDATE] I know this compiles just fine: /*1*/ int main() { /*2*/ for (int i = 7; i;){break;} /*3*/ if (int i = 7) {} /*4*/ }

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• floats in NSArray

  - by JordanC

  I have an NSArray of floats which I did by encapsulating the floats using [NSNumber numberWithFloat:myFloat] ; Then I passed that array somewhere else and I need to pull those floats out of the array and perform basic arithmatic. When I try [myArray objectAtIndex:i] ; The compiler complains that I'm trying to perform arithmatic on a type id. It also won't let me cast to float or double. Any ideas? This seems like it should be an easy problem. Maybe it will come to me after another cup of coffee, but some help would be appreciated. Thanks.

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• ASP.NET MVC4: How to convert an IEnumerable to a string for ViewBag

  - by sehummel

  This is what I'm trying to do, but it doesn't work: HardwareType hwt = new HardwareType { HType = "PC" }; IEnumerable<Hardware> Pcs = db.Hardware.Where(h => h.HardwareType.Contains(hwt)); ViewBag.Pcs = Pcs.ToString(); So how do I convert my IEnumerable to a string (or other primitive data type) so the compiler won't give me an error when I try to use it in my Razor? @foreach (var item in ViewBag.Pcs) { <li><a href="#" class="btn"><i class="icon-hdd"></i> @item.HType</a></li> }

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• How to catch any exception (System.Exception) without a warning in F#?

  - by LLS

  I tried to catch an Exception but the compiler gives warning: This type test or downcast will always hold let testFail () = try printfn "Ready for failing..." failwith "Fails" with | :? System.ArgumentException -> () | :? System.Exception -> () The question is: how to I do it without the warning? (I believe there must be a way to do this, otherwise there should be no warning) Like C# try { Console.WriteLine("Ready for failing..."); throw new Exception("Fails"); } catch (Exception) { }

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• Clojure warn-on-reflection and type hints

  - by Ralph

  In the following code, I am getting a warning on reflection: (ns com.example (:import [org.apache.commons.cli CommandLine Option Options PosixParser])) (def *help-option* "help") (def *host-option* "db-host") (def *options* (doto (Options.) (.addOption "?" *help-option* false "Show this usage information") (.addOption "h" *host-option* true "Name of the database host"))) (let [^CommandLine command-line (.. (PosixParser.) (parse *options* (into-array String args))) db-host (.getOptionValue command-line "h")] ; WARNING HERE ON .getOptionValue ; Do stuff with db-host ) I have a type hint on command-line. Why the warning? I am using Clojure 1.2 on OS X 10.6.6 (Apple VM). I assume that I do not get a warning on (.addOption ...) because the compiler knows that (Options.) is a org.apache.commons.cli.Options).

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• Static initialization of a struct with class members

  - by JS Bangs

  I have a struct that's defined with a large number of vanilla char* pointers, but also an object member. When I try to statically initialize such a struct, I get a compiler error. typedef struct { const char* pszA; // ... snip ... const char* pszZ; SomeObject obj; } example_struct; // I only want to assign the first few members, the rest should be default example_struct ex = { "a", "b" }; SomeObject has a public default constructor with no arguments, so I didn't think this would be a problem. But when I try to compile this (using VS), I get the following error: error C2248: 'SomeObject::SomeObject' : cannot access private member declared in class 'SomeObject' Any idea why?

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