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• Multithreaded linked list traversal

  - by Rob Bryce

  Given a (doubly) linked list of objects (C++), I have an operation that I would like multithread, to perform on each object. The cost of the operation is not uniform for each object. The linked list is the preferred storage for this set of objects for a variety of reasons. The 1st element in each object is the pointer to the next object; the 2nd element is the previous object in the list. I have solved the problem by building an array of nodes, and applying OpenMP. This gave decent performance. I then switched to my own threading routines (based off Windows primitives) and by using InterlockedIncrement() (acting on the index into the array), I can achieve higher overall CPU utilization and faster through-put. Essentially, the threads work by "leap-frog'ing" along the elements. My next approach to optimization is to try to eliminate creating/reusing the array of elements in my linked list. However, I'd like to continue with this "leap-frog" approach and somehow use some nonexistent routine that could be called "InterlockedCompareDereference" - to atomically compare against NULL (end of list) and conditionally dereference & store, returning the dereferenced value. I don't think InterlockedCompareExchangePointer() will work since I cannot atomically dereference the pointer and call this Interlocked() method. I've done some reading and others are suggesting critical sections or spin-locks. Critical sections seem heavy-weight here. I'm tempted to try spin-locks but I thought I'd first pose the question here and ask what other people are doing. I'm not convinced that the InterlockedCompareExchangePointer() method itself could be used like a spin-lock. Then one also has to consider acquire/release/fence semantics... Ideas? Thanks!

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• Is the C++ compiler optimizer allowed to break my destructor ability to be called multiple times?

  - by sharptooth

  We once had an interview with a very experienced C++ developer who couldn't answer the following question: is it necessary to call the base class destructor from the derived class destructor in C++? Obviously the answer is no, C++ will call the base class destructor automagically anyway. But what if we attempt to do the call? As I see it the result will depend on whether the base class destructor can be called twice without invoking erroneous behavior. For example in this case: class BaseSafe { public: ~BaseSafe() { } private: int data; }; class DerivedSafe { public: ~DerivedSafe() { BaseSafe::~BaseSafe(); } }; everything will be fine - the BaseSafe destructor can be called twice safely and the program will run allright. But in this case: class BaseUnsafe { public: BaseUnsafe() { buffer = new char[100]; } ~BaseUnsafe () { delete[] buffer; } private: char* buffer; }; class DerivedUnsafe { public: ~DerivedUnsafe () { BaseUnsafe::~BaseUnsafe(); } }; the explicic call will run fine, but then the implicit (automagic) call to the destructor will trigger double-delete and undefined behavior. Looks like it is easy to avoid the UB in the second case. Just set buffer to null pointer after delete[]. But will this help? I mean the destructor is expected to only be run once on a fully constructed object, so the optimizer could decide that setting buffer to null pointer makes no sense and eliminate that code exposing the program to double-delete. Is the compiler allowed to do that?

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• useer degined Copy ctor, and copy-ctors further down the chain - compiler bug ? programers brainbug

  - by J.Colmsee

  Hi. i have a little problem, and I am not sure if it's a compiler bug, or stupidity on my side. I have this struct : struct BulletFXData { int time_next_fx_counter; int next_fx_steps; Particle particles[2];//this is the interesting one ParticleManager::ParticleId particle_id[2]; }; The member "Particle particles[2]" has a self-made kind of smart-ptr in it (resource-counted texture-class). this smart-pointer has a default constructor, that initializes to the ptr to 0 (but that is not important) I also have another struct, containing the BulletFXData struct : struct BulletFX { BulletFXData data; BulletFXRenderFunPtr render_fun_ptr; BulletFXUpdateFunPtr update_fun_ptr; BulletFXExplosionFunPtr explode_fun_ptr; BulletFXLifetimeOverFunPtr lifetime_over_fun_ptr; BulletFX( BulletFXData data, BulletFXRenderFunPtr render_fun_ptr, BulletFXUpdateFunPtr update_fun_ptr, BulletFXExplosionFunPtr explode_fun_ptr, BulletFXLifetimeOverFunPtr lifetime_over_fun_ptr) :data(data), render_fun_ptr(render_fun_ptr), update_fun_ptr(update_fun_ptr), explode_fun_ptr(explode_fun_ptr), lifetime_over_fun_ptr(lifetime_over_fun_ptr) { } /* //USER DEFINED copy-ctor. if it's defined things go crazy BulletFX(const BulletFX& rhs) :data(data),//this line of code seems to do a plain memory-copy without calling the right ctors render_fun_ptr(render_fun_ptr), update_fun_ptr(update_fun_ptr), explode_fun_ptr(explode_fun_ptr), lifetime_over_fun_ptr(lifetime_over_fun_ptr) { } */ }; If i use the user-defined copy-ctor my smart-pointer class goes crazy, and it seems that calling the CopyCtor / assignment operator aren't called as they should. So - does this all make sense ? it seems as if my own copy-ctor of struct BulletFX should do exactly what the compiler-generated would, but it seems to forget to call the right constructors down the chain. compiler bug ? me being stupid ? Sorry about the big code, some small example could have illustrated too. but often you guys ask for the real code, so well - here it is :D

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• Strange pattern matching with functions instancing Show

  - by Sean D

  So I'm writing a program which returns a procedure for some given arithmetic problem, so I wanted to instance a couple of functions to Show so that I can print the same expression I evaluate when I test. The trouble is that the given code matches (-) to the first line when it should fall to the second. {-# OPTIONS_GHC -XFlexibleInstances #-} instance Show (t -> t-> t) where show (+) = "plus" show (-) = "minus" main = print [(+),(-)] returns [plus,plus] Am I just committing a motal sin printing functions in the first place or is there some way I can get it to match properly? edit:I realise I am getting the following warning: Warning: Pattern match(es) are overlapped In the definition of `show': show - = ... I still don't know why it overlaps, or how to stop it.

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• Is there a way to find out whether a class is a direct base of another class?

  - by user176168

  Hi I'm wondering whether there is a way to find out whether a class is a direct base of another class i.e. in boost type trait terms a is_direct_base_of function. As far as I can see boost doesn't see to support this kind of functionality which leads me to think that its impossible with the current C++ standard. The reason I want it is to do some validation checking on two macro's that are used for a reflection system to specify that one class is derived from another e.g. header.h: #define BASE A #define DERIVED B class A {}; class B : public A { #include <rtti.h> }; rtti.h: // I want to check that the two macro's are correct with a compile time assert Rtti<BASE, DERIVED> m_rtti; Although the macro's seem unnecessary in this simple example in my real world scenario rtti.h is a lot more complex. One possible avenue would be to compare the size of the this pointer with the size of a this pointer cast to the base type and some how trying to figure out whether its the size of the base class itself away or something (yeah your right I don't know how that would work either! lol)

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• How to convert a list object to bigdecimal in prepared statement?

  - by user1103504

  I am using prepared statement for bulk insertion of records. Iam iterating a list which contains values and their dataTypes differ. One of the data type is BigDecimal and when i try to set calling preparedstatement, it is throwing null pointer exception. My code int count = 1; for (int j = 0; j < list.size(); j++) { if(list.get(j) instanceof Timestamp) { ps.setTimestamp(count, (Timestamp) list.get(j)); } else if(list.get(j) instanceof java.lang.Character) { ps.setString(count, String.valueOf(list.get(j))); } else if(list.get(j) instanceof java.math.BigDecimal) { ps.setBigDecimal(count, (java.math.BigDecimal)list.get(j)); } else { ps.setObject(count, list.get(j)); } count++; } I tried 2 ways to convert, casting the object and tried to create a new object of type BigDecimal ps.setBigDecimal(count, new BigDecimal(list.get(j).toString)); both donot solve my problem. It is throwing null pointer exception. help is appreciated. Thanks

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• grouping objects to achieve a similar mean property for all groups

  - by cytochrome

  I have a collection of objects, each of which has a numerical 'weight'. I would like to create groups of these objects such that each group has approximately the same arithmetic mean of object weights. The groups won't necessarily have the same number of members, but the size of groups will be within one of each other. In terms of numbers, there will be between 50 and 100 objects and the maximum group size will be about 5. Is this a well-known type of problem? It seems a bit like a knapsack or partition problem. Are efficient algorithms known to solve it? As a first step, I created a python script that achieves very crude equivalence of mean weights by sorting the objects by weight, subgrouping these objects, and then distributing a member of each subgroup to one of the final groups. I am comfortable programming in python, so if existing packages or modules exist to achieve part of this functionality, I'd appreciate hearing about them. Thank you for your help and suggestions.

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• Why does VS2005 skip execution of lines when debugging managed C++ without optimizations?

  - by Sakin

  I ran into a rather odd behavior that I don't even know how to start describing. I wrote a piece of managed C++ code that makes calls to native methods. A (very) simplified version of the code would look like this (I know it looks like a full native function, just assume there is managed stuff being done all over the place): int somefunction(ptrHolder x) { // the accessptr method returns a native pointer if (x.accessptr() != nullptr) // I tried this with nullptr, NULL, 0) { try { x->doSomeNativeVeryImportantStuff(); // or whatever, doesn't matter } catch (SomeCustomExceptionClass &) { return 0; } } SomeOtherNativeClass::doStaticMagic(); return 1; } I compiled this code without optimizations using the /clr flag (VS.NET 2005, SP2) and when running it in the debugger I get to the if statement, since the pointer is actually null, I don't enter the if, but surprisingly, the cursor jumps directly to the return 1 statement, ignoring the doStaticMagic() method completely!!! When looking at the assembly code, I see that it really jumps directly to that line. If I force the debugger to enter the if block, I also jump to the return 1 statement after I press F10. Any ideas why this is happening? Thanks, Ariel

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• Using an embedded DB (SQLite / SQL Compact) for Message Passing within an app?

  - by wk1989

  Hello, Just out of curiosity, for applications that have a fairly complicated module tree, would something like sqlite/sql compact edition work well for message passing? So if I have modules containing data such as: \SubsystemA\SubSubSysB\ModuleB\ModuleDataC, \SubSystemB\SubSubSystemC\ModuleA\ModuleDataX Using traditional message passing/routing, you have to go through intermediate modules in order to pass a message to ModuleB to request say ModuleDataC. Instead of doing that, if we we simply store "\SubsystemA\SubSubSysB\ModuleB\ModuleDataC" in a sqlite database, getting that data is as simple as a sql query and needs no routing and passing stuff around. Has anyone done this before? Even if you haven't, do you foresee any issues & performance impact? The only concern I have right now would be the passing of custom types, e.g. if ModuleDataC is a custom data structure or a pointer, I'll need some way of storing the data structure into the DB or storing the pointer into the DB. Thanks, JW EDIT One usage case I haven't thought about is when you want to send a message from ModuleA to ModuleB to get ModuleB to do something rather than just getting/setting data. Is it possible to do this using an embedded DB? I believe callback from the DB would be needed, how feasible is this?

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• What's the silliest programming mistake you've ever done? [closed]

  - by bgo

  Despite not being a professional programmer, i use python or c to accomplish simple tasks. Once I needed a nasty equation to use with various data for physics lab. I thought that it would take a few hours if i used some mechanic calculator. Then i've written a python script to make things easier but something was wrong. After finding my mistakes one of which was forgetting to put ":", i realized that it had already took 2-3 hours, not to mention inventing satanic arithmetic techniques since i thought the errors was caused by my math :) Sometimes when you don't see it, you don't see it! Now i can find the syntax errors without thinking for a moment :) Any similar annoying mistakes that you've experienced?

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• HashMap Memory Leak because of Dynamic Array

  - by Jake M

  I am attempting to create my own HashMap to understand how they work. I am using an array of Linked Lists to store the values(strings) in my hashmap. I am creating the array like this: Node** list; Instead of this: Node* list[nSize]; This is so the array can be any size at runtime. But I think I am getting a memory leak because of how I am doing this. I dont know where the error is but when I run the following simple code the .exe crashes. Why is my application crashing and how can I fix it? Note: I am aware that using a vector would be much better than an array but this is just for learning and I want to challenge myself to create the hashmap using a 'Dynamic' Array. PS: is that the correct term(Dynamic Array) for the kind of array I am using? struct Node { // to implement }; class HashMap { public: HashMap(int dynSize) { *list = new Node[dynSize]; size = dynSize; for (int i=0; i<size; i++) list[i] = NULL; cout << "END\n"; } ~HashMap() { for (int i=0; i<size; i++) delete list[i]; } private: Node** list; // I could use a vector here but I am experimenting with a pointer to an array(pointer), also its more elegant int size; }; int main() { // When I run this application it crashes. Where is my memory leak? HashMap h(5); system("PAUSE"); return 0; }

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• Dependency injection in C++

  - by Yorgos Pagles

  This is also a question that I asked in a comment in one of Miško Hevery's google talks that was dealing with dependency injection but it got buried in the comments. I wonder how can the factory / builder step of wiring the dependencies together can work in C++. I.e. we have a class A that depends on B. The builder will allocate B in the heap, pass a pointer to B in A's constructor while also allocating in the heap and return a pointer to A. Who cleans up afterwards? Is it good to let the builder clean up after it's done? It seems to be the correct method since in the talk it says that the builder should setup objects that are expected to have the same lifetime or at least the dependencies have longer lifetime (I also have a question on that). What I mean in code: class builder { public: builder() : m_ClassA(NULL),m_ClassB(NULL) { } ~builder() { if (m_ClassB) { delete m_ClassB; } if (m_ClassA) { delete m_ClassA; } } ClassA *build() { m_ClassB = new class B; m_ClassA = new class A(m_ClassB); return m_ClassA; } }; Now if there is a dependency that is expected to last longer than the lifetime of the object we are injecting it into (say ClassC is that dependency) I understand that we should change the build method to something like: ClassA *builder::build(ClassC *classC) { m_ClassB = new class B; m_ClassA = new class A(m_ClassB, classC); return m_ClassA; } What is your preferred approach?

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• Read in double type from txt file - C++

  - by Greenhouse Gases

  Hi there I'm in the midst of a university project and have decided to implement a method that can accept information from a text file (in this instance called "locations.txt"). input from the text file will look like this: London 345 456 Madrid 234 345 Beinjing 345 456 Frankfurt 456 567 The function looks like this currently (and you will notice I am missing the While condition to finish adding input when reaches end of text in locations.txt, i tried using eof but this didnt work?!). Also get function expects a char and so cant accept input thats a double which is what the latitude and longitude are defined as... void populateList(){ ifstream inputFile; inputFile.open ("locations.txt"); temp = new locationNode; // declare the space for a pointer item and assign a temporary pointer to it while(HASNT REACHED END OF TEXT FILE!!) { inputFile.getline(temp-nodeCityName, MAX_LENGTH); // inputFile.get(temp-nodeLati, MAX_LENGTH); // inputFile.get(temp-nodeLongi, MAX_LENGTH); temp-Next = NULL; //set to NULL as when one is added it is currently the last in the list and so can not point to the next if(start_ptr == NULL){ // if list is currently empty, start_ptr will point to this node start_ptr = temp; } else { temp2 = start_ptr; // We know this is not NULL - list not empty! while (temp2-Next != NULL) { temp2 = temp2-Next; // Move to next link in chain until reach end of list } temp2->Next = temp; } } inputFile.close(); } Any help you can provide would be most useful. If I need to provide anymore detail I will do, I'm in a bustling canteen atm and concentrating is hard!!

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• Templates, Function Pointers and C++0x

  - by user328543

  One of my personal experiments to understand some of the C++0x features: I'm trying to pass a function pointer to a template function to execute. Eventually the execution is supposed to happen in a different thread. But with all the different types of functions, I can't get the templates to work. #include `<functional`> int foo(void) {return 2;} class bar { public: int operator() (void) {return 4;}; int something(int a) {return a;}; }; template <class C> int func(C&& c) { //typedef typename std::result_of< C() >::type result_type; typedef typename std::conditional< std::is_pointer< C >::value, std::result_of< C() >::type, std::conditional< std::is_object< C >::value, std::result_of< typename C::operator() >::type, void> >::type result_type; result_type result = c(); return result; } int main(int argc, char* argv[]) { // call with a function pointer func(foo); // call with a member function bar b; func(b); // call with a bind expression func(std::bind(&bar::something, b, 42)); // call with a lambda expression func( [](void)->int {return 12;} ); return 0; } The result_of template alone doesn't seem to be able to find the operator() in class bar and the clunky conditional I created doesn't compile. Any ideas? Will I have additional problems with const functions?

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• C++, using one byte to store two variables

  - by 2di

  Hi All I am working on representation of the chess board, and I am planning to store it in 32 bytes array, where each byte will be used to store two pieces. (That way only 4 bits are needed per piece) Doing it in that way, results in a overhead for accessing particular index of the board. Do you think that, this code can be optimised or completely different method of accessing indexes can be used? c++ char getPosition(unsigned char* c, int index){ //moving pointer c+=(index>>1); //odd number if (index & 1){ //taking right part return *c & 0xF; }else { //taking left part return *c>>4; } } void setValue(unsigned char* board, char value, int index){ //moving pointer board+=(index>>1); //odd number if (index & 1){ //replace right part //save left value only 4 bits *board = (*board & 0xF0) + value; }else { //replacing left part *board = (*board & 0xF) + (value<<4); } } int main() { char* c = (char*)malloc(32); for (int i = 0; i < 64 ; i++){ setValue((unsigned char*)c, i % 8,i); } for (int i = 0; i < 64 ; i++){ cout<<(int)getPosition((unsigned char*)c, i)<<" "; if (((i+1) % 8 == 0) && (i > 0)){ cout<<endl; } } return 0; } I am equally interested in your opinions regarding chess representations, and optimisation of the method above, as a stand alone problem. Thanks a lot

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• C++ private inheritance and static members/types

  - by WearyMonkey

  I am trying to stop a class from being able to convert its 'this' pointer into a pointer of one of its interfaces. I do this by using private inheritance via a middle proxy class. The problem is that I find private inheritance makes all public static members and types of the base class inaccessible to all classes under the inheriting class in the hierarchy. class Base { public: enum Enum { value }; }; class Middle : private Base { }; class Child : public Middle { public: void Method() { Base::Enum e = Base::value; // doesn't compile BAD! Base* base = this; // doesn't compile GOOD! } }; I've tried this in both VS2008 (the required version) and VS2010, neither work. Can anyone think of a workaround? Or a different approach to stopping the conversion? Also I am curios of the behavior, is it just a side effect of the compiler implementation, or is it by design? If by design, then why? I always thought of private inheritance to mean that nobody knows Middle inherits from Base. However, the exhibited behavior implies private inheritance means a lot more than that, in-fact Child has less access to Base than any namespace not in the class hierarchy!

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• Is it possible to compute the minimum of three numbers by using two comparisons at the same time?

  - by Milo Hou

  I've been trying to think up of some way that I could do two comparisons at the same time to find the greatest/least of three numbers. Arithmetic operations on them are considered "free" in this case. That is to say, the classical way of finding the greater of two, and then comparing it to the third number isn't valid in this case because one comparison depends on the result of the other. Is it possible to use two comparisons where this isn't the case? I was thinking maybe comparing the differences of the numbers somehow or their products or something, but came up with nothing. Just to reemphasize, two comparisons are still done, just that neither comparison relies on the result of the other comparison. EDIT: what about: boolA = A^2 + B^2 < C^2 boolB = A > B if boolA then max=C else if boolB then max=A else max=B

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• sIFR multiple calls don't work :(

  - by roxane

  Hello ! I'm trying to use sIFR on my menu and on my headers... but only one call works... the others that I put after never appear :(... Can somebody help me ? Note : I'm using the r436 and below is my code ... var segoe = { src: 'pathto/segoe.swf', ratios: [7, 1.58, 8, 1.49, 10, 1.5, 11, 1.45, 16, 1.46, 21, 1.44, 22, 1.41, 27, 1.42, 30, 1.41, 32, 1.4, 35, 1.41, 36, 1.4, 38, 1.39, 41, 1.4, 58, 1.39, 65, 1.38, 66, 1.39, 102, 1.38, 104, 1.37, 106, 1.38, 107, 1.37, 108, 1.38, 109, 1.37, 110, 1.38, 112, 1.37, 114, 1.38, 120, 1.37, 121, 1.38, 1.37] }; sIFR.activate(segoe); sIFR.replace(segoe, { selector: '#header ul li a span.title', css: ['.sIFR-root { text-align: center; font-size:13px; letter-spacing:2; font-weight: bold; text-transform:uppercase; cursor: pointer; color:#FFFFFF; background-color:#0C2C39; margin:0 3px; }'], wmode: 'transparent' }); sIFR.replace(segoe, { selector: 'h1', css: ['.sIFR-root { text-align: center; font-size:13px; letter-spacing:2; font-weight: bold; text-transform:uppercase; cursor: pointer; color:#000; background-color:#fff; margin:0 3px; }'] });

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• Stretch ListBox Items hit area to full width if the ListBox?

  - by Nicholas

  I've looked around for an answer on this, but the potential duplicates are more concerned with presentation than interaction. I have a basic list box, and each item's content is a simple string. The ListBox itself is stretched to fill it's grid container, but each ListBoxItem's hitarea does not mirror the ListBox width. It looks as if the hitarea (pointer contact area) for each item is only the width of the text content. How do I make this stretch all the way across, regardless of the text size. I've set HorizontalContentAlignment to Stretch, but this doesn't solve my problem. My only other guess is that the content is actually stretching, but the background is invisible and so not capturing the mouse pointer. <ListBox Grid.Row="1" x:Name="ProjectsListBox" DisplayMemberPath="Name" ItemsSource="{Binding Path=Projects}" SelectedItem="{Binding Path=SelectedProject}" HorizontalContentAlignment="Stretch"/> The XAML is pretty straight forward on this. If I mouse over the text in one of the items, then the entire width of the item becomes active. I guess I just need to know how to create an interactive background that is invisible.

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• casting char[][] to char** causes segfault?

  - by Earlz

  Ok my C is a bit rusty but I figured I'd make my next(small) project in C so I could polish back up on it and less than 20 lines in I already have a seg fault. This is my complete code: #define ROWS 4 #define COLS 4 char main_map[ROWS][COLS+1]={ "a.bb", "a.c.", "adc.", ".dc."}; void print_map(char** map){ int i; for(i=0;i<ROWS;i++){ puts(map[i]); //segfault here } } int main(){ print_map(main_map); //if I comment out this line it will work. puts(main_map[3]); return 0; } I am completely confused as to how this is causing a segfault. What is happening when casting from [][] to **!? That is the only warning I get. rushhour.c:23:3: warning: passing argument 1 of ‘print_map’ from incompatible pointer type rushhour.c:13:7: note: expected ‘char **’ but argument is of type ‘char (*)[5]’ Are [][] and ** really not compatible pointer types? They seem like they are just syntax to me.

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• Passing C++ object to C++ code through Python?

  - by cornail

  Hi all, I have written some physics simulation code in C++ and parsing the input text files is a bottleneck of it. As one of the input parameters, the user has to specify a math function which will be evaluated many times at run-time. The C++ code has some pre-defined function classes for this (they are actually quite complex on the math side) and some limited parsing capability but I am not satisfied with this construction at all. What I need is that both the algorithm and the function evaluation remain speedy, so it is advantageous to keep them both as compiled code (and preferrably, the math functions as C++ function objects). However I thought of glueing the whole simulation together with Python: the user could specify the input parameters in a Python script, while also implementing storage, visualization of the results (matplotlib) and GUI, too, in Python. I know that most of the time, exposing C++ classes can be done, e.g. with SWIG but I still have a question concerning the parsing of the user defined math function in Python: Is it possible to somehow to construct a C++ function object in Python and pass it to the C++ algorithm? E.g. when I call f = WrappedCPPGaussianFunctionClass(sigma=0.5) WrappedCPPAlgorithm(f) in Python, it would return a pointer to a C++ object which would then be passed to a C++ routine requiring such a pointer, or something similar... (don't ask me about memory management in this case, though :S) The point is that no callback should be made to Python code in the algorithm. Later I would like to extend this example to also do some simple expression parsing on the Python side, such as sum or product of functions, and return some compound, parse-tree like C++ object but let's stay at the basics for now. Sorry for the long post and thx for the suggestions in advance.

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• Which tool can list writing access to a specific variable in C?

  - by Lichtblitz

  Unfortunately I'm not even sure how this sort of static analysis is called. It's not really control flow analysis because I'm not looking for function calls and I don't really need data flow analysis because I don't care about the actual values. I just need a tool that lists the locations (file, function) where writing access to a specific variable takes place. I don't even care if that list contained lines that are unreachable. I could imagine that writing a simple parser could suffice for this task but I'm certain that there must be a tool out there that does this simple analysis. As a poor student I would appreciate free or better yet open source tools and if someone could tell me how this type of static analysis is actually called, I would be equally grateful! EDIT: I forgot to mention there's no pointer arithmetic in the code base.

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• Why do I get "request for member in something not a struct or union" from this code?

  - by pyroxene

  I'm trying to teach myself C by coding up a linked list. I'm new to pointers and memory management and I'm getting a bit confused. I have this code: /* Remove a node from the list and rejiggle the pointers */ void rm_node(struct node **listP, int index) { struct node *prev; struct node *n = *listP; if (index == 0) { *listP = *listP->next; free(n); return; } for (index; index > 0; index--) { n = n->next; if (index == 2) { prev = n; } } prev->next = n->next; free(n); } to remove an element from the list. If I want to remove the first node, I still need some way of referring to the list, which is why the listP arg is a double pointer, so it can point to the first element of the list and allow me to free the node that used to be the head. However, when I try to dereference listP to access the pointer to the next node, the compiler tells me error: request for member ‘next’ in something not a structure or union . What am I doing wrong here? I think I might be hopelessly mixed up..?

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• template specialization for static member functions; howto?

  - by Rolle

  I am trying to implement a template function with handles void differently using template specialization. The following code gives me an "Explicit specialization in non-namespace scope" in gcc: template <typename T> static T safeGuiCall(boost::function<T ()> _f) { if (_f.empty()) throw GuiException("Function pointer empty"); { ThreadGuard g; T ret = _f(); return ret; } } // template specialization for functions wit no return value template <> static void safeGuiCall<void>(boost::function<void ()> _f) { if (_f.empty()) throw GuiException("Function pointer empty"); { ThreadGuard g; _f(); } } I have tried moving it out of the class (the class is not templated) and into the namespace but then I get the error "Explicit specialization cannot have a storage class". I have read many discussions about this, but people don't seem to agree how to specialize function templates. Any ideas?

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• "Address of" (&) an array / address of being ignored be gcc?

  - by dbarbosa

  Hi, I am a teaching assistant of a introductory programming course, and some students made this type of error: char name[20]; scanf("%s",&name); which is not surprising as they are learning... What is surprising is that, besides gcc warning, the code works (at least this part). I have been trying to understand and I wrote the following code: void foo(int *str1, int *str2) { if (str1 == str2) printf("Both pointers are the same\n"); else printf("They are not the same\n"); } int main() { int test[50]; foo(&test, test); if (&test == test) printf("Both pointers are the same\n"); else printf("They are not the same\n"); } Compiling and executing: $ gcc test.c -g test.c: In function ‘main’: test.c:12: warning: passing argument 1 of ‘foo’ from incompatible pointer type test.c:13: warning: comparison of distinct pointer types lacks a cast $ ./a.out Both pointers are the same Both pointers are the same Can anyone explain why they are not different? I suspect it is because I cannot get the address of an array (as I cannot have & &x), but in this case the code should not compile.

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