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  • Pointers in C vs No pointers in PHP

    - by AnnaBanana
    Both languages have the same syntax. Why does C have the weird * character that denotes pointers (which is some kind of memory address of the variable contents?), when PHP doesn't have it and you can do pretty much the same things in PHP that you can do in C, without pointers? I guess the PHP compiler handles this internally, why doesn't C do the same? Doesn't this add unneeded complexity in C? For example I don't understand them :)

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  • C++ Pointers: Number of levels of Indirection

    - by A B
    In a C++ program that doesn't contain legacy C code, is there a guideline regarding the maximum number of levels of indirection that should be used in the source code? I know that in C (as opposed to C++), some programmers have used pointers to pointers for a multiple dimension array, but for the case of arrays, there are data structures in C++ that can be used to avoid the pointers to pointers. Are users who still create pointers to pointers (or more than this) trying to use pointers to pointers only for performance ETC. reasons? I have tried NOT to use any more than a pointer to a pointer, only in the case that a pointer needed modification; does anyone have any other official or unofficial guidelines or rules regarding the number of levels of indirection?

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  • Why does void in C mean not void?

    - by Naftuli Tzvi Kay
    In strongly-typed languages like Java and C#, void (or Void) as a return type for a method seem to mean: This method doesn't return anything. Nothing. No return. You will not receive anything from this method. What's really strange is that in C, void as a return type or even as a method parameter type means: It could really be anything. You'd have to read the source code to find out. Good luck. If it's a pointer, you should really know what you're doing. Consider the following examples in C: void describe(void *thing) { Object *obj = thing; printf("%s.\n", obj->description); } void *move(void *location, Direction direction) { void *next = NULL; // logic! return next; } Obviously, the second method returns a pointer, which by definition could be anything. Since C is older than Java and C#, why did these languages adopt void as meaning "nothing" while C used it as "nothing or anything (when a pointer)"?

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  • C++ smart pointers: sharing pointers vs. sharing data

    - by Eli Bendersky
    In this insightful article, one of the Qt programmers tries to explain the different kinds of smart pointers Qt implements. In the beginning, he makes a distinction between sharing data and sharing the pointers themselves: First, let’s get one thing straight: there’s a difference between sharing pointers and sharing data. When you share pointers, the value of the pointer and its lifetime is protected by the smart pointer class. In other words, the pointer is the invariant. However, the object that the pointer is pointing to is completely outside its control. We don’t know if the object is copiable or not, if it’s assignable or not. Now, sharing of data involves the smart pointer class knowing something about the data being shared. In fact, the whole point is that the data is being shared and we don’t care how. The fact that pointers are being used to share the data is irrelevant at this point. For example, you don’t really care how Qt tool classes are implicitly shared, do you? What matters to you is that they are shared (thus reducing memory consumption) and that they work as if they weren’t. Frankly, I just don't undersand this explanation. There was a clarification plea in the article comments, but I didn't find the author's explanation sufficient. If you do understand this, please explain. What is this distinction, and how are other shared pointer classes (i.e. from boost or the new C++ standards) fit into this taxonomy? Thanks in advance

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  • How to implement an interface member that returns void in F#

    - by Drew Noakes
    Imagine the following interface in C#: interface IFoo { void Bar(); } How can I implement this in F#? All the examples I've found during 30 minutes of searching online show only examples that have return types which I suppose is more common in a functional style, but something I can't avoid in this instance. Here's what I have so far: type Bar() = interface IFoo with member this.Bar = void Fails with FS0010: Unexpected keyword 'void' in expression.

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  • Void pointer values comparing C++

    - by user2962977
    My actual question is it really possible to compare values contained in two void pointers, when you actually know that these values are the same type? For example int. void compVoids(void *firstVal, void *secondVal){ if (firstVal < secondVal){ cout << "This will not make any sense as this will compare addresses, not values" << endl; } } Actually I need to compare two void pointer values, while outside the function it is known that the type is int. I do not want to use comparison of int inside the function. So this will not work for me as well: if (*(int*)firstVal > *(int*)secondVal) Any suggestions? Thank you very much for help!

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  • What's a nice explanation for pointers?

    - by Macneil
    In your own studies (on your own, or for a class) did you have an "ah ha" moment when you finally, really understood pointers? Do you have an explanation you use for beginner programmers that seems particularly effective? For example, when beginners first encounter pointers in C, they might just add &s and *s until it compiles (as I myself once did). Maybe it was a picture, or a really well motivated example, that made pointers "click" for you or your student. What was it, and what did you try before that didn't seem to work? Were any topics prerequisites (e.g. structs, or arrays)? In other words, what was necessary to understand the meaning of &s and *, when you could use them with confidence? Learning the syntax and terminology or the use cases isn't enough, at some point the idea needs to be internalized. Update: I really like the answers so far; please keep them coming. There are a lot of great perspectives here, but I think many are good explanations/slogans for ourselves after we've internalized the concept. I'm looking for the detailed contexts and circumstances when it dawned on you. For example: I only somewhat understood pointers syntactically in C. I heard two of my friends explaining pointers to another friend, who asked why a struct was passed with a pointer. The first friend talked about how it needed to be referenced and modified, but it was just a short comment from the other friend where it hit me: "It's also more efficient." Passing 4 bytes instead of 16 bytes was the final conceptual shift I needed.

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  • Problem using void pointer as a function argument

    - by Nazgulled
    Hi, I can't understand this result... The code: void foo(void * key, size_t key_sz) { HashItem *item = malloc(sizeof(HashItem)); printf("[%d]\n", (int)key); ... item->key = malloc(key_sz); memcpy(item->key, key, key_sz); } void bar(int num) { foo(&num, sizeof(int)); } And I do this call: bar(900011009); But the printf() output is: [-1074593956] I really need key to be a void pointer, how can I fix this?

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  • pointers to functions

    - by DevAno1
    I have two basic Cpp tasks, but still I have problems with them. First is to write functions mul1,div1,sub1,sum1, taking ints as arguments and returning ints. Then I need to create pointers ptrFun1 and ptrFun2 to functions mul1 and sum1, and print results of using them. Problem starts with defining those pointers. I thought I was doing it right, but devcpp gives me errors in compilation. #include <iostream> using namespace std; int mul1(int a,int b) { return a * b; } int div1(int a,int b) { return a / b; } int sum1(int a,int b) { return a + b; } int sub1(int a,int b) { return a - b; } int main() { int a=1; int b=5; cout << mul1(a,b) << endl; cout << div1(a,b) << endl; cout << sum1(a,b) << endl; cout << sub1(a,b) << endl; int *funPtr1(int, int); int *funPtr2(int, int); funPtr1 = sum1; funPtr2 = mul1; cout << funPtr1(a,b) << endl; cout << funPtr2(a,b) << endl; system("PAUSE"); return 0; } 38 assignment of function int* funPtr1(int, int)' 38 cannot convertint ()(int, int)' to `int*()(int, int)' in assignment Task 2 is to create array of pointers to those functions named tabFunPtr. How to do that ?

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  • Using pointers in PHP.

    - by Babiker
    I ask this question because i learned that in programming and designing, you must have a good reason for decisions. I am php learner and i am at a crossroad here, i am using simple incrementation to try to get what im askin across. I am certainly not here to start a debate about the pros/cons of pointers but when it comes to php, which is the better programming practice: function increment(&$param) { $param++; } Or function increment($param){ return $param++; } $param = increment($param);

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  • handling refrence to pointers/double pointers using SWIG [C++ to Java]

    - by Siddu
    My code has an interface like class IExample { ~IExample(); //pure virtual methods ...}; a class inheriting the interface like class CExample : public IExample { protected: CExample(); //implementation of pure virtual methods ... }; and a global function to create object of this class - createExample( IExample *& obj ) { obj = new CExample(); } ; Now, I am trying to get Java API wrapper using SWIG, the SWIG generated interface has a construcotr like - IExample(long cPtr, boolean cMemoryOwn) and global function becomes createExample(IExample obj ) The problem is when i do, IExample exObject = new IExample(LogFileLibraryJNI.new_plong(), true /*or false*/ ); createExample( exObject ); The createExample(...) API at C++ layer succesfully gets called, however, when call returns to Java layer, the cPtr (long) variable does not get updated. Ideally, this variable should contain address of CExample object. I read in documentation that typemaps can be used to handle output parameters and pointer references as well; however, I am not able to figure out the suitable way to use typemaps to resolve this problem, or any other workaround. Please suggest if i am doing something wrong, or how to use typemap in such situation?

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  • introducing pointers to a large software project

    - by stefan
    I have a fairly large software project written in c++. In there, there is a class foo which represents a structure (by which i don't mean the programmers struct) in which foo-objects can be part of a foo-object. Here's class foo in simplest form: class Foo { private: std::vector<unsigned int> indices; public: void addFooIndex(unsigned int); unsigned int getFooIndex(unsigned int); }; Every foo-object is currently stored in an object of class bar. class Bar { private: std::vector<Foo> foos; public: void addFoo(Foo); std::vector<Foo> getFoos(); } So if a foo-object should represent a structure with a "inner" foo-object, I currently do Foo foo; Foo innerFoo; foo.addFooIndex(bar.getFoos().size() - 1); bar.addFoo(innerFoo); And to get it, I obviously use: Foo foo; for ( unsigned int i = 0; i < foo.getFooIndices().size(); ++i ) { Foo inner_foo; assert( foo.getFooIndices().at(i) < bar.getFoos().size() ); inner_foo = bar.getFoos().at(foo.getFooIndices().at(i)); } So this is not a problem. It just works. But it's not the most elegant solution. I now want to make the inner foos to be "more connected" with the foo-object. It would be obviously to change class foo to: class Foo { private: std::vector<Foo*> foo_pointers; public: void addFooPointer(Foo*); std::vector<Foo*> getFooPointers(); }; So now, for my question: How to gently change this basic class without messing up the whole code? Is there a "clean way"?

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  • Functions returning pointers

    - by fg nu
    C++ noob here. I have a very basic question about a construct I found in the C++ book I am reading. // class declaration class CStr { char sData[256]; public: char* get(void); }; // implementation of the function char* CStr::get(void) { return sData; } So the Cstr::get function is obviously meant to return a character pointer, but the function is passing what looks like the value (return sData). Does C++ know to return the address of the returned object? My guess would have been that the function definition would be return &sData.

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  • C++ simple arrays and pointers question

    - by nashmaniac
    So here's the confusion, let's say I declare an array of characters char name[3] = "Sam"; and then I declare another array but this time using pointers char * name = "Sam"; What's the difference between the two? I mean they work the same way in a program. Also how does the latter store the size of the stuff that someone puts in it, in this case 3 characters? Also how is it different from char * name = new char[3]; If those three are different where should they be used I mean in what circumstances?

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  • What does this mean: warning: converting from ‘void (ClassName::*)()’ to ‘void (*)()’

    - by Brendan Long
    I have a member function in a class that has a callback, but the callback isn't strictly neccessary, so it has a default callback, which is empty. It seems to work fine, but I get an annoying warning: warning: converting from ‘void (ClassName::*)()’ to ‘void (*)()’ I'm trying to figure out what it means and how to turn it off (or fix it if I really am doing something wrong). Here's some simple code: class ClassName{ public: void doSomething(void (*callbackFunction)() = (void(*)()) &ClassName::doNothing){ callbackFunction(); } void doNothing(){} }; int main(){ ClassName x; x.doSomething(); return 0; } Note: If I do this (without explicitly casting it as a void(*)()): void doSomething(void (*callbackFunction)() = &ClassName::doNothing) I get this: main.cpp:3: error: default argument for parameter of type ‘void (*)()’ has type ‘void (ClassName::*)()’

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  • What's so bad about pointers in C++?

    - by Martin Beckett
    To continue the discussion in Why are pointers not recommended when coding with C++ Suppose you have a class that encapsulates objects which need some initialisation to be valid - like a network socket. // Blah manages some data and transmits it over a socket class socket; // forward declaration, so nice weak linkage. class blah { ... stuff TcpSocket *socket; } ~blah { // TcpSocket dtor handles disconnect delete socket; // or better, wrap it in a smart pointer } The ctor ensures that socket is marked NULL, then later in the code when I have the information to initialise the object. // initialising blah if ( !socket ) { // I know socket hasn't been created/connected // create it in a known initialised state and handle any errors // RAII is a good thing ! socket = new TcpSocket(ip,port); } // and when i actually need to use it if (socket) { // if socket exists then it must be connected and valid } This seems better than having the socket on the stack, having it created in some 'pending' state at program start and then having to continually check some isOK() or isConnected() function before every use. Additionally if TcpSocket ctor throws an exception it's a lot easier to handle at the point a Tcp connection is made rather than at program start. Obviously the socket is just an example, but I'm having a hard time thinking of when an encapsulated object with any sort of internal state shouldn't be created and initialised with new.

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  • Invalid conversion from int to int** C++

    - by user69514
    Not sure why I'm getting this error. I have the following: int* arr = new int[25]; int* foo(){ int* i; cout << "Enter an integer:"; cin >> *i; return i; } void test(int** myInt){ *myInt = foo(); } This call here is where I get the error: test(arr[0]); //here i get invalid conversion from int to int**

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  • How is precedence determined in C pointers?

    - by ankur.trapasiya
    I've come across two pointer declarations that I'm having trouble understanding. My understanding of precedence rules goes something like this: Operator Precedence Associativity (), [ ] 1 Left to Right *, identifier 2 Right to Left Data type 3 But even given this, I can't seem to figure out how to evaluate the following examples correctly: First example float * (* (*ptr)(int))(double **,char c) My evaluation: *(ptr) (int) *(*ptr)(int) *(*(*ptr)(int)) Then, double ** char c Second example unsigned **( * (*ptr) [5] ) (char const *,int *) *(ptr) [5] *(*ptr)[5] *(*(*ptr)[5]) **(*(*ptr)[5]) How should I read them?

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  • Weird behavior when using pointers [migrated]

    - by Kinan Al Sarmini
    When I run this code on MS VS C++ 2010: #include <iostream> int main() { const int a = 10; const int *b = &a; int *c = (int *)b; *c = 10000; std::cout << c << " " << &a << std::endl; std::cout << *c << " " << a << " " << *(&a) << std::endl; return 0; } The output is: 0037F784 0037F784 10000 10 10 The motivation for writing that code was this sentence from "The C++ Programming Language" by Stroustrup: "It is possible to explicitly remove the restrictions on a pointer to const by explicit type conversion". I know that trying to modify a constant is conceptually wrong, but I find this result quite weird. Can anyone explain the reason behind it?

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  • How to use void*

    - by Rondogiannis Aristophanes
    I am imlementing a simple merge function and I have got stuck, as the compiler gives me errors that I cannot explain. Here is my merge function: void merge(void *a, int beg, int middle, int end, int (*cmp)(const void*, const void* { std::stack<void*> first; std::stack<void*> second; for(int i = beg; i < middle; i++) { first.push(a+i); } for(int i = middle; i < end; i++) { second.push(a+i); } for(int i = beg; i < end; i++) { if(first.empty()) { void *tmp = second.top(); second.pop(); a+i = tmp; } else if(second.empty()) { void *tmp = first.top(); first.pop(); a+i = tmp; } else if(cmp(first.top(), second.top())) { void *tmp = first.top(); first.pop(); a+i = tmp; } else { void *tmp = second.top(); second.pop(); a+i = tmp; } } } And here is the error: sort.h: In function `void merge(void*, int, int, int, int (*)(const void*, const void*))': sort.h:9: error: pointer of type `void *' used in arithmetic sort.h:12: error: pointer of type `void *' used in arithmetic sort.h:19: error: pointer of type `void *' used in arithmetic sort.h:19: error: non-lvalue in assignment sort.h:23: error: pointer of type `void *' used in arithmetic sort.h:23: error: non-lvalue in assignment sort.h:27: error: pointer of type `void *' used in arithmetic sort.h:27: error: non-lvalue in assignment sort.h:31: error: pointer of type `void *' used in arithmetic sort.h:31: error: non-lvalue in assignment Can anyone help me? TIA.

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  • What do I return if the return type of a method is Void? (Not void!)

    - by DR
    Due to the use of Generics in Java I ended up in having to implement a function having Void as return type: public Void doSomething() { //... } and the compiler demands that I return something. For now I'm just returning null, but I'm wondering if that is good coding practice... I've also tried Void.class, void, Void.TYPE, new Void(), no return at all, but all that doesn't work at all. (For more or less obvious reasons) (See this answer for details) So what am I supposed to return if the return type of a function is Void? What's the general use of the Void class? EDIT: Just to spare you the downvotes: I'm asking about V?oid, not v?oid. The class Void, not the reserved keyword void.

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