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• How can Java assignment be made to point to an object instead of making a copy?

  - by Matthew Piziak

  In a class, I have: private Foo bar; public Constructor(Foo bar) { this.bar = bar; } Instead of creating a copy of bar from the object provided in the parameter, is it possible to include a pointer to bar in the constructor such that changing the original bar changes the field in this object? Another way of putting it: int x = 7; int y = x; x = 9; System.out.print(y); //Prints 7. It is possible to set it up so that printing y prints 9 instead of 7?

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• What's the meaning of 'char (*p)[5];'?

  - by jpmelos

  people. I'm trying to grasp the differences between these three declarations: char p[5]; char *p[5]; char (*p)[5]; I'm trying to find this out by doing some tests, because every guide of reading declarations and stuff like that has not helped me so far. I wrote this little program and it's not working (I've tried other kinds of use of the third declaration and I've ran out of options): #include <stdio.h> #include <string.h> #include <stdlib.h> int main(void) { char p1[5]; char *p2[5]; char (*p3)[5]; strcpy(p1, "dead"); p2[0] = (char *) malloc(5 * sizeof(char)); strcpy(p2[0], "beef"); p3[0] = (char *) malloc(5 * sizeof(char)); strcpy(p3[0], "char"); printf("p1 = %s\np2[0] = %s\np3[0] = %s\n", p1, p2[0], p3[0]); return 0; } The first and second works alright, and I've understood what they do. What is the meaning of the third declaration and the correct way to use it? Thank you!

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• What does this pointer-heavy C code do?

  - by justRadojko

  Could someone explain to me what should two following lines do: s.httpheaderline[s.httpheaderlineptr] = *(char *)uip_appdata; ++((char *)uip_appdata); This is taken from uIP code for microcontrollers. s - structure httpheaderline - http packet presented as a string httpheadrlineptr - integer value uip_appdata - received ethernet packet (string) If some more info is needed please let me know. BTW. Eclipse is reporting an error on the second line with message Invalid lvalue in increment so i'm trying to figure out how to solve this.

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• What does the Asterisk * mean in Objective-C?

  - by Thanks

  Is it true, that the Asterisk always means "Hey, that is a pointer!" And an Pointer always holds an memory adress? (Yes I know for the exception that a * is used for math operation) For Example: NSString* myString; or SomeClass* thatClass; or (*somePointerToAStruct).myStructComponent = 5; I feel that there is more I need to know about the Asterirsk (*) than that I use it when defining an Variable that is a pointer to a class. Because sometimes I already say in the declaration of an parameter that the Parameter variable is a pointer, and still I have to use the Asterisk in front of the Variable in order to access the value. That recently happened after I wanted to pass a pointer of an struct to a method in a way like [myObj myMethod:&myStruct], I could not access a component value from that structure even though my method declaration already said that there is a parameter (DemoStruct*)myVar which indeed should be already known as a pointer to that demostruct, still I had always to say: "Man, compiler. Listen! It IIISSS a pointer:" and write: (*myVar).myStructComponentX = 5; I really really really do not understand why I have to say that twice. And only in this case. When I use the Asterisk in context of an NSString* myString then I can just access myString however I like, without telling the compiler each time that it's a pointer. i.e. like using *myString = @"yep". It just makes no sense to me.

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• Misunderstanding function pointer - passing it as an argument

  - by Stef

  I want to pass a member function of class A to class B via a function pointer as argument. Please advise whether this road is leading somewhere and help me fill the pothole. #include <iostream> using namespace std; class A{ public: int dosomeA(int x){ cout<< "doing some A to "<<x <<endl; return(0); } }; class B{ public: B(int (*ptr)(int)){ptr(0);}; }; int main() { A a; int (*APtr)(int)=&A::dosomeA; B b(APtr); return 0; } This brilliant piece of code leaves me with the compiler error: cannot convert int (A::*)(int)' toint (*)(int)' in initialization Firstly I want it to compile. Secondly I don't want dosomeA to be STATIC.

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• Void pointer cast C++ and GTK

  - by Tarantula

  See this GTK callback function: static gboolean callback(GtkWidget *widget, GdkEventButton *event, gpointer *data) { AnyClass *obj = (AnyClass*) data; // using obj works } (please note the gpointer* on the data). And then the signal is connected using: AnyClass *obj2 = new AnyClass(); gtk_signal_connect(/*GTK params (...)*/, callback, obj2); See that the *AnyClass is going to be cast to gpointer* (void**). In fact, this is working now. The callback prototype in GTK documentation is "gpointer data" and not "gpointer *data" as shown in code, what I want to know is: how this can work ? Is this safe ?

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• What is wrong with my version of strchr?

  - by Eduard Saakashvili

  My assignment is to write my own version of strchr, yet it doesn't seem to work. Any advice would be much appreciated. Here it is: char *strchr (const char *s, int c) //we are looking for c on the string s { int dog; //This is the index on the string, initialized as 0 dog = 0; int point; //this is the pointer to the location given by the index point = &s[dog]; while ((s[dog] != c) && (s[dog] != '\0')) { //it keeps adding to dog until it stumbles upon either c or '\0' dog++; } if (s[dog]==c) { return point; //at this point, if this value is equal to c it returns the pointer to that location } else { return NULL; //if not, this means that c is not on the string } }

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• Can I get a raw pointer from boost's weak_ptr?

  - by Czubaka

  Is it possible to get a raw pointer from boost::weak_ptr? Boost's shared_ptr has get() method and "-" operator. Is there some rationale behind weak_ptr not having the same functionality?

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• C++: getting the address of the start of an std::vector ?

  - by shoosh

  Sometimes it is useful to use the starting address of an std::vector and temporarily treat that address as the address of a regularly allocated buffer. For instance replace this: char* buf = new char[size]; fillTheBuffer(buf, size); useTheBuffer(buf, size); delete[] buf; With This: vector<char> buf(size); fillTheBuffer(&buf[0], size); useTheBuffer(&buf[0], size); The advantage of this is of course that the buffer is deallocated automatically and I don't have to worry about the delete[]. The problem I'm having with this is when size == 0. In that case the first version works ok. An empty buffer is "allocated" and the subsequent functions do nothing size they get size == 0. The second version however fails if size == 0 since calling buf[0] may rightly contain an assertion that 0 < size. So is there an alternative to the idiom &buf[0] that returns the address of the start of the vector even if the vector is empty? I've also considered using buf.begin() but according to the standard it isn't even guaranteed to return a pointer.

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• C++ standard: dereferencing NULL pointer to get a reference?

  - by shoosh

  I'm wondering about what the C++ standard says about code like this: int* ptr = NULL; int& ref = *ptr; int* ptr2 = &ref; In practice the result is that ptr2 is NULL but I'm wondering, is this just an implementation detail or is this well defined in the standard? Under different circumstances a dereferencing of a NULL pointer should result in a crash but here I'm dereferencing it to get a reference which is implemented by the compiler as a pointer so there's really no actual dereferencing of NULL.

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• What is wrong in this c++ code?

  - by narayanpatra

  Why this coder do not show error #include <iostream> int main() { using namespace std; unsigned short int myInt = 99; unsigned short int * pMark = 0; cout << myInt << endl; pMark = &myInt; *pMark = 11; cout << "*pMark:\t" << *pMark << "\nmyInt:\t" << myInt << endl; return 0; } But this one shows : #include<iostream> using namespace std; int addnumber(int *p, int *q){ cout << *p = 12 << endl; cout << *q = 14 << endl; } #include<iostream> using namespace std; int addnumber(int *p, int *q){ cout << *p = 12 << endl; cout << *q = 14 << endl; } int main() { int i , j; cout << "enter the value of first number"; cin >> i; cout << "enter the value of second number"; cin >> j; addnumber(&i, &j); cout << i << endl; cout << j << endl; } In both the code snippets, I am assigning *pointer=somevalue. In first code it do not show any error but it shows error in the line cout << *p = 12 << endl; cout << *q = 14 << endl; What mistake I am doing ?

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• char pointer array in c#

  - by james

  consider the following c++ code #include "stdafx.h" #include<iostream> using namespace std; void ping(int,char* d[]); void ping(int a,char *b[]) { int size; size=sizeof(b)/sizeof(int); // total size of array/size of array data type //cout<<size; for(int i=0;i<=size;i++) cout<<"ping "<<a<<b[i]<<endl; } int _tmain(int argc, _TCHAR* argv[]) { void (*funcptr)(int,char* d[]); char* c[]={"a","b"}; funcptr= ping; funcptr(10,c); return 0; } how can i implement the same in c#.. m new to c#. how can i have char pointer array in c#?

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• Object allocation in C++

  - by Poiuyt

  char *myfunc() { char *temp = "string"; return temp; } In this piece of code, where does the allocation of the object pointed to by temp happen and what would be its scope? Is this function a valid way to return a char* pointer?

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• Reading function pointer syntax

  - by bobobobo

  Everytime I look at a C function pointer, my eyes glaze over. I can't read them. From here, here are 2 examples of function pointer TYPEDEFS: typedef int (*AddFunc)(int,int); typedef void (*FunctionFunc)(); Now I'm used to something like: typedef vector<int> VectorOfInts ; Which I read as typedef vector<int> /* as */ VectorOfInts ; But I can't read the above 2 typedefs. The bracketing and the asterisk placement, it's just not logical. Why is the * beside the word AddFunc..?

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• enable_shared_from_this and inheritance

  - by DeadMG

  I've got a type which inherits from enable_shared_from_this<type>, and another type that inherits from this type. Now I can't use the shared_from_this method because it returns the base type and in a specific derived class method I need the derived type. Is it valid to just construct a shared_ptr from this directly? Edit: In a related question, how can I move from an rvalue of type shared_ptr<base> to a type of shared_ptr<derived>? I used dynamic_cast to verify that it really was the correct type, but now I can't seem to accomplish the actual move.

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• What happens when you deallocate a reference twice or more in c++?

  - by flopex

  int main(){ Employee *e = new Employee(); delete e; delete e; ... delete e; return 0; }

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• What happens when you deallocate a pointer twice or more in C++?

  - by flopex

  int main(){ Employee *e = new Employee(); delete e; delete e; ... delete e; return 0; }

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• C++: Print only one char

  - by Martijn Courteaux

  Hi, When I read one char* from std::cin and I want to write it to std::cout, it prints until it finds a \0 in memory. So what did was: char c; cin >> c; char* pChar = &c; pChar++; *pChar = '\0'; println(&c); // My own method: void println(char * str) { cout << str << endl; } But I don't think this is a safe action. Is there a safer way to do this?

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• Better variant of getting the output dinamically-allocated array from the function?

  - by Raigomaru

  Here is to variants. First: int n = 42; int* some_function(int* input) { int* result = new int[n]; // some code return result; } void main() { int* input = new int[n]; int* output = some_function(input); delete[] input; delete[] output; } Here the function returns the memory, allocated inside the function. Second variant: int n = 42; void some_function(int* input, int* output) { // some code } void main() { int* input = new int[n]; int* output = new int[n]; some_function(input, output); delete[] input; delete[] output; } Here the memory is allocated outside the function. Now I use the first variant. But I now that many built-in c++ functions use the second variant. The first variant is more comfortable (in my opinion). But the second one also has some advantages (you allocate and delete memory in the same block). Maybe it's a silly question but what variant is better and why?

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• Add 64 bit offset to a pointer

  - by Novox

  In F#, there's the NativePtr module, but it seems to only support 32 bit offsets for its’ add/get/set functions, just like System.IntPtr does. Is there a way to add a 64 bit offset to a native pointer (nativeptr<'a) in F#? Of course I could convert all addresses to 64 bit integers, do normal integer operations and then convert the result again to nativeptr<'a, but this would cost additional add and imul instructions. I really want the AGUs to perform the address calculations. For instance, using unsafe in C# you could do something like void* ptr = Marshal.AllocHGlobal(...).ToPointer(); int64 offset = ...; T* newAddr = (T*)ptr + offset; // T has to be an unmanaged type Well actually you can't, because there is no "unmanaged" constraint for type parameters, but at least you can do general pointer arithmetic in a non-generic way. In F# we finally got the unmanaged constraint; but how do I do the pointer arithmetic?

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• Can I access type int (*)[] with [][]?

  - by Framester

  Hi coming from this question "What does (int (*)[])var1 stand for?" I tried to access the result of the cast like a multidimensional array. But I get following error: "assignment from incompatible pointer type" followed by a segmentation fault. I tried also some other variations, but none of them worked. How can I access the elements in var1 in the function example directly? Thank you! #include <stdlib.h> int i(int n,int m,int var1[n][m]) { var1[0][0]=5; return var1[0][0]; } int example() { int *var1 = malloc(100); // works int var2; var2 = i(10,10,(int (*)[])var1); printf("var2=%i",var2); //doesn't work I int *var3; var3=(int (*)[])var1; //"assignment from incompatible pointer type" printf("var3[0][0]=%i",var3[0][0]); //doesn't work II int *var4; var4=var1; printf("var4[0][0]=%i",var4[0][0]); //" error: subscripted value is neither array nor pointer" //doesn't work III int **var5; var5=var1; printf("var5[0][0]=%i",var5[0][0]); // assignment from incompatible pointer type return(1); } int main(){ int a; a=example(); return(1); }

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• C++: Validate a pointer?

  - by Rosarch

  If I have a pointer, like char* foo, is there any way to determine if foo points to a valid location in memory? (So, foo is not NULL, and delete has not yet been called on it.)

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• What is better: to delete pointer or set it with a new value?

  - by user63898

  Hi simple question in c++ , say i have a loop and i have function that returns pointer to item so i have to define inner loop pointer so my question is what to do with the pointer inside the loop , delete it ? or to set it with new value is good for example: for(int i =0;i<count();i++) { ptrTmp* ptr = getItemPtr(); // do somthing with the ptr ... // what to do here ? to delete the poinetr or not? delete ptr; // ?? }

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• How to convert a void pointer to array of classes

  - by user99545

  I am trying to convert a void pointer to an array of classes in a callback function that only supports a void pointer as a means of passing paramaters to the callback. class person { std::string name, age; }; void callback (void *val) { for (int i = 0; i < 9; i++) { std::cout << (person [])val[i].name; } } int main() { person p[10]; callback((void*)p); } My goal is to be able to pass an array of the class person to the callback which then prints out the data such as their name and age. However, the compile does not like what I am doing and complains that error: request for member 'name' in 'val', which is of non-class type 'void*' How can I go about doing this?

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• base pointer to derived class

  - by Jay

  Suppose there are Base class and Derived class. Base *A = new Base; Here A is a pointer point to Base class, and new constructs one that A points to. I also saw Base *B = new Derived; How to explain this? B is a pointer to Base Class, and a Derived class constructed and pointed by B? If there is a function derived from Base class, say, Virtual void f(), and it's been overridden in Derived class, then B->f() will invoke which version of the function? version in Base class, or version that overridden in Derived Class. What if there is a new function void g()in Derived, is B->g() going to invoke this function properly? One more is, is int *a = new double; or int *a = new int; legal?

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