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• How to return a copy of the data in C++

  - by Josh Curren

  I am trying to return a new copy of the data in a C++ Template class. The following code is getting this error: invalid conversion from ‘int*’ to ‘int’. If I remove the new T then I am not returning a copy of the data but a pointer to it. template<class T> T OrderedList<T>::get( int k ) { Node<T>* n = list; for( int i = 0; i < k; i++ ) { n=n->get_link(); } return new T( n->get_data() ); // This line is getting the error ********** }

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• Check if a pointer points to allocated memory on the heap.

  - by Ugo

  Ok, I know this question seems to have been asked many times on stackoverflow. but please read Well the answer for any address is "No you can't" but the question here is to know if a pointer points to a piece of memory allocated with malloc/new. Actually I think it could be easily implemented overriding malloc/free and keeping track of allocated memory ranges. Do you know a memory management library providing this specific tool ?

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• Class lookup structure array in C++

  - by wyatt

  I'm trying to create a structure array which links input strings to classes as follows: struct {string command; CommandPath cPath;} cPathLookup[] = { {"set an alarm", AlarmCommandPath}, {"send an email", EmailCommandPath}, {"", NULL} }; which will be used as follows: CommandPath *cPath = NULL; string input; getline(cin, input); for(int i = 0; cPathLookup[i] != ""; i++) { if(cPathLookup[i].command == input) cPath = new cPathLookup[i].cPath; } Obviously, this code is meaningless, but I think my intention is apparent - depending on input, I'd like cPath to be initialized as either a new AlarmCommandPath or a new EmailCommandPath. I could handle it with a function returning an instance depending on input, but a whole sequence of ifs just seems inelegant. I should also note that, in case it's not apparent and important, that AlarmCommandPath and EmailCommandPath are derived from CommandPath, and CommandPath is an abstract class. Thanks for any help you can offer. EDIT: I just noticed that, in spite of CommandPath being abstract, I have a declaration: CommandPath *cPath = NULL; in working code. Why does that compile?

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• Acessing a struct member, using a pointer to a vector of structs. Error:base operand of '->' has non-pointer type

  - by Matt Munson

  #include <iostream> #include <vector> using namespace std; struct s_Astruct { vector <int> z; }; int main () { vector <s_Astruct> v_a; for(int q=0;q<10;q++) { v_a.push_back(s_Astruct()); for(int w =0;w<5;w++) v_a[q].z.push_back(8); } vector <s_Astruct> * p_v_a = & v_a; cout << p_v_a[0]->z[4]; //error: base operand of '->' has non-pointer type //'__gnu_debug_def::vector<s_Astruct, std::allocator<s_Astruct> >' } There seems to be some issue with this sort of operation that I don't understand. In the code that I'm working on I actually have things like p_class-vector[]-vector[]-int; and I'm getting a similar error.

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• Pointer to current function

  - by pythonic metaphor

  Is there any way to get a pointer to the current function, maybe through gcc extensions or some other trickery?

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• C++: Question about freeing memory

  - by Martijn Courteaux

  On Learn C++, they wrote this to free memory: int *pnValue = new int; // dynamically allocate an integer *pnValue = 7; // assign 7 to this integer delete pnValue; pnValue = 0; My question is: "Is the last statement needed to free the memory correctly, completly?" I thought that the pointer *pnValue was still on the stack and new doesn't make any sense to the pointer. And if it is on the stack it will be cleaned up when the application leaves the scope (where the pointer is declared in), isn't it?

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• using structures with multidimentional tables

  - by gem

  I have a table of structures and this structures are 2 dimentional table of constants. can you teach me on how to get the values in the table of constants. (note following is just example) typedef struct { unsigned char ** Type1; unsigned char ** Type2; } Formula; typedef struct { Formula tformula[size]; } table; const table Values = { (unsigned char**) &(default_val1), (unsigned char**) &(default_val2) }; const unsigned char default_val1[4][4] = { {0,1,2,3}, {4,5,6,7}, {8,9,0,11}, {12,13,14,15} } const unsigned char default_val2[4][4] = { {15,16,17,13}, {14,15,16,17}, {18,19,10,21}, {22,23,24,25} }

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• How can I return to a string address and then assign it to a new string?

  - by Y_Y

  I have 1 function that I want to return the address of an assigned string to the main function and assign an new string pointer with the same address so that the new string will have the contents of the old string. For example: unknown_datatype function() { char *old = "THE STRING"; return old; } int main() { char *snew = ""; snew = function(); return 0; } *unknown_datatype means I don't know that to put there... *How can I approach this without changing anything in the main() method

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• How do I return the indices of a multidimensional array element in C?

  - by Eddy

  Say I have a 2D array of random boolean ones and zeroes called 'lattice', and I have a 1D array called 'list' which lists the addresses of all the zeroes in the 2D array. This is how the arrays are defined: define n 100 bool lattice[n][n]; bool *list[n*n]; After filling the lattice with ones and zeroes, I store the addresses of the zeroes in list: for(j = 0; j < n; j++) { for(i = 0; i < n; i++) { if(!lattice[i][j]) // if element = 0 { list[site_num] = &lattice[i][j]; // store address of zero site_num++; } } } How do I extract the x,y coordinates of each zero in the array? In other words, is there a way to return the indices of an array element through referring to its address?

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• glibc detected ./.a.out: free(): invalid pointer

  - by ExtremeBlue

  typedef struct _PERSON { size_t age; unsigned char* name; }PERSON; int init(PERSON** person) { (* person) = (PERSON *) malloc(sizeof(struct _PERSON)); (* person)->age = 1; (* person)->name = (unsigned char *) malloc(sizeof(4)); (* person)->name = "NAME"; return 0; } void close(PERSON** person) { (* person)->age = 0; if((* person)->name != NULL) { free((* person)->name); } if((* person) != NULL) { free((* person)); } } int main(int argc, char* argv[]) { PERSON* p; init(&p); printf("%d\t%s\n", (int) p->age, p->name); close(&p); return 0; } 1 NAME *** glibc detected *** ./a.out: free(): invalid pointer: 0x000000000040079c *** ======= Backtrace: ========= /lib/libc.so.6(+0x774b6)[0x7fa9027054b6] /lib/libc.so.6(cfree+0x73)[0x7fa90270bc83] ./a.out(close+0x3d)[0x400651] ./a.out[0x40069f] /lib/libc.so.6(__libc_start_main+0xfe)[0x7fa9026acd8e] ./a.out[0x4004f9] ... 7fa8fc000000-7fa8fc021000 rw-p 00000000 00:00 0 7fa8fc021000-7fa900000000 ---p 00000000 00:00 0 7fa902478000-7fa90248d000 r-xp 00000000 08:12 23068732 /lib/libgcc_s.so.1 7fa90248d000-7fa90268c000 ---p 00015000 08:12 23068732 /lib/libgcc_s.so.1 7fa90268c000-7fa90268d000 r--p 00014000 08:12 23068732 /lib/libgcc_s.so.1 7fa90268d000-7fa90268e000 rw-p 00015000 08:12 23068732 /lib/libgcc_s.so.1 7fa90268e000-7fa902808000 r-xp 00000000 08:12 23068970 /lib/libc-2.12.1.so 7fa902808000-7fa902a07000 ---p 0017a000 08:12 23068970 /lib/libc-2.12.1.so 7fa902a07000-7fa902a0b000 r--p 00179000 08:12 23068970 /lib/libc-2.12.1.so 7fa902a0b000-7fa902a0c000 rw-p 0017d000 08:12 23068970 /lib/libc-2.12.1.so 7fa902a0c000-7fa902a11000 rw-p 00000000 00:00 0 7fa902a11000-7fa902a31000 r-xp 00000000 08:12 23068966 /lib/ld-2.12.1.so 7fa902c25000-7fa902c28000 rw-p 00000000 00:00 0 7fa902c2e000-7fa902c31000 rw-p 00000000 00:00 0 7fa902c31000-7fa902c32000 r--p 00020000 08:12 23068966 /lib/ld-2.12.1.so 7fa902c32000-7fa902c33000 rw-p 00021000 08:12 23068966 /lib/ld-2.12.1.so 7fa902c33000-7fa902c34000 rw-p 00000000 00:00 0 7fff442d5000-7fff442f6000 rw-p 00000000 00:00 0 [stack] 7fff44308000-7fff44309000 r-xp 00000000 00:00 0 [vdso] ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall] Aborted

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• c Pointer to pointer, or passing list to functions

  - by user361808

  Hi, I am new to c programming. Could anyone please tell me what's wrong with the following program? typedef struct Person_s { int age; char name[40]; } Person_t; int process_list(int *countReturned, Person_t **p_list) { Person_t *rowPtr=0; //the actual program will fethc data from DB int count =1; if(!((*p_list) = (Person_t *) malloc(sizeof(Person_t)))) { return -1; } rowPtr = *p_list; rowPtr[count-1].age =19; strcpy(rowPtr[count-1].name,"Prince Dastan"); *countReturned = count; return 0; } int main(int argc, char *argv[]) { Person_t *tmpPerson=0; Person_t **p_list=0; int *count=0; int i; process_list(count,p_list); tmpPerson = *p_list; for(i=0; i< *count; i++) { printf("Name: %s , age: %d\n",tmpPerson->name,tmpPerson->age); tmpPerson++; } //free(tmpPerson); return 0; }

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• C problem, left of '->' must point to class/struct/union/generic type ??

  - by Patrick

  Hello! Trying to understand why this doesn't work. I keep getting the following errors: left of '-nextNode' must point to class/struct/union/generic type (Also all the lines with a - in the function new_math_struct) Header file #ifndef MSTRUCT_H #define MSTRUCT_H #define PLUS 0 #define MINUS 1 #define DIVIDE 2 #define MULTIPLY 3 #define NUMBER 4 typedef struct math_struct { int type_of_value; int value; int sum; int is_used; struct math_struct* nextNode; } ; typedef struct math_struct* math_struct_ptr; #endif C file int get_input(math_struct_ptr* startNode) { /* character, input by the user */ char input_ch; char* input_ptr; math_struct_ptr* ptr; math_struct_ptr* previousNode; input_ptr = &input_ch; previousNode = startNode; /* as long as input is not ok */ while (1) { input_ch = get_input_character(); if (input_ch == ',') // Carrage return return 1; else if (input_ch == '.') // Illegal character return 0; if (input_ch == '+') ptr = new_math_struct(PLUS, 0); else if (input_ch == '-') ptr = new_math_struct(MINUS, 0); else if (input_ch == '/') ptr = new_math_struct(DIVIDE, 0); else if (input_ch == '*') ptr = new_math_struct(MULTIPLY, 0); else ptr = new_math_struct(NUMBER, atoi(input_ptr)); if (startNode == NULL) { startNode = previousNode = ptr; } else { previousNode->nextNode = ptr; previousNode = ptr; } } return 0; } math_struct_ptr* new_math_struct(int symbol, int value) { math_struct_ptr* ptr; ptr = (math_struct_ptr*)malloc(sizeof(math_struct_ptr)); ptr->type_of_value = symbol; ptr->value = value; ptr->sum = 0; ptr->is_used = 0; return ptr; } char get_input_character() { /* character, input by the user */ char input_ch; /* get the character */ scanf("%c", &input_ch); if (input_ch == '+' || input_ch == '-' || input_ch == '*' || input_ch == '/' || input_ch == ')') return input_ch; // A special character else if (input_ch == '\n') return ','; // A carrage return else if (input_ch < '0' || input_ch > '9') return '.'; // Not a number else return input_ch; // Number } The header for the C file just contains a reference to the struct header and the definitions of the functions. Language C.

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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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• 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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• 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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• How to declare a pointer to a variable as a parameter of a function in C++?

  - by Keand64

  I have a function that takes a pointer to a D3DXVECTOR3, but I have no reason to declare this beforehand. The most logical solution to me was using new: Function( //other parameters, new D3DXVECTOR3(x, y, 0)); but I don't know how I would go about deleting it, beign intitialized in a function. My next thought was to use the & operator, like so: Function( //other parameters, &D3DVECTOR3(x, y, 0)); but I don't know if this is a valid way to go about doing this. (It doesn't get an error, but neither does int *x; x = 50;). So should I use new, &, or some other technique I'm overlooking?

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• C pointer question, dereferencing crash

  - by skynorth

  Why do this work? int *var; while(scanf("%d", &var) && *var != 0) printf("%d \n", var); While this does not? int *var; while(scanf("%d", &var) && var != 0) printf("%d \n", var); Doesn't * (dereference operator) give you the value pointed by the pointer? So why does *var != 0 crash the program, while var != 0 does not?

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• Does (size_t)((char *)0) ever not evaluate to 0?

  - by Bruce Christensen

  According to the responses in "Why subtract null pointer in offsetof()?" (and my reading of K&R), the C standard doesn't require that (size_t)((char *)0) == 0. Still, I've never seen a situation where casting a null pointer to an integer type evaluates to anything else. If there is a compiler or scenario where (size_t)((char *)0) != 0, what is it?

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• how do you set a property of a control to an address of a function in xaml?

  - by ambog36

  Hi, I have a control that has a "Filter" property that expects a function that defines how the contents of the control should be filtered. so far i am setting the filter in code behind as such: MyControl.Filter = AddressOf Filters.MyFilter In this example MyFilter is a shared function in the Filters class with the following signature: Public Shared Function MyFilter(ByVal obj As Object, ByVal text As String) As Boolean I would like to set this in xaml. I was thinking of setting the Filters.MyFilter as a static resource and setting it that way: ...Filter="{StaticResource myFilter}"/ but i cant set Filters.MyFilter as a static resource. Any ideas on how to achieve this? Thanks,

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• Call c++ function pointer from c#

  - by Sam

  Is it possible to call a c(++) static function pointer like this typedef int (*MyCppFunc)(void* SomeObject); from c#? void CallFromCSharp(MyCppFunc funcptr, IntPtr param) { funcptr(param); } I need to be able to callback from c# into some old c++ classes. C++ is managed, but the classes are not ref classes (yet). So far I got no idea how to call a c++ function pointer from c#, is it possible?

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• Virtual functions - base class pointer

  - by user980411

  I understood why a base class pointer is made to point to a derived class object. But, I fail to understand why we need to assign to it, a base class object, when it is a base class object by itself. Can anyone please explain that? #include <iostream> using namespace std; class base { public: virtual void vfunc() { cout << "This is base's vfunc().\n"; } }; class derived1 : public base { public: void vfunc() { cout << "This is derived1's vfunc().\n"; } }; int main() { base *p, b; derived1 d1; // point to base p = &b; p->vfunc(); // access base's vfunc() // point to derived1 p = &d1; p->vfunc(); // access derived1's vfunc() return 0; }

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• learning C++ from java, trying to make a linked list.

  - by kyeana

  I just started learning c++ (coming from java) and am having some serious problems with doing anything :P Currently, i am attempting to make a linked list, but must be doing something stupid cause i keep getting "void value not ignored as it ought to be" compile errors (i have it marked where it is throwing it bellow). If anyone could help me with what im doing wrong, i would be very grateful :) Also, I am not used to having the choice of passing by reference, address, or value, and memory management in general (currently i have all my nodes and the data declared on the heap). If anyone has any general advice for me, i also wouldn't complain :P Key code from LinkedListNode.cpp LinkedListNode::LinkedListNode() { //set next and prev to null pData=0; //data needs to be a pointer so we can set it to null for //for the tail and head. pNext=0; pPrev=0; } /* * Sets the 'next' pointer to the memory address of the inputed reference. */ void LinkedListNode::SetNext(LinkedListNode& _next) { pNext=&_next; } /* * Sets the 'prev' pointer to the memory address of the inputed reference. */ void LinkedListNode::SetPrev(LinkedListNode& _prev) { pPrev=&_prev; } //rest of class Key code from LinkedList.cpp #include "LinkedList.h" LinkedList::LinkedList() { // Set head and tail of linked list. pHead = new LinkedListNode(); pTail = new LinkedListNode(); /* * THIS IS WHERE THE ERRORS ARE. */ *pHead->SetNext(*pTail); *pTail->SetPrev(*pHead); } //rest of class

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• Is apparent NULL pointer dereference in C actually pointer arithmetic?

  - by karthik A

  hey ive got this piece of code. It dereferences a null pointer here. But then there is an and with unsigned int. I really dont understand the whole part. Can someone explain the output.?? struct hi { long a; int b; long c; }; int main() { struct hi ob={3,4,5}; struct hi *ptr=&ob; int num= (unsigned int) & (((struct hi *)0)->b); printf("%d",num); printf("%d",*(int *)((char *)ptr + (unsigned int) & (((struct hi *)0)->b))); } The output I get is 44. But how does it work?

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• how to write an address (pointer) in a text file that can be read and dereferenced in C

  - by hatorade

  I would like to write an address in a text file, which can be read by fscanf and dereferenced by C after reading the pointer. How do I write the address in the file?

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• Assigning a pointer variable to a const int in C++?

  - by John

  I'm wondering if anyone can explain the following to me: If I write int i = 0; float* pf = i; I get a compile error (gcc 4.2.1): error: invalid conversion from ‘int’ to ‘float*’ Makes sense - they are obviously two completely different types. But if instead I write const int i = 0; float* pf = i; It compiles without error. Why should the 'const' make a difference on the right hand side of the assignment? Isn't part of the idea of the 'const' keyword to be able to enforce type constraints for constant values? Any explanation I have been able to come up with feels kind of bogus. And none of my explanations also explain the fact that const int i = 1; float* pf = i; fails to compile. Can anyone offer an explanation?

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