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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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• 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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• 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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• Can somebody please explain this recursive function for me?

  - by capncoolio

  #include <stdio.h> #include <stdlib.h> void reprint(char *a[]) { if(*a) { printf("%d ",a); reprint(a+1); printf("%s ",*a); } } int main() { char *coll[] = {"C", "Objective", "like", "don't", "I", NULL}; reprint(coll); printf("\n"); return EXIT_SUCCESS; } As the more experienced will know, this prints the array in reverse. I don't quite understand how! I need help understanding what reprint(char *a[]) does. I understand pointer arithmetic to a degree, but from inserting printf's here and there, I've determined that the function increments up to the array end, and then back down to the start, only printing on the way down. However, I do not understand how it does this; all I've managed to understand by looking at the actual code is that if *a isn't NULL, then call reprint again, at the next index. Thanks guys!

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• segmentation fault when using pointer to pointer

  - by user3697730

  I had been trying to use a pointer to pointer in a function,but is seems that I am not doing the memory allocation correctly... My code is: #include<stdio.h> #include<math.h> #include<ctype.h> #include<stdlib.h> #include<string.h> struct list{ int data; struct list *next; }; void abc (struct list **l,struct list **l2) { *l2=NULL; l2=(struct list**)malloc( sizeof(struct list*)); (*l)->data=12; printf("%d",(*l)->data); (*l2)->next=*l2; } int main() { struct list *l,*l2; abc(&l,&l2); system("pause"); return(0); } This code compiles,but I cannot run thw program..I get a segmentation fault..What should I do?Any help would be appreciated!

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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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• 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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• 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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• 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 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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• How to pass a member function to a function used in another member function?

  - by Tommaso Ferrari

  I found something about my problem, but I don't already understand very well. I need to do something like this: class T{ double a; public: double b; void setT(double par){ a=par; }; double funct(double par1) { return par1/a; } void exec(){ b=extfunct(funct, 10); } } double extfunct(double (*f)(double),double par2){ return f(par2)+5; } Operation and function are only for example, but the structure is that. The reason of this structure is that I have a precostituited class which finds the minimum of a gived function (it's extfunct in the example). So I have to use it on a function member of a class. I understood the difference between pointer to function and pointer to member function, but I don't understand how to write it. Thanks, and sorry for the poor explanation of the problem.

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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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• collect string in loop and printout all the string outside loop

  - by user1508163

  I'm newbie here and there is some question that I want have some lesson from you guys. For example: #include <stdio.h> #include<stdlib.h> #include<ctype.h> void main() { char name[51],selection; do { printf("Enter name: "); fflush(stdin); gets(name); printf("Enter another name?(Y/N)"); scanf("%c",&selection); selection=toupper(selection); }while (selection=='Y'); //I want to printout the entered name here but dunno the coding printf("END\n"); system("pause"); } As I know when the loops perform will overwrite the variable then how I perform a coding that will printout all the name user entered? I have already ask my tutor and he is ask me to use pointer, can anyone guide me in this case?

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• Is there a way to have a bit bucket pointer? (C/C++)

  - by Crazy Chenz

  Is there a way to have a bit bucket pointer? A lot of IO (specifically input related) system calls return data to a buffer of a specific size. Is there a trick or way to make a sorta bit bucket pointer, so I can accept any amount of data that will be thrown away. Doing something like "char tmp[INT_MAX]" is crazy. The behavior I am looking for is something like /dev/null, only in a pointer world. Not to hopeful on this.... just curious. Thanks, Chenz UPDATE: Perhaps mmap-ing /dev/null. Forgot about that when I asked the question.

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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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• Evaluation of (de)reference operators

  - by Micha

  I have an (uncommented...) source file which I'm trying to understand. static const Map *gCurMap; static std::vector<Map> mapVec; then auto e = mapVec.end(); auto i = mapVec.begin(); while(i!=e) { // ... const Map *map = gCurMap = &(*(i++)); // ... } I don't understand what &(*(i++)) does. It does not compile when just using i++, but to me it looks the same, because I'm "incrementing" i, then I'm requesting the value at the given address and then I'm requesting the address of this value?!

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• C++ finding pointer errors

  - by Tony

  Hello! I am currently on a project which has been buggy for a long while, now I suspect, among other things, that there is pointer errors in the code. The program is written in native C++ using COM and uses out-of-process COM servers. Can anybody give me some tips as how one would go about finding these errors? Is there specific things to keep in mind, ways to do it, etc...

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• Set argument pointer to point to new memory inside a function (without returning it) IN C

  - by user321605

  Hello, Hopefully my title was descriptive enough to attract the right help. I want to write a function that will return 1 thing, and modify a provided pointer in another. My current function declaration is . . . char * afterURL replaceURLS(char * body) What I want to do is copy all of body's data into a new string, and set body to point to this new data. I then want afterURL to point to a location within the new string. My issue is getting the actual pointer that is passed in to this function to point to the new data. Thanks in advance! Rob

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• How to guard against memory leaks?

  - by just_wes

  I was recently interviewing for a C++ position, and I was asked how I guard against creating memory leaks. I know I didn't give a satisfactory answer to that question, so I'm throwing it to you guys. What are the best ways to guard against memory leaks? Thanks!

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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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• assignment from incompatible pointer type

  - by Hristo

  I have set up the following struct: typedef struct _thread_node_t { pthread_t thread; struct thread_node_t *next; } thread_node_t; ... and then I have defined: // create thread to for incoming connection thread_node_t *thread_node = (thread_node_t*) malloc(sizeof(thread_node_t)); pthread_create(&(thread_node->thread), NULL, client_thread, &csFD); thread_node->next = thread_arr; // assignment from incompatible pointer type thread_arr = thread_node; where thread_arr is thread_node_t *thread_arr = NULL; I don't understand why the compiler is complaining. Maybe I'm misunderstanding something.

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• C++ - Difference between (*). and -> ?

  - by Rachel

  Is there any difference in performance - or otherwise - between: ptr->a(); and (*ptr).a(); ?

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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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