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  • C++ - passing references to boost::shared_ptr

    - by abigagli
    If I have a function that needs to work with a shared_ptr, wouldn't it be more efficient to pass it a reference to it (so to avoid copying the shared_ptr object)? What are the possible bad side effects? I envision two possible cases: 1) inside the function a copy is made of the argument, like in ClassA::take_copy_of_sp(boost::shared_ptr<foo> &sp) { ... m_sp_member=sp; //This will copy the object, incrementing refcount ... } 2) inside the function the argument is only used, like in Class::only_work_with_sp(boost::shared_ptr<foo> &sp) //Again, no copy here { ... sp->do_something(); ... } I can't see in both cases a good reason to pass the boost::shared_ptr by value instead of by reference. Passing by value would only "temporarily" increment the reference count due to the copying, and then decrement it when exiting the function scope. Am I overlooking something? Andrea. EDIT: Just to clarify, after reading several answers : I perfectly agree on the premature-optimization concerns, and I alwasy try to first-profile-then-work-on-the-hotspots. My question was more from a purely technical code-point-of-view, if you know what I mean.

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  • [C] this code, its work fine and return what i want, but its hang before print it ??

    - by Rami Jarrar
    I make this program :: #include<stdio.h> char *raw_input(char *msg); main() { char *s; *s = *raw_input("Message Here Is: "); printf("Return Done.."); printf(s); } char *raw_input(char *msg){ char *d; printf("%s", msg); scanf("%s",&d); return d; } What this do is, it print my message and scan for input from the user, then print it,, but whats the problem in print the input from the user ???

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  • Normal pointer vs Auto pointer (std::auto_ptr)

    - by AKN
    Code snippet (normal pointer) int *pi = new int; int i = 90; pi = &i; int k = *pi + 10; cout<<k<<endl; delete pi; [Output: 100] Code snippet (auto pointer) Case 1: std::auto_ptr<int> pi(new int); int i = 90; pi = &i; int k = *pi + 10; //Throws unhandled exception error at this point while debugging. cout<<k<<endl; //delete pi; (It deletes by itself when goes out of scope. So explicit 'delete' call not required) Case 2: std::auto_ptr<int> pi(new int); int i = 90; *pi = 90; int k = *pi + 10; cout<<k<<endl; [Output: 100] Can someone please tell why it failed to work for case 1?

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  • Whats the problem with int *p=23;

    - by piemesons
    Yesterday in my interview I was asked this question. (At that time I was highly pressurized by so many abrupt questions). int *p; *p=23; printf('%d',*p); Is there any problem with this code? I explained him that you are trying to assign value to a pointer to whom memory is not allocated. But the way he reacted, it was like I am wrong. Although I got the job but after that he said Mohit think about this question again. I don't know what he was trying to say. Please let me know is there any problem in my answer?

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  • declaring a 2D array of pointer objects

    - by Tyler Stennette
    I'm having a tough time figuring out how to instantiate a 2D array of pointer objects. Here is how I'm doing it: Pieces* chessBoard[9][9]; When I want to set it to an actual object pointer, I'm doing the following: chessBoard[1][1] = new Rook(p1Rook); Rook is a class that inherits attributes from the Pieces class and p1Rook is a char variable set to 'R'. This class also implements virtual functions (not pure virtual) from Pieces such as move() or getPiece() that are unique to the particular chess piece. However, when I compile my program, I get the following error: ChessBoard.cpp:69: error: expected type-specifier before ‘Rook’ ChessBoard.cpp:69: error: cannot convert ‘int*’ to ‘Pieces*’ in assignment Can someone please explain what I should change to get rid of this rather annoying persistent error? I would appreciate it.

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  • Mutual class instances in C++

    - by SepiDev
    Hi guys. What is the issue with this code? Here we have two files: classA.h and classB.h classA.h: #ifndef _class_a_h_ #define _class_a_h_ #include "classB.h" class B; //???? class A { public: A() { ptr_b = new B(); //???? } virtual ~A() { if(ptr_b) delete ptr_b; //???? num_a = 0; } int num_a; B* ptr_b; //???? }; #endif //_class_a_h_ classB.h: #ifndef _class_b_h_ #define _class_b_h_ #include "classA.h" class A; //???? class B { public: B() { ptr_a = new A(); //???? num_b = 0; } virtual ~B() { if(ptr_a) delete ptr_a; //???? } int num_b; A* ptr_a; //???? }; #endif //_class_b_h_ when I try to compile it, the compiler (g++) says: classB.h: In constructor ‘B::B()’: classB.h:12: error: invalid use of incomplete type ‘struct A’ classB.h:6: error: forward declaration of ‘struct A’ classB.h: In destructor ‘virtual B::~B()’: classB.h:16: warning: possible problem detected in invocation of delete operator: classB.h:16: warning: invalid use of incomplete type ‘struct A’ classB.h:6: warning: forward declaration of ‘struct A’ classB.h:16: note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined.

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  • Write pointer to file in C

    - by Sergey
    I have a stucture: typedef structure student { char *name; char *surname; int age; } Student; I need to write it to binary file. Student *s = malloc(sizeof(*s)); I fill my structure with data and then i write in to the file: fwrite(s, sizeof(*s), 1, fp); In my file doesnt exist a name and surname, it have an adresses of char*. How can i write to file a word, not an adresses?

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  • Pointer arithmetic.

    - by Knowing me knowing you
    Having code: int** a = new int*[2]; a[0] = new int(1); a[1] = new int(2); cout << "a[0] " << a[0] << '\n'; cout << "a[1] " << a[1] << '\n'; cout << "a[2] " << a[2] << '\n'; cout << "a[0] + 1 " << a[0] + 1 << '\n';//WHY THIS ISN'T == a[1] ? cout << "*(a + 1): " << *(a + 1) << '\n'; //WHY THIS IS == a[1] ? cout << "a[0] - a[1] " << static_cast<int>(a[0] - a[1])<< '\n';//WHY THIS IS == 16 not 4? cout << sizeof(int**); Questions are included right next to relevant lines in code.

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  • Create thread is not accepting the member functon

    - by prabhakaran
    I am trying to create a class for network programming. This will create a general purpose socket with thread. But when I tried to creat the thread using createthread(). The third argument is producing errors. And from the net I came to know that I can't use the member functions as a argument to the createthread(). Is there any thing by which I can achieve this.

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  • Help with data retrieval MACRO

    - by Andrei Ciobanu
    Hello, given the following structure: struct nmslist_elem_s { nmptr data; struct nmslist_elem_s *next; }; typedef struct nmslist_elem_s nmslist_elem; Where: typedef void* nmptr; Is it possible to write a MACRO that retrieves the data from the element and cast it to the right type: MACRO(type, element) that expands to *((type*)element->data). For example for int, i would need something like this: *((int*)(element->data)) .

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  • C Pointer Question: &(*struct->struct)

    - by NTek
    I have a struct defined with the structure as follows (names are different) struct str1 { int field1; struct str2; } And I have a *str1 in a function. I'd like to get a pointer to str2. So I tried &(str1->str2) and was hoping this would return a pointer to str2. Is this incorrect? It doesn't seem to be working. How would I get a pointer to str2 if given a pointer to str1?

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  • how to cast c++ smart pointer up and down

    - by user217428
    two clients communicate to each other on top of a message layer in the message body, I need include a field pointing to any data type From client A, I send the field as a shared_ptr to the message layer. I define this field as a shared_ptr in the message layer. But how can I convert this field back to shared_ptr in client B? Or should I define shared_ptr in message layer as something else? Thanks

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  • Passing a template func. as a func. ptr to an overloaded func. - is there a way to compile this code

    - by LoudNPossiblyRight
    Just a general c++ curiosity: This code below shouldn't compile because it's impossible to know which to instantiate: temp(const int&) or temp(const string&) when calling func(temp) - this part i know. What i would like to know is if there is anything i can do to the line marked PASSINGLINE to get the compiler to deduce that i want FPTR1 called and not FPTR2 ? #include<iostream> using std::cout; using std::endl; /*FPTR1*/ void func(void(*fptr)(const int&)){ fptr(1001001);} /*FPTR2*/ void func(void(*fptr)(const string&)){ fptr("1001001"); } template <typename T> void temp(const T &t){ cout << t << endl; } int main(){ /*PASSINGLINE*/ func(temp); return 0; } Thank you.

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  • Passing a 2D array to a function in C

    - by Tyler Treat
    I have, essentially, a matrix of data (lets say ints) that I would like to store in a 2D array in which the dimensions are not known until runtime (say x columns and y rows). I want to populate the array in a function, so I assume I need to do something like this: int main(int argc, char **argv) { int y = atoi(argv[1]); int x = atoi(argv[2]); int **matrix = malloc(x * sizeof(int*)); populateMatrix(matrix, y, x); return 0; } void populateMatrix(**matrix, int y, int x) { int i, j; for (i = 0; i < y; i++) { for (j = 0; j < x; j++) { matrix[i][j] = i * j; // populated with trivial data to keep it simple } } } Obviously this doesn't work, but I'm not sure how to do what I'm describing exactly.

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  • Pointer to local variable

    - by Radek Šimko
    May I have any acces to local variable in different function? If may, how? void replaceNumberAndPrint(int array[3]) { printf("%i\n", array[1]); printf("%i\n", array[1]); } int * getArray() { int myArray[3] = {4, 65, 23}; return myArray; } int main() { replaceNumberAndPrint(getArray()); } The output of the piece of code above: 65 4202656 What am i doing wrong? What the "4202656" means?? Do I have to copy the whole array in the replaceNumberAndPrint() function to be able to access to it more than first times?

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  • What is the merit of the "function" type (not "pointer to function")

    - by anatolyg
    Reading the C++ Standard, i see that there are "function" types and "pointer to function" types: typedef int func(int); // function typedef int (*pfunc)(int); // pointer to function typedef func* pfunc; // same as above I have never seen the function types used outside of examples (or maybe i didn't recognize their usage?). Some examples: func increase, decrease; // declares two functions int increase(int), decrease(int); // same as above int increase(int x) {return x + 1;} // cannot use the typedef when defining functions int decrease(int x) {return x - 1;} // cannot use the typedef when defining functions struct mystruct { func add, subtract, multiply; // declares three member functions int member; }; int mystruct::add(int x) {return x + member;} // cannot use the typedef int mystruct::subtract(int x) {return x - member;} int main() { func k; // the syntax is correct but the variable k is useless! mystruct myobject; myobject.member = 4; cout << increase(5) << ' ' << decrease(5) << '\n'; // outputs 6 and 4 cout << myobject.add(5) << ' ' << myobject.subtract(5) << '\n'; // 9 and 1 } Seeing that the function types support syntax that doesn't appear in C (declaring member functions), i guess they are not just a part of C baggage that C++ has to support for backward compatibility. So is there any use for function types, other than demonstrating some funky syntax?

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  • bubble sort on array of c structures not sorting properly

    - by xmpirate
    I have the following program for books record and I want to sort the records on name of book. the code isn't showing any error but it's not sorting all the records. #include "stdio.h" #include "string.h" #define SIZE 5 struct books{ //define struct char name[100],author[100]; int year,copies; }; struct books book1[SIZE],book2[SIZE],*pointer; //define struct vars void sort(struct books *,int); //define sort func main() { int i; char c; for(i=0;i<SIZE;i++) //scanning values { gets(book1[i].name); gets(book1[i].author); scanf("%d%d",&book1[i].year,&book1[i].copies); while((c = getchar()) != '\n' && c != EOF); } pointer=book1; sort(pointer,SIZE); //sort call i=0; //printing values while(i<SIZE) { printf("##########################################################################\n"); printf("Book: %s\nAuthor: %s\nYear of Publication: %d\nNo of Copies: %d\n",book1[i].name,book1[i].author,book1[i].year,book1[i].copies); printf("##########################################################################\n"); i++; } } void sort(struct books *pointer,int n) { int i,j,sorted=0; struct books temp; for(i=0;(i<n-1)&&(sorted==0);i++) //bubble sort on the book name { sorted=1; for(j=0;j<n-i-1;j++) { if(strcmp((*pointer).name,(*(pointer+1)).name)>0) { //copy to temp val strcpy(temp.name,(*pointer).name); strcpy(temp.author,(*pointer).author); temp.year=(*pointer).year; temp.copies=(*pointer).copies; //copy next val strcpy((*pointer).name,(*(pointer+1)).name); strcpy((*pointer).author,(*(pointer+1)).author); (*pointer).year=(*(pointer+1)).year; (*pointer).copies=(*(pointer+1)).copies; //copy back temp val strcpy((*(pointer+1)).name,temp.name); strcpy((*(pointer+1)).author,temp.author); (*(pointer+1)).year=temp.year; (*(pointer+1)).copies=temp.copies; sorted=0; } *pointer++; } } } My Imput The C Programming Language X Y Z 1934 56 Inferno Dan Brown 1993 453 harry Potter and the soccers stone J K Rowling 2012 150 Ruby On Rails jim aurther nil 2004 130 Learn Python Easy Way gmaps4rails 1967 100 And the output ########################################################################## Book: Inferno Author: Dan Brown Year of Publication: 1993 No of Copies: 453 ########################################################################## ########################################################################## Book: The C Programming Language Author: X Y Z Year of Publication: 1934 No of Copies: 56 ########################################################################## ########################################################################## Book: Ruby On Rails Author: jim aurther nil Year of Publication: 2004 No of Copies: 130 ########################################################################## ########################################################################## Book: Learn Python Easy Way Author: gmaps4rails Year of Publication: 1967 No of Copies: 100 ########################################################################## ########################################################################## Book: Author: Year of Publication: 0 No of Copies: 0 ########################################################################## We can see the above sorting is wrong? What I'm I doing wrong?

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  • Another dynamic memory allocation bug.

    - by m4design
    I'm trying to allocate memory for a multidimensional array (8 rows, 3 columns). Here's the code for the allocation (I'm sure the error is clear for you) char **ptr = (char **) malloc( sizeof(char) * 8); for (i = 0; i < 3; i++) ptr[i] = (char *) malloc( sizeof(char) * 3); The crash happens when I reference this: ptr[3][0]; Unhandled exception at 0x0135144d in xxxx.exe: 0xC0000005: Access violation writing location 0xabababab. Are there any recommended references/readings for this kind of subject? Thanks.

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  • C++, function pointer to the template function pointer

    - by Ian
    I am having a pointer to the common static method class MyClass { private: static double ( *pfunction ) ( const Object *, const Object *); ... }; pointing to the static method class SomeClass { public: static double getA ( const Object *o1, const Object *o2); ... }; Initialization: double ( *MyClass::pfunction ) ( const Object *o1, const Object *o2 ) = &SomeClass::getA; I would like to convert this pointer to the static template function pointer: template <class T> static T ( *pfunction ) ( const Object <T> *, const Object <T> *); //Compile error where: class SomeClass { public: template <class T> static double getA ( const Object <T> *o1, const Object <T> *o2); ... }; But there is some error... Thanks for your help...

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  • c++ File input/output

    - by Myx
    Hi: I am trying to read from a file using fgets and sscanf. In my file, I have characters on each line of the while which I wish to put into a vector. So far, I have the following: FILE *fp; fp = fopen(filename, "r"); if(!fp) { fprintf(stderr, "Unable to open file %s\n", filename); return 0; } // Read file int line_count = 0; char buffer[1024]; while(fgets(buffer, 1023, fp)) { // Increment line counter line_count++; char *bufferp = buffer; ... while(*bufferp != '\n') { char *tmp; if(sscanf(bufferp, "%c", tmp) != 1) { fprintf(stderr, "Syntax error reading axiom on " "line %d in file %s\n", line_count, filename); return 0; } axiom.push_back(tmp); printf("put %s in axiom vector\n", axiom[axiom.size()-1]); // increment buffer pointer bufferp++; } } my axiom vector is defined as vector<char *> axiom;. When I run my program, I get a seg fault. It happens when I do the sscanf. Any suggestions on what I'm doing wrong?

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  • How to read some bytes from BYTE*

    - by chekalin-v
    I have BYTE pointer. For example the length of this BYTE array is 10. How can I read 4 bytes from 3 position BYTE array? Now I doing it so BYTE *source = "1234567890\0"; BYTE* tmp = new BYTE[4+1](); for(int i=0; i<4; i++) { tmp[i] = source[i+3]; }

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  • Double indirection and structures passed into a function

    - by ZPS
    I am curious why this code works: typedef struct test_struct { int id; } test_struct; void test_func(test_struct ** my_struct) { test_struct my_test_struct; my_test_struct.id=267; *my_struct = &my_test_struct; } int main () { test_struct * main_struct; test_func(&main_struct); printf("%d\n",main_struct->id); } This works, but pointing to the memory address of a functions local variable is a big no-no, right? But if i used a structure pointer and malloc, that would be the correct way, right? void test_func(test_struct ** my_struct) { test_struct *my_test_struct; my_test_struct = malloc(sizeof(test_struct)); my_test_struct->id=267; *my_struct = my_test_struct; } int main () { test_struct * main_struct; test_func(&main_struct); printf("%d\n",main_struct->id); }

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  • What does it mean to pass a &variable to a function? E.g., string& insert ( size_t pos1, const strin

    - by Bob Montgomery
    I understand passing a pointer, and returning a pointer: char * strcat ( char * destination, const char * source ); You're passing a variable that contains the address to a char; returning the same. But what does it mean to pass something using the reference operator? Or to return it? string& insert ( size_t pos1, const string& str ); I mean, I understand what actually happens, I just don't understand the notation. Why isn't the notation this instead: string * insert ( size_t pos1, const string * str ); //made up I presume it has something to do with passing/returning the instance of a class, but what? Is this syntax valid; if not why not and if so what does it mean? char & strcat ( char & destination, const char & source ); //made up (all of the function declarations, except the last made-up two, are from http://www.cplusplus.com )

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