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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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  • make it simpler and efficient

    - by gcc
    temp1=*tutar[1]; //i hold input in char *tutar[] if(temp1!='x'||temp1!='n') arrays[1]=malloc(sizeof(int)*num_arrays); //if second input is int a=0; n=i; for(i=1;i<n;++i) { temp1=*tutar[i]; if(temp1=='d') { ++i; j=atoi(tutar[i]); free(arrays[j]); continue; } if(temp1=='x') break; if(temp1=='n')//if it is n { a=0; ++j; arrays[j]=malloc(sizeof(int)*num_arrays);//create and allocate continue; } ++a; if(a>num_arrays) //resize the array arrays[j]=realloc(arrays[j],sizeof(int)*(num_arrays+a)); *(arrays[j]+a-1)=atoi(tutar[i]); printf("%d",arrays[1][1]); } arrays is pointer when you see x exit you see n create (old one is new array[a] new one is array[i+1]) you see d delete arrays[i] according to int after d first number is size of max arrays and where is the error in code input is composed from int and n d x i make a program -taking input(first input must be int) -according to input(there is comman in input like n or d or j , i fill array with number and use memory efficiently -j is jumb to array[x] ( x is int coming after j in input)

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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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  • Any pitfalls using char* instead of void* when writing cross platform code?

    - by UberMongoose
    Is there any pitfalls when using char*'s to write cross platform code that does memory access? I'm working on a play memory allocator to better understand how to debug memmory issues. I have come to believe char*'s are preferable because of the ability to do pointer arithmetic and derefernce them over void*'s, is that true? Do the following assumptions always hold true on different common platforms? sizeof(char) == 1 sizeof(char*) == sizeof(void*) sizeof(char*) == sizeof(size_t)

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  • Cannot determine why pointer variable will not address elements in a string in this program?

    - by Smith Will Suffice
    I am attempting to utilize a pointer variable to access elements of a string and there are issues with my code generating a compilation error: #include <stdio.h> #define MAX 29 char arrayI[250]; char *ptr; int main(void) { ptr = arrayI; puts("Enter string to arrayI: up to 29 chars:\n"); fgets(arrayI, MAX, stdin); printf("\n Now printing array by pointer:\n"); printf("%s", *ptr); ptr = arrayI[1]; //(I set the pointer to the second array char element) printf("%c", *ptr); //Here is where I was wanting to use my pointer to //point to individual array elements. return 0; } My compiler crieth: [Warning] assignment makes pointer from integer without a cast [enabled by default] I do not see where my pointer was ever assigned to the integer data type? Could someone please explain why my attempt to implement a pointer variable is failing? Thanks all!

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  • Using traversal by pointer to check whether a string is repeated

    - by Bob John
    bool repeat_char(char *s, int n); //R: s is a C-string of at least n non-NUL characters and n > 0 //E: returns true if the first n characters are fully repeated throughout the string s, false // otherwise. I'm having trouble implementing this function using traversal by pointer. I was thinking that I could extract the first n characters from s, then use that in a comparison with s, but I'm not sure how I could do that. If I'm traversing through s one character at a time, how can I check that it matches a block of text, such as the first n characters of s? Thanks!

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  • in c++ what is bettr to delete poiner or set it with 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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  • Declaring pointers; asterisk on the left or right of the space between the type and name?

    - by GenTiradentes
    I've seen mixed versions of this in a lot of code. (This applies to C and C++, by the way.) People seem to declare pointers in one of two ways, and I have no idea which one is correct, of if it even matters. The first way it to put the asterisk adjacent the type name, like so: someType* somePtr; The second way is to put the asterisk adjacent the name of the variable, like so: someType *somePtr; This has been driving me nuts for some time now. Is there any standard way of declaring pointers? Does it even matter how pointers are declared? I've used both declarations before, and I know that the compiler doesn't care which way it is. However, the fact that I've seen pointers declared in two different ways leads me to believe that there's a reason behind it. I'm curious if either method is more readable or logical in some way that I'm missing.

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  • How do I marshal a pointer to an array of pointers to structures?

    - by Daniel Stutzbach
    I have a C function with the following signature: int my_function(int n, struct player **players) players is a pointer to an array of pointers to struct player objects. n is the number of pointers in the array. The function does not modify the array nor the contents of the structures, and it does not retain any pointers after returning. I tried the following: [DllImport("mylibary.dll")] static extern int my_function(int n, [In, MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 0)] player_in []players); However, that marshals the data as a pointer to an array of structures, not a pointer to an array of pointers to structures.

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  • Does Ctypes Structures and POINTERS automatically free the memory when the Python object is deleted?

    - by jsbueno
    When using Python CTypes there are the Structures, that allow you to clone c-structures on the Python side, and the POINTERS objects that create a sofisticated Python Object from a memory address value and can be used to pass objects by reference back and forth C code. What I could not find on the documentation or elsewhere is what happens when a Python object containing a Structure class that was de-referenced from a returning pointer from C Code (that is - the C function alocated memory for the structure) is itself deleted. Is the memory for the original C structure freed? If not how to do it? Furthermore -- what if the Structure contains Pointers itself, to other data that was also allocated by the C function? Does the deletion of the Structure object frees the Pointers onits members? (I doubt so) Else - -how to do it? Trying to call the system "free" from Python for the Pointers in the Structure is crashing Python for me. In other words, I have this structure filled up by a c Function call: class PIX(ctypes.Structure): """Comments not generated """ _fields_ = [ ("w", ctypes.c_uint32), ("h", ctypes.c_uint32), ("d", ctypes.c_uint32), ("wpl", ctypes.c_uint32), ("refcount", ctypes.c_uint32), ("xres", ctypes.c_uint32), ("yres", ctypes.c_uint32), ("informat", ctypes.c_int32), ("text", ctypes.POINTER(ctypes.c_char)), ("colormap", ctypes.POINTER(PIXCOLORMAP)), ("data", ctypes.POINTER(ctypes.c_uint32)) ] And I want to free the memory it is using up from Python code.

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  • C++: How to build an events / messaging system without void pointers?

    - by Jarx
    I'd like to have a dynamic messaging system in my C++ project, one where there is a fixed list of existing events, events can be triggered anywhere during runtime, and where you can subscribe callback functions to certain events. There should be an option for arguments passed around in those events. For example, one event might not need any arguments (EVENT_EXIT), and some may need multiple ones (EVENT_PLAYER_CHAT: Player object pointer, String with message) The first option for making this possible is allowing to pass a void pointer as argument to the event manager when triggering an event, and receiving it in the callback function. Although: I was told that void pointers are unsafe and I shouldn't use them. How can I keep (semi) dynamic argument types and counts for my events whilst not using void pointers?

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  • Is it possible to store pointers in shared memory without using offsets?

    - by Joseph Garvin
    When using shared memory, each process may mmap the shared region into a different area of their address space. This means that when storing pointers within the shared region, you need to store them as offsets of the start of the shared region. Unfortunately, this complicates use of atomic instructions (e.g. if you're trying to write a lock free algorithm). For example, say you have a bunch of reference counted nodes in shared memory, created by a single writer. The writer periodically atomically updates a pointer 'p' to point to a valid node with positive reference count. Readers want to atomically write to 'p' because it points to the beginning of a node (a struct) whose first element is a reference count. Since p always points to a valid node, incrementing the ref count is safe, and makes it safe to dereference 'p' and access other members. However, this all only works when everything is in the same address space. If the nodes and the 'p' pointer are stored in shared memory, then clients suffer a race condition: x = read p y = x + offset Increment refcount at y During step 2, p may change and x may no longer point to a valid node. The only workaround I can think of is somehow forcing all processes to agree on where to map the shared memory, so that real pointers rather than offsets can be stored in the mmap'd region. Is there any way to do that? I see MAP_FIXED in the mmap documentation, but I don't know how I could pick an address that would be safe.

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  • Assigning a 2D array (of pointers) to a variable in an object for access in C++?

    - by MrMormon
    I'm sorry if I didn't pick a descriptive or concise name. A lot of questions sound similar, but I haven't been able to find what I'm looking for. What I want to do is store a 2D array of pointers somewhere and assign a variable in some object to that array to be able to access it. Here's some example code that has the same compile error I'm getting with a bigger project. #include <iostream> using namespace std; struct X{ float * b[8][8]; X(){ *(b[1][5]) = 1; cout << *(b[1][5]) << endl; } void Set(float * c[8][8]){ b = c; cout << *(b[1][5]) << endl; } }; main(){ float * a[8][8]; *(a[1][5]) = 2; X obj; obj.Set(a); } What I want to happen in this code is that an X object starts with its own 2D array, whose value pointed to by b[1][5] should be printed as "1". Then the main method's 2D array, a, is passed to the object's Set() method and assigned to its array variable. The value pointed to by b[1][5] should then be printed as "2". However, I can't figure out what type the Set() parameter, c, should be. I get error: incompatible types in assignment of ‘float* (*)[8]’ to ‘float* [8][8]’ when I try to compile. As for why I want to do this, I'm trying to use an array of pointers to objects, not floats, but it's the same error.

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  • Using a function with reference as a function with pointers?

    - by epatel
    Today I stumbled over a piece of code that looked horrifying to me. The pieces was chattered in different files, I have tried write the gist of it in a simple test case below. The code base is routinely scanned with FlexeLint on a daily basis, but this construct has been laying in the code since 2004. The thing is that a function implemented with a parameter passing using references is called as a function with a parameter passing using pointers...due to a function cast. The construct has worked since 2004 on Irix and now when porting it actually do work on Linux/gcc too. My question now. Is this a construct one can trust? I can understand if compiler constructors implement the reference passing as it was a pointer, but is it reliable? Are there hidden risks? Should I change the fref(..) to use pointers and risk braking anything in the process? What to you think? #include <iostream> using namespace std; // ---------------------------------------- // This will be passed as a reference in fref(..) struct string_struct { char str[256]; }; // ---------------------------------------- // Using pointer here! void fptr(const char *str) { cout << "fptr: " << str << endl; } // ---------------------------------------- // Using reference here! void fref(string_struct &str) { cout << "fref: " << str.str << endl; } // ---------------------------------------- // Cast to f(const char*) and call with pointer void ftest(void (*fin)()) { void (*fcall)(const char*) = (void(*)(const char*))fin; fcall("Hello!"); } // ---------------------------------------- // Let's go for a test int main() { ftest((void (*)())fptr); // test with fptr that's using pointer ftest((void (*)())fref); // test with fref that's using reference return 0; }

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  • Is it worth setting pointers to NULL in a destructor?

    - by Rob
    Imagine I have a class that allocates memory (forget about smart pointers for now): class Foo { public: Foo() : bar(new Bar) { } ~Foo() { delete bar; } void doSomething() { bar->doSomething(); } private: Bar* bar; }; As well as deleting the objects in the destructor is it also worth setting them to NULL? I'm assuming that setting the pointer to NULL in the destructor of the example above is a waste of time.

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  • Compute the AES-encryption key given the plaintext and its ciphertext?

    - by Null Pointers etc.
    I'm tasked with creating database tables in Oracle which contain encrypted strings (i.e., the columns are RAW). The strings are encrypted by the application (using AES, 128-bit key) and stored in Oracle, then later retrieved from Oracle and decrypted (i.e., Oracle itself never sees the unencrypted strings). I've come across this one column that will be one of two strings. I'm worried that someone will notice and presumably figure out what those two values to figure out the AES key. For example, if someone sees that the column is either Ciphertext #1 or #2: Ciphertext #1: BF,4F,8B,FE, 60,D8,33,56, 1B,F2,35,72, 49,20,DE,C6. Ciphertext #2: BC,E8,54,BD, F4,B3,36,3B, DD,70,76,45, 29,28,50,07. and knows the corresponding Plaintexts: Plaintext #1 ("Detroit"): 44,00,65,00, 74,00,72,00, 6F,00,69,00, 74,00,00,00. Plaintext #2 ("Chicago"): 43,00,68,00, 69,00,63,00, 61,00,67,00, 6F,00,00,00. can he deduce that the encryption key is "Buffalo"? 42,00,75,00, 66,00,66,00, 61,00,6C,00, 6F,00,00,00. I'm thinking that there should be only one 128-bit key that could convert Plaintext #1 to Ciphertext #1. Does this mean I should go to a 192-bit or 256-bit key instead, or find some other solution? (As an aside, here are two other ciphertexts for the same plaintexts but with a different key.) Ciphertext #1 A ("Detroit"): E4,28,29,E3, 6E,C2,64,FA, A1,F4,F4,96, FC,18,4A,C5. Ciphertext #2 A ("Chicago"): EA,87,30,F0, AC,44,5D,ED, FD,EB,A8,79, 83,59,53,B7.

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  • How do I implement something like pointers in javascript?

    - by Shaun
    I know that javascript doesn't have pointers in terms of a variable referring to a place in memory but what I have is a number of variables which are subject to change and dependent on each other. For example: Center (x,y) = (offsetLeft + width/scale , offsetTop + height/scale) As of now I have rewritten the equation in terms of each individual variable and after any changes I call the appropriate update function. For example: If scale changes, then then the center, height, and width stay the same. So I call updateoffset() { offsetLeft = centerx - width/scale; offsetTop = centery - height/scale; } Is this the easiest way to update each of these variables when any of them changes?

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