C/C++, troubles with pointers and functions - c++

I have a little question about understand how pointers and functions work.
I want to see how a function looks like qsort(), but I need to use my own function to swap elements and to compare elements. I am very surprised to know that my function does not swap data...
My code:
//prototypes file: other.h
void Sort(char* pcFirst, int nNumber, int size, void (*Swap)(void*, void*), int (*Compare)(void*, void*) ); //sorts any arrays
void SwapInt(void* p1, void* p2); // swap pointers
int CmpInt(void* p1, void* p2); // compare poineters
//realisation file: other.cpp
#include "other.h"
void Sort(char* pcFirst, int nNumber, int size,
void (*Swap)(void*, void*), int (*Compare)(void*, void*) )
{
int i;
for( i = 1; i < nNumber; i++)
for(int j = nNumber - 1; j >= i; j--)
{
char* pCurrent = pcFirst + j * size;
char* pPrevious = pcFirst + (j - 1) * size;
if( (*Compare)( pPrevious, pCurrent ) > 0 )// if > 0 then Swap
{
(*Swap)( pPrevious, pCurrent );
}
}
}
void SwapInt(void* p1, void* p2)
{
int * ptmp1 = static_cast<int*>(p1);
int * ptmp2 = static_cast<int*>(p2);
int * ptmp = ptmp1;
ptmp1 = ptmp2;
ptmp2 = ptmp;
}
int CmpInt(void* p1, void* p2)
{
int nResult;
int * ptmp1 = static_cast<int*>(p1);
int * ptmp2 = static_cast<int*>(p2);
nResult = (*ptmp1 - *ptmp2);
return nResult;
}
//main file: lab.cpp
#include <tchar.h>
#include <iostream>
#include <cstdio>
#include <cmath>
#include "other.h"
int _tmain()
{
int nAr[] = {33,44,55,22,11}; //array for sort
int nTotal = sizeof(nAr) / sizeof(int); //number of elements
for ( int i = 0; i < nTotal; i++)
{
printf("%d ",nAr[i]); // result of cycle is 33 44 55 22 11
}
Sort(reinterpret_cast<char*>(&nAr[0]), nTotal, sizeof(int), SwapInt, CmpInt);
for ( int i = 0; i < nTotal; i++)
{
printf("%d ",nAr[i]); // result of cycle is 33 44 55 22 11 too =(
}
}
Why does the array not change?
In the debugger I can see that all pointers change, and get correct values, but in main my array is not changed.

pointers point to objects
the code
int * ptmp = ptmp1;
ptmp1 = ptmp2;
ptmp2 = ptmp;
changes some pointer values locally in the function, and that's all.
in order to swap the values of two objects, pass them by reference:
void swap_values_of( int& a, int& b )
{
int const original_a = a;
a = b;
b = original_a;
}
you can also do that, less safely, with pointer arguments, then taking care to swap the values pointed to instead of the pointers themselves.
but except for purposes of learning, use std::swap instead
not asked for, but... if you change the current Microsoft-specific
int _tmain()
to just standard
int main()
then the code will (much more likely) work also in e.g. Linux.
just a tip

You may look at various combinations as these.....
#include<iostream>
#include<stdio.h>
#include<malloc.h>
//Call by Address
void SwapIntAddr(int* ptmp1, int* ptmp2)
{
int ptmp;
ptmp = *ptmp1;
*ptmp1 = *ptmp2;
*ptmp2 = ptmp;
}
//Call by Reference
void SwapIntRef(int& ptmp1, int& ptmp2)
{
int ptmp;
ptmp = ptmp1;
ptmp1 = ptmp2;
ptmp2 = ptmp;
}
//Call by Reference but in pointer level
void SwapPtrRef(int*& ptmp1, int*& ptmp2)
{
int* ptmp;
ptmp = ptmp1;
ptmp1 = ptmp2;
ptmp2 = ptmp;
}
//Call by Address but in Pointer level.
void SwapPtrAddr(int** ptmp1,int** ptmp2)
{
int** ptmp = (int**) malloc(sizeof(int*));
*ptmp = *ptmp1;
*ptmp1 = *ptmp2;
*ptmp2 = *ptmp;
}
int main(){
int a = 3, b= 5;
int* p1 = &a;
int* p2 = &b;
SwapIntAddr(p1,p2);
printf("%d %d\n",*p1,*p2);
SwapIntRef(*p1,*p2);
printf("%d %d\n",*p1,*p2);
SwapPtrRef(p1,p2);
printf("%d %d\n",*p1,*p2);
SwapPtrAddr(&p1,&p2);
printf("%d %d\n",*p1,*p2);
return 0;
}

Your SwapInt function swaps some pointers, not ints. Since all those pointers are local to SwapInt, it has no actual effect. Probably you meant to do something with the ints *ptmp1 and *ptmp2.

What you are actually doing is swapping pointers. What you are trying to do is to swap values, where that pointers point to. At least that comes from your program logic.
So your code could be something like this:
void SwapInt(void* p1, void* p2)
{
int * ptmp1 = static_cast<int*>(p1);
int * ptmp2 = static_cast<int*>(p2);
int ptmp = *ptmp1;
*ptmp1 = *ptmp2;
*ptmp2 = ptmp;
}

Related

Passing pointers of vectors with structures C++

I've declared the following function in C++
void setCellInfo (CELL_MESH* target, int Global_ID, int node0,vector<NODE_MESH *>* NodeStore, vector<CELL_MESH *>* CellStore) {
CellStore->push_back(target); //No Errors
target->Global_ID = Global_ID; //No Errors
if (node0 != 0) {
target->node[0] = NodeStore[vector<NODE_MESH *>::size_type(node0)]->ID; //ERROR 1
target->node_pointers[0] = NodeStore[vector<NODE_MESH *>::size_type(node0)]; //ERROR 2
}
}
ERROR1: Gives me a "No member named 'ID' in 'std::vector<NODE_MESH *>'" for the target->node[] attributions although its the entities from the pointers within the vector that have this ID member. Since I'm trying to get a specific entity in the vector using NodeStore[value], I would think it would work.
ERROR2: Gives me "Assigning to 'NODE_MESH *' from incompatible type 'vector<NODE_MESH *>'" for all the target->node_pointers attributions. This seems to be the same problem but with pointers directly (without the ID member).
the NodeStore and CellStore vectors a defined as follows outside the function
vector<NODE_MESH*> NodeStore;
vector<CELL_MESH*> CellStore;
I then try to use the function like this, with 'i' being the int Global_ID and 'nodes_x*y+x' being some integer.
CELL_MESH *newCell = new CELL_MESH;
setCellInfo (&newCell, i, nodes_x*y+x, &NodeStore, &CellStore);
I've tried many different alterations to pointers but can't get it work. Would you know how to ?
Here's a simplified complete version:
#include <vector>
using namespace std;
typedef struct NODE_MESH{
int ID;
}NODE_MESH;
typedef struct CELL_MESH{
int Global_ID;
NODE_MESH* node_pointers[4];
int node[4];
}CELL_MESH;
vector<NODE_MESH*> NodeStore;
vector<CELL_MESH*> CellStore;
double nodes_y = 5;
double nodes_x = 4;
int cells_y = 4;
int cells_x = 3;
void setCellInfo (CELL_MESH* target, int Global_ID, int node0,vector<NODE_MESH *>* NodeStore, vector<CELL_MESH *>* CellStore) {
CellStore->push_back(target); //No Errors
target->Global_ID = Global_ID; //No Errors
if (node0 != 0) {
target->node[0] = NodeStore[vector<NODE_MESH *>::size_type(node0)]->ID; //ERROR 1
target->node_pointers[0] = NodeStore[vector<NODE_MESH *>::size_type(node0)]; //ERROR 2
}
}
int main() {
int i = 0;
for (double y = 0; y < nodes_y; y++) {
for (double x = 0; x < nodes_x; x++) {
NODE_MESH *newNode = new NODE_MESH;
NodeStore.push_back(newNode);
newNode -> ID = i;
i++;
}
}
i = 0;
for (int y = 0; y < cells_y; y++) { //nodes_y since horizontal faces are aligned with nodes horizontaly (same y)
for (int x = 0; x < cells_x; x++) { //x coordinate for horizontal faces is in-between nodes so 0.5 with count for faces
CELL_MESH *newCell = new CELL_MESH;
setCellInfo (newCell, i, nodes_x*y+x, &NodeStore, &CellStore);
i++;
}
}
return 0;
}
Given a variable T* t, the syntax t[x] is equivalent to *(t+x), which is the cause of this confusion. Concretely, NodeStore[vector<NODE_MESH *>::size_type(node0)] is of type vector<NODE_MESH *>& instead of an element of the NodeStore as you expected.
Change your code to take variables by reference instead:
void setCellInfo (CELL_MESH* target, int Global_ID, int node0,vector<NODE_MESH *>& NodeStore, vector<CELL_MESH *>& CellStore) {
CellStore->push_back(target); //No Errors
target->Global_ID = Global_ID; //No Errors
if (node0 != 0) {
target->node[0] = NodeStore[node0]->ID;
target->node_pointers[0] = NodeStore[node0];
}
}
The call is then simply
setCellInfo (&newCell, i, nodes_x*y+x, NodeStore, CellStore);
Alternatively, you will need to dereference the pointer before indexing:
(*NodeStore)[node0]->ID
Final Solution:
void setCellInfo (CELL_MESH* target, int Global_ID, int node0,vector<NODE_MESH *>& NodeStore, vector<CELL_MESH *>& CellStore) {
CellStore.push_back(target);
target->Global_ID = Global_ID;
if (node0 != 0) {
target->node[0] = NodeStore[node0]->ID;
target->node_pointers[0] = NodeStore[node0];
}
}
With this when using it:
CELL_MESH *newCell = new CELL_MESH;
setCellInfo (newCell, i, nodes_x*y+x, NodeStore, CellStore);

Initialize and declare dynamically multiple variables of random entities in a loop in C++

This is my code:
#include <cstdlib>
#include <ctime>
#include <stdio.h>
#include <vector>
#define ENTITY(A) entity##A
#define ALM(A) alm##A
struct TEntity{
private:
int sumx;
int sumy;
const char * rep;
int m_ix;
int m_iy;
public:
TEntity(int x, int y, int sum_x, int sum_y, const char * txt);
};
TEntity::TEntity(int x, int y, int sum_x, int sum_y, const char * txt) {
m_ix = x;
m_iy = y;
sumx = sum_x;
sumy = sum_y;
rep = txt;
}
class IAlmacenable {
private:
void * element;
public:
IAlmacenable(void * e);
IAlmacenable();
void * getValue();
};
IAlmacenable::IAlmacenable(void *e) {
element = e;
}
IAlmacenable::IAlmacenable() {
element = nullptr;
}
void * IAlmacenable::getValue() {
return element;
}
class TList {
private:
std::vector<IAlmacenable*> elementos;
int position;
public:
TList();
int Size();
int Push(IAlmacenable* psz);
};
TList::TList() {
elementos = std::vector<IAlmacenable*>();
position = 0;
}
int TList::Size() {
return elementos.size();
}
int TList::Push(IAlmacenable* psz) {
int res = 0;
if (elementos.size() >= elementos.max_size()) {
res = -1;
}
else {
elementos.push_back(psz);
}
return res;
}
int main(){
srand(time(NULL));
TList *list = new TList();
//we can put entities in the list and the rest will be filled up to 5
int size = list->Size();
for(int i = size; i<5;i++){
const char c[] = {(rand() % 2 ? 65 + rand() % 25 : 97 + rand() % 25), '\0'};
TEntity ENTITY(i)(rand() % 10, rand() % 10, rand() % 5, rand() % 5, c);
IAlmacenable ALM(i)(&ENTITY(i));
list->Push(&ALM(i));
size++;
}
//do things like printing their value...
delete list;
return 0;
}
I need to create a new variable everytime it run the "TEntity ENTITY(i)" line,
the problem is that it creates the same variable always, I think it is because it creates the variable entityi and therefore it is overwriting on the same variable, besides it seems that the random it generates is always the same number since all entities have the same values ​​in all its parameters. The c variable create a const char * random variable between a-z, A-Z , I don't put the print code because it is unnecessary, so what can I do? Is there any way to dynamically create variables of entities whose values ​​are random?
EDIT
Here is the new code fixed (the macros have been eliminated since they were not necessary and the necessary code has been included to be able to execute it) but there is still the same problem that they are generated with the same parameters (since they are still the same variable):
#include <cstdlib>
#include <ctime>
#include <stdio.h>
#include <vector>
#include <conio.h>
#include <windows.h>
struct TEntity{
private:
int sumx;
int sumy;
const char * rep;
int m_ix;
int m_iy;
public:
TEntity(int x, int y, int sum_x, int sum_y, const char * txt);
void movimiento();
void pinta();
};
TEntity::TEntity(int x, int y, int sum_x, int sum_y, const char * txt) {
m_ix = x;
m_iy = y;
sumx = sum_x;
sumy = sum_y;
rep = txt;
}
void TEntity::movimiento() {
m_ix += sumx;
m_iy += sumy;
}
void TEntity::pinta() {
gotoxy(static_cast<short int>(m_ix), static_cast<short int>(m_iy));
printf("%s", rep);
}
void gotoxy(short int x, short int y)
{
COORD pos = {x, y};
HANDLE output = GetStdHandle(STD_OUTPUT_HANDLE);
SetConsoleCursorPosition(output, pos);
}
void clear()
{
system("cls");
}
class IAlmacenable {
private:
void * element;
public:
IAlmacenable(void * e);
IAlmacenable();
void * getValue();
};
IAlmacenable::IAlmacenable(void *e) {
element = e;
}
IAlmacenable::IAlmacenable() {
element = nullptr;
}
void * IAlmacenable::getValue() {
return element;
}
class TList {
private:
std::vector<IAlmacenable*> elementos;
int position;
public:
TList();
int Size();
int Push(IAlmacenable* psz);
IAlmacenable* First();
IAlmacenable* Next();
};
TList::TList() {
elementos = std::vector<IAlmacenable*>();
position = 0;
}
int TList::Size() {
return elementos.size();
}
int TList::Push(IAlmacenable* psz) {
int res = 0;
if (elementos.size() >= elementos.max_size()) {
res = -1;
}
else {
elementos.push_back(psz);
}
return res;
}
IAlmacenable* TList::First() {
IAlmacenable* res;
if (elementos.empty()) {
res = nullptr;
}
else {
res = elementos.front();
position = 1;
}
return res;
}
IAlmacenable* TList::Next() {
IAlmacenable* res;
if (elementos.empty()) {
res = nullptr;
}
else {
int pos = position;
int size = elementos.size();
if (pos < size) {
res = elementos.at(position);
position++;
}
else {
res = this->First();
}
}
return res;
}
int main(){
srand(time(NULL));
TList *list = new TList();
//we can put entities in the list and the rest will be filled up to 5
int size = list->Size();
for(int i = size; i<5;i++){
const char c[] = {(rand() % 2 ? 65 + rand() % 25 : 97 + rand() % 25), '\0'};
TEntity *entity = new TEntity(rand() % 10, rand() % 10, rand() % 5, rand() % 5, c);
IAlmacenable *alm = new IAlmacenable(entity);
list->Push(alm);
size++;
}
while(true){
clear();
for (int i = 0; i < size; i++) {
reinterpret_cast<TEntity *>(list->Next()->getValue())->pinta();
reinterpret_cast<TEntity *>(list->Next()->getValue())->movimiento();
}
Sleep(2000);
}
delete list;
return 0;
}
There is some confusion here.
Some points:
The macro is not fit-for-purpose, as you already know; you're just creating a variable name entityi each time;
That doesn't matter! The object only exists for the duration of the loop iteration anyway; C++ doesn't let you create multiple objects with the same name at the same time. In fact you can get rid of the entire macro stuff and just call the object entity;
Now that that's out of the way, you're getting repeated results because you're storing a pointer to each iteration of that local variable — on each occasion, that's a dangling pointer to an object that's been destroyed. Don't store dangling pointers!
You can either:
Dynamically allocate the objects that you're adding to the list, or
Store actual objects rather than pointers-to-objects.
Either way, the local-scope name is irrelevant and certainly need not change repeatedly for each loop iteration.

How to obtain sizes of arrays in a loop

I am aligning several arrays in order and performing some sort of classification. I created an array to hold other arrays in order to simplify the operations that I want to perform.
Sadly, my program crashed when I ran it and I went on to debug it to finally realize that the sizeof operator is giving me sizes of pointers and not arrays within the loop.So I resorted to the cumbersome solution and my program worked.
How can I avoid this cumbersome method? I want to calculate within a loop!
#include <iostream>
#include <string>
#define ARRSIZE(X) sizeof(X) / sizeof(*X)
int classify(const char *asset, const char ***T, size_t T_size, size_t *index);
int main(void)
{
const char *names[] = { "book","resources","vehicles","buildings" };
const char *books[] = { "A","B","C","D" };
const char *resources[] = { "E","F","G" };
const char *vehicles[] = { "H","I","J","K","L","M" };
const char *buildings[] = { "N","O","P","Q","R","S","T","U","V" };
const char **T[] = { books,resources,vehicles,buildings };
size_t T_size = sizeof(T) / sizeof(*T);
size_t n, *index = new size_t[T_size];
/* This will yeild the size of pointers not arrays...
for (n = 0; n < T_size; n++) {
index[n] = ARRSIZE(T[n]);
}
*/
/* Cumbersome solution */
index[0] = ARRSIZE(books);
index[1] = ARRSIZE(resources);
index[2] = ARRSIZE(vehicles);
index[3] = ARRSIZE(buildings);
const char asset[] = "L";
int i = classify(asset, T, T_size, index);
if (i < 0) {
printf("asset is alien !!!\n");
}
else {
printf("asset ---> %s\n", names[i]);
}
delete index;
return 0;
}
int classify(const char *asset, const char ***T, size_t T_size, size_t *index)
{
size_t x, y;
for (x = 0; x < T_size; x++) {
for (y = 0; y < index[x]; y++) {
if (strcmp(asset, T[x][y]) == 0) {
return x;
}
}
}
return -1;
}
As you are including <string> and <iostream> I assume that the question is about C++ and not C. To avoid all this complication, simply use containers. E.g:
#include <vector>
std::vector<int> vect = std::vector<int>(3,0);
std::cout << vect.size() << std::endl; // prints 3
One solution if you are coding in C is to terminate your array with a special item, like NULL
const char *books[] = { "A","B","C","D", NULL };
size_t size(const char *arr[])
{
const char **p = arr;
while (*p)
{
p++;
}
return p - arr;
}
You can specify the array size explizit:
size_t n, index[] = {ARRSIZE(books), ARRSIZE(resources), ARRSIZE(vehicles), ARRSIZE(vehicles)};
or if you want to avoid double typing you can you X-Macros to roll out everything:
#define TBL \
X(books) \
X(resources) \
X(vehicles) \
X(buildings)
const char **T[] = {
#define X(x) x,
TBL
};
#undef X
size_t n, index[] = {
#define X(x) ARRSIZE(x),
TBL
};
which produces the same. See Running Demo.

Passing and Assigning New Value to Pointer C++

I'm passing a pointer to a function. I'd like to assign a new address to the passed pointer inside the function, and I'd like that address to be used after the function returns. I'm not sure if this is possible, but I'd like to do:
int main()
{
int i = 100, j = 200;
int * intPtr = &i;
foo(intPtr, j);
// I want intPtr to point to j, which contains 200 after returning from foo.
}
void foo( int * fooPtr, int & newInt )
{
int * newIntPtr = &newInt;
fooPtr = newIntPtr;
}
Is this possible, or will intPtr not maintain the new assignment after returning from foo? Could this work (if it doesn't: why)?
Pass a reference to the pointer:
void foo( int *& fooPtr, int & newInt )
The reason why your method does not work is that you're passing the pointer by-value. Passing by-value creates a temporary within the function, so as soon as the function returns, any changes to the temporary go away.
It is no different than this:
void foo(int x)
{
x = 10;
}
int main()
{
int a = 0;
foo( a );
// a is still 0, not 10
}
The a is passed by value, so the foo() function changes the parameter to 10 within the function. However, you will see that a in main does not change to 10 after the function returns.
To change a, you need to pass the int by reference:
void foo(int& x)
{
x = 10;
}
int main()
{
int a = 0;
foo( a );
// a is now 10
}
Pass a pointer of the pointer and assign to it
int main()
{
int i = 100, j = 200;
int * intPtr = &i;
foo( &intPtr, j );
// I want intPtr to point to j, which contains 200 after returning from foo.
}
void foo( int ** fooPtr, int & newInt )
{
int * newIntPtr = newInt;
*fooPtr = newIntPtr;
}
If you programing in pure C you can do like this
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void foo(int **, int *);
int main()
{
int i = 100, j = 200;
int * intPtr = &i;
int *intPtr2=&j;
foo( &intPtr, intPtr2 );
// I want intPtr to point to j, which contains 200 after returning from foo.
printf("%d",*intPtr);
}
void foo( int ** fooPtr, int * newInt )
{
int * newIntPtr = newInt;
*fooPtr = newIntPtr;
}

Converting C++ implementation of vector in C

I have written the following code in C++, however found out that I have to convert it in C. I am not C or even C++ programmer, please help.
Can someone help me change this method to C directives, specifically vector implementation, following will not compile I have removed complexity to keep it simple. Thanks in anticipation.
__declspec(dllexport) std::vector<MY_STRUCT> WINAPI ABC(char *strVal)
{
MY_STRUCT f;
std::vector<MY_STRUCT> list = std::vector<MY_STRUCT>();
while (*dddd)
{ /*do the following for every feature in license file*/
f.attrib_num = fi.attrib_num;
f.attrib_lic = fi.attrib_lic;
list.push_back(f);
} /* end while(conf) */
dddd++;
printf("\n");
} /* end while (*dddd) */
return flist;
}
Here is implementation (also usage) of dynamic array of structs in C. You can adapt it to your structure; I used to post it on code review before
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct
{
int ID;
char * name;
} Student;
// array of structs
typedef struct
{
Student *array;
size_t used;
size_t size;
} Array;
void initArray(Array *a, size_t initialSize)
{
int i = 0;
// Allocate initial space
a->array = malloc(initialSize * sizeof(Student));
a->used = 0; // no elements used
a->size = initialSize; // available nr of elements
// Initialize all values of the array to 0
for(i = 0; i<initialSize; i++)
{
memset(&a->array[i],0,sizeof(Student));
}
}
// Add element to array
void addElement(Array *a, Student element)
{
int i = 0;
if (a->used == a->size)
{
a->size *= 2;
a->array = realloc(a->array, a->size * sizeof(Student));
// Initialize the last/new elements of the reallocated array
for(i = a->used; i<a->size; i++)
{
memset(&a->array[i],0,sizeof(Student));
}
}
// Copy name
a->array[a->used].name = (char*)malloc(strlen(element.name) + 1);
strcpy(a->array[a->used].name, element.name);
// Copy ID
a->array[a->used].ID=element.ID;
a->used++;
}
void freeArray(Array *a)
{
int i = 0;
// Free all name variables of each array element first
for(i=0; i<a->used; i++)
{
free(a->array[i].name);
a->array[i].name=NULL;
}
// Now free the array
free(a->array);
a->array = NULL;
a->used = 0;
a->size = 0;
}
int main(int argc, const char * argv[])
{
Array a;
Student x,y,z;
x.ID = 20;
x.name=malloc(strlen("stud1") + 1);
strcpy(x.name,"stud1");
y.ID = 30;
y.name=malloc(strlen("student2") + 1);
strcpy(y.name,"student2");
z.ID = 40;
z.name=malloc(strlen("student3") + 1);
strcpy(z.name,"student3");
// Init array, don't forget
initArray(&a, 5);
// Add elements
addElement(&a, x);
addElement(&a, y);
addElement(&a, z);
// Print elements
printf("%d\n", a.array[0].ID);
printf("%s\n", a.array[0].name);
printf("%d\n", a.array[1].ID);
printf("%s\n", a.array[1].name);
printf("%d\n", a.array[2].ID);
printf("%s\n", a.array[2].name);
// Free array
// don't forget
freeArray(&a);
free(x.name);
free(y.name);
free(z.name);
return 0;
}