Databases
StackOverflow2013
Postsdbo
POarray of questions on pointers, double pointers, malloc, realloc and new (to set the record straight)
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POarray of questions on pointers, double pointers, malloc, realloc and new (to set the record straight)
    primarykey
    data
    text
    <p>I'm catching up on pointers. I wrote down a few lines of code to test the different ways I could dynamically allocate a 2d array (posted at the bottom)</p> <p>My questions are the following:</p> <ol> <li>Should I use malloc in c++, or new? If I use new, can I still use realloc?</li> <li>When should I use realloc? What are the implications of using it in terms of performance and bugs?</li> <li>Out of the examples bellow, which objectNew version should I use? If the answer depends on the application, what are does it depend on?</li> </ol> <p>Many thanks, Matt</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; struct myObject { int data; myObject(int i) { data = i; } myObject() { data = 0; } }; int main(){ int r = 7; int c = 6; printf("Objects using NEW===============================\n"); //notice the triple pointer being assigned a new double pointer array myObject*** objectNew = new myObject** [r]; for(int i=0;i&lt;r;i++) { //objectNew is a 1D array of double pointers, however if we assign another layer of pointers, it becomes a 2D array of pointers to myObject objectNew[i] = new myObject* [c]; for(int j=0;j&lt;c;j++){ objectNew[i][j] = new myObject(10*i+j); //notice that we dereference data (-&gt;) printf("objectNew[%2d][%2d]=%02d\n",i,j,objectNew[i][j]-&gt;data); } } delete objectNew; printf("Objects using NEW version 2===============================\n"); //notice the triple pointer being assigned a new double pointer array myObject** objectNew2 = new myObject* [r]; for(int i=0;i&lt;r;i++) { //objectNew is a 1D array of double pointers, however if we assign another layer of pointers, it becomes a 2D array of pointers to myObject objectNew2[i] = new myObject [c]; for(int j=0;j&lt;c;j++){ objectNew2[i][j] = myObject(10*i+j); //notice that we dereference data (-&gt;) printf("objectNew2[%2d][%2d]=%02d\n",i,j,objectNew2[i][j].data); } } delete objectNew2; printf("Objects using MALLOC===============================\n"); //notice the double pointer being allocated double pointers the size of pointers to myObject myObject** objectMalloc =(myObject**) malloc(sizeof(myObject*)*r); for(int i=0;i&lt;r;i++) { //now we are assigning array of pointers the size of myObject to each double pointer objectMalloc[i] = (myObject*) malloc(sizeof(myObject)*c); for(int j=0;j&lt;c;j++){ objectMalloc[i][j] = myObject(10*i+j); //notice that we access data without dereferencing (.) printf("objectMalloc[%2d][%2d]=%02d\n",i,j,objectMalloc[i][j].data); } } free((void*) objectMalloc); //same as Malloc printf("Objects using CALLOC===============================\n"); myObject** objectCalloc = (myObject**) calloc(r,sizeof(myObject*)); for(int i=0;i&lt;r;i++) { objectCalloc[i] = (myObject*) calloc(c,sizeof(myObject)); for(int j=0;j&lt;c;j++){ objectCalloc[i][j] = myObject(10*i+j); printf("objectCalloc[%2d][%2d]=%02d\n",i,j,objectCalloc[i][j].data); } } free((void*) objectCalloc); printf("Int using NEW===============================\n"); //int is not an object int** intNew = new int* [r]; for(int i=0;i&lt;r;i++) { intNew[i] = new int[c]; for(int j=0;j&lt;c;j++){ intNew[i][j] = 10*i+j; printf("intNew[%2d][%2d]=%02d\n",i,j,intNew[i][j]); } } delete intNew; printf("Int using malloc===============================\n"); int** intMalloc =(int**) malloc(sizeof(int*)*r); for(int i=0;i&lt;r;i++) { intMalloc[i] =(int*) malloc(sizeof(int)*c); for(int j=0;j&lt;c;j++){ intMalloc[i][j] = 10*i+j; printf("intMalloc[%2d][%2d]=%02d\n",i,j,intMalloc[i][j]); } } free((void*) intMalloc); getchar(); return 0; } </code></pre>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USwkl
    1. USmatthewaveryusa
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POarray of questions on pointers, double pointers, malloc, realloc and new (to set the record straight)
      1. This table or related slice is empty.
      1. VTUpMod
    1. This table or related slice is empty.
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload