c++ delete dynamic matrix

Question

I have this code to allocate and initialize:

nw = new int*[V];

for (w = 0; w < V; w++) {

nw[w] = new int[K];

for (k = 0; k < K; k++) 

  nw[w][k] = 0;
}

and this to free memory:

if (nw) {
 for (int w = 0; w < V; w++) {  
  if (nw[w]) 
delete nw[w];      
}

The program compile and runs, but, when its try to deallocate memory, fails. The program not always fails at the same value of w.

Any ideas?

Unrelated: `delete`ing null pointers has no ill side-effects, so the check is completely redundant and unecessary. — R. Martinho Fernandes, Dec 04 '12 at 17:14

hmjd · Answer 1 · 2012-12-04T17:50:08.923

8

When new[] use delete[], so change to:

delete[] nw[w];

and remember to delete[] nw;.

Note that the individual assignment of 0 to each int in the array can be replaced with:

nw[w] = new int[K](); // This value initializes the array, in this
                //^^     case sets all values to zero.

You can avoid explicitly handling dynamic allocation with std::vector<std::vector<int>>:

std::vector<std::vector<int>> nw(V, std::vector<int>(K));

edited Dec 04 '12 at 17:50

answered Dec 04 '12 at 17:14

hmjd

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I change to: delete[] nw[w]; and the error continues. I make debug and it crash. For some values of w runs well, but not for others – user1705996 Dec 04 '12 at 17:29
@user1705996, see http://ideone.com/Ju5MvX for an example. Is `nw` a class member? If so, is it _definitely_ being initialised? – hmjd Dec 04 '12 at 17:40
nw is an atribute of a class: class model { public: ... int ** nw; ... } I'm printing the values and all values are 0. (One think that i don't understand is: for example: I execute it, and it crash when the value of w is 30, and if I execute it another time, crash for a diferent value of w, for example 15 or 40) – user1705996 Dec 04 '12 at 17:58
@user1705996, random behaviour is indicative of undefined behaviour. My guess is that `nw` is not being initialised in all versions of the constructor _or_ the object that contains `nw` is being copied and you are using the default copy constructor and default assignment operator (see http://stackoverflow.com/questions/4172722/what-is-the-rule-of-three). My advice is use `std::vector` instead. – hmjd Dec 04 '12 at 18:01
Do you know... whats going on if i don't implement the destructor?(when i reboot my computer the memory that i reserved its free?) – user1705996 Dec 05 '12 at 10:16
@user1705996, when the program exits the memory will be reclaimed by the OS. However, this is _not_ how to solve this problem. Remove chunks of your code while the error repeats and then start adding parts back in to isolate the cause. – hmjd Dec 05 '12 at 10:22

score 3 · Answer 2 · edited Dec 04 '12 at 17:20

3

I think you meant to use delete[], not delete:

for (int w = 0; w < V; w++)
    delete[] nw[w];
delete[] nw;

edited Dec 04 '12 at 17:20

Mooing Duck

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answered Dec 04 '12 at 17:15

Xymostech

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I change to: delete[] nw[w]; and the error continues. I make debug and it crash. For some values of w runs well, but not for others – user1705996 Dec 04 '12 at 17:32

c++ delete dynamic matrix

2 Answers2