How to initialize array later when using private object variable (after initializing the object)

Question

I want to first initialize "dummy" array when i create a object. I have this in private part of my class.

// Initialize dummy array when object is initialized.
int* matrix_[0][0];

Then i want later to initialize new array to replace the dummy one (one that has actual size). I have method for this in my class:

void set_map_size(int width, int height) {
    int* pm[width][height];
    matrix_ = pm;
}

So the problem is when I try to initialize this array after the creation of the object. I want other methods / other objects to have access to this array.

Maybe I should have pointer? Initialize pointer as nullptr when object is created and then change pointer to point to array?

int* matrix_[0][0]; doesn't look right, is this standard? Shouldn't you use malloc or new instead of a local automatic variable in that function? To avoid all these, consider using std::vector if you can — Aykhan Hagverdili
– Aykhan Hagverdili, Commented Apr 6, 2019 at 16:02
It probably doesn't work. I was just trying to illustrate what I would like to achieve. @Ayxan — Tuki
– Tuki, Commented Apr 6, 2019 at 16:07
@Ayxan int* matrix_[0][0] isn't right, it declares a 0 length 2d array of pointers. But malloc should pretty much never be used in c++, and raw new only very rarely has a place. — George
– George, Commented Apr 6, 2019 at 16:08
I would like to have there something has place holder so that i could replace it later with actual 2d array when i know the size. At the initialization of the object I don't yet know the size of the array I want to use. — Tuki
– Tuki, Commented Apr 6, 2019 at 16:10
I'd keep things simple if I were you, and go with @Ayxan s suggestion of using std::vector. — George
– George, Commented Apr 6, 2019 at 16:11

Christophe · Accepted Answer · 2019-04-06 16:10:16Z

1

The right way to do this in C++ is to use vectors.

vector<vector<int>> matrix_;

You may then dynamically resize your matrix

void set_map_size(int width, int height) {
    matrix_.resize(height); 
    for (auto &x : matrix_) 
       x.resize(width);
}

Because arrays are of fixed size and variable length array are not standard C++ (even if some compilers accept it).

answered Apr 6, 2019 at 16:10

Christophe

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3 Comments

Aykhan Hagverdili Over a year ago

The thing is whenever you need a variable array of some sort in C++, the answer is always std::vector

Aykhan Hagverdili Over a year ago

@George Vectors have very similar performance and handling a dynamically allocated array is enough of a pain that unless you absolutely need that infinitesimal performance gain in your high-performance library code, you should go with std::vector

Christophe Over a year ago

@George If performance is at stake, the 2D vector should be flattened to a 1D vector. This could make a significant difference. If it would nevertheless appear that there would be a performance issue, you could still use a pointer to data() in the time critical section. But do this only if the performance issue was really confirmed by measurement, because "premature optimization is the root of all evil"

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