How to initialize an array class with constructor C++

Question

i'm trying to initialize an array of a class, but i can only do this if i specify every object of the array, for example:

class Pixel{
private:
    int color;
public:
    Pixel(int color):color(color){}
};

class Screen{
private:
    Pixel screen[1920][1080];   
public:
    Screen(){
    //code//
    //i was trying to use for, to determinate each pixel color from a certain area
    //but i cannot, bc i would have to do something like this:
    screen[1920][1080] = {{Pixel(1),Pixel(1),Pixel(1),Pixel(1)...1080 times},{Pixel(2)
    ,Pixel(2), Pixel(2)...1080 times}...1918 times}
    //as you can see, is nearly impossible to do this in this way.
    }
};

there is a native way to initialize this array with fors or something like this?

(that code is just an EXAMPLE with the exact same logic, its not from my project)

You can write a default constructor for your `Pixel` class. Or better yet just give the parameter in your ctor a default value — perivesta, Jan 17 '22 at 22:11
Also, your 2D array has an aspect ratio of 9:16 whereas you might really want 16:9 as in the TVs and monitors, etc. — digito_evo, Jan 17 '22 at 22:23
You can use a `for` loop (two nested, in fact) to assign initial values in the constructor — M.M, Jan 17 '22 at 23:12

einpoklum · Answer 1 · 2022-01-17T23:29:03.437

There are several issues with your code:

Your Pixel type requires explicit initalization. Is that wise? @perivesta suggests providing a default constructor; or a default parameter to your existing constructor.

Another option would be reconsidering whether you even need a proper Pixel class at all. Perhaps it might be enough to just have
```
using Pixel = std::int32_t;
```
or whichever size a pixel is supposed to have. If a pixel needs fancy methods, then a struct wrapping the basic value, but without specifying any constructors or destructors, i.e. using the rule of zero
As @digito_evo explains - that's too large of an array to place on the stack; your program might crash for this reason alone, even if you were able to initialize your array. Consider having your Screen class allocate space on the heap, e.g. via std::unique_ptr member, or an std::vector like @NathanOliver suggests (both of them get allocated on the heap).

Read more about the heap and the stack: What and where are the stack and heap?
Use of magic numbers: 1920, 1080 - don't just type them in, say what they mean... have static class (constexpr) constants for these dimensions. For more on this point, see the C++ coding guideline against magic numbers.

score 1 · Answer 2 · answered Jan 17 '22 at 23:10

Your 2D array screen is too big (about 8 MB!!) to fit on the stack. You certainly don't want a stack overflow in your program. Therefore use a vector instead.

Also, a color variable doesn't need to be of type int. What are you going to do with 32 bits really?? Usually, 8 bits is sufficient so I switched to unsigned char.

Since you want a for-loop, have a look at this:

#include <vector>


class Pixel
{
public:
    Pixel( const unsigned char color = 0 )
    : m_color( color )
    {
    }

private:
    unsigned char m_color;
};

class Screen
{
public:
    Screen( const std::size_t rowCount = 1920, const std::size_t colCount = 1080 )
    : screen( rowCount, std::vector<Pixel>( colCount ) )
    {
        for ( std::size_t row { }; row < rowCount; ++row )
        {
            for ( std::size_t col { }; col < colCount; ++col )
            {
                screen[ row ][ col ] = Pixel( static_cast<unsigned char>( row ) + 1 );
            }
        }
    }

private:
    std::vector< std::vector<Pixel> > screen; // a vector of vectors
};

int main( )
{
    Screen scrn;
}

How to initialize an array class with constructor C++

2 Answers2