2D array of object in C++

Question

I have a problem with assign object 2d [5][5] array to object.

This is my array definition

class PrintRectangle : public QWidget
{

public:
    bool clicked[5][5] = {};
    teacher *tech;
    perceptron *p;

};

And fragment perceptron class

class perceptron
{
public:
    perceptron& operator=(const perceptron&);
};

When I try to assign object to my perceptron *p

PrintRectangle::PrintRectangle(QWidget *parent) : QWidget(parent)
{
    tech = new teacher(clicked);

    *p = new perceptron[5][5];

    for(int i=0; i<5; i++)
    {
        for(int j=0; j<5; j++)
        {
            p[i][j] = new perceptron();
            p[i][j].randweight();
        }
    }

    double learnConst = 0.1;
    tech->learnPerceptrons(p);
}

I get an error

    E:\Sieci Neuronowe\Perceptron\printrectangle.cpp:10: error: no match for 'operator=' (operand types are 'perceptron' and 'perceptron (*)[5]')
         *p = new perceptron[5][5];
            ^

E:\Sieci Neuronowe\Perceptron\printrectangle.cpp:16: error: no match for 'operator[]' (operand types are 'perceptron' and 'int')
             p[i][j] = new perceptron();
                 ^

I have only

perceptron& perceptron::operator=(const perceptron&){

    return * this;
}

In my perceptron class. How can I correct this? I don't cleary understend pointers.

`*p = new perceptron[5][5]` - this does not initialise a 2D array, the allocation must be done somehow else. — Polb, Nov 23 '16 at 23:20
http://stackoverflow.com/questions/936687/how-do-i-declare-a-2d-array-in-c-using-new — Polb, Nov 23 '16 at 23:22

R Sahu · Accepted Answer · 2016-11-23T23:45:52.230

1

*p = new perceptron[5][5];

is wrong for the following reasons.

Type mismatch.

Type of *p is perceptron.
Type of new perceptron[5][5]; is perception (*)[5].

There is no conversion from perception (*)[5] to perceptron.
Dereferencing p.

Dereferencng p, i.e. *p is going to be valid at run time only after you have allocated memory for p.

Solution:

You can fix your memory allocation and type mismatch problems but I strongly recommend using containers from the standard library.

class PrintRectangle : public QWidget
{
  public:

    std::vector<teacher> tech;               // 1D array
    std::vector<std::vector<perceptron>> p;  // 2D array.
};

You can initialize them in the constructor with:

PrintRectangle::PrintRectangle(QWidget *parent) :
   QWidget(parent),
   tech(clicked),
   p(5, std::vector<perceptron>(5))
{
   ...
}

edited Nov 23 '16 at 23:45

answered Nov 23 '16 at 23:24

R Sahu

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Ok, but how can I change my for loop for this? – lukassz Nov 23 '16 at 23:27
@lukassz, you can access `tech` like you would access a 1D array and `p` like a 2D array. Take a look at http://en.cppreference.com/w/cpp/container/vector to get started with using `std::vector`. – R Sahu Nov 23 '16 at 23:30
Ok, why this is the better way? – lukassz Nov 23 '16 at 23:32
@lukassz, it removes the complexity of managing dynamically allocated memory from your code. That's already taken care of by the implementors of the standard library. – R Sahu Nov 23 '16 at 23:34
Ok. I assign to `tech` only one element `tech = new teacher(clicked);` it's good to make `tech` vector? – lukassz Nov 23 '16 at 23:36
@lukassz, I am not sure what you mean by that. – R Sahu Nov 23 '16 at 23:37
Ok, when I use your constructor I get an error `E:\Sieci Neuronowe\Perceptron\printrectangle.cpp:12: error: cannot convert 'bool (*)[5]' to 'teacher*' in initialization p(std::vector(5), 5) ^` – lukassz Nov 23 '16 at 23:39
`clicked` is a field in `PrintRectangle` class `bool clicked[5][5] = {};` – lukassz Nov 23 '16 at 23:40
@lukassz, now you are bringing up issues that are not mentioned in the original post. That's why we emphasize posting a [mcve]. – R Sahu Nov 23 '16 at 23:42
Yes but your code It's not working `E:\Sieci Neuronowe\Perceptron\printrectangle.cpp:11: error: no matching function for call to 'std::vector >::vector(std::vector, int)' p(std::vector(5), 5) ^` – lukassz Nov 23 '16 at 23:44
@lukassz, that was a mistake on my part. It's fixed now. – R Sahu Nov 23 '16 at 23:46
Ok, it works thanks. Now I sould put `new perceptron();` into vector in loop? – lukassz Nov 23 '16 at 23:48
Or I must make `operator=` but I get `E:\Sieci Neuronowe\Perceptron\printrectangle.cpp:24: błąd: no match for 'operator=' (operand types are '__gnu_cxx::__alloc_traits >::value_type {aka perceptron}' and 'perceptron*') p[i][j] = new perceptron; ^` – lukassz Nov 23 '16 at 23:49
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/128885/discussion-between-lukassz-and-r-sahu). – lukassz Nov 24 '16 at 00:11

2D array of object in C++

1 Answers1