CODEWITHSUNDEEP
c++arraysfunctionpointers
Doej
Doej

Reputation: 65

Return a pointer to 2D array from a function - C++

So what I am trying to achieve is to return a pointer to a 2D array from the function so it could accessed in main(). I know there are some C++ libraries that does it for you like std::vector but I am trying to avoid dynamic memory allocation since I am working on embedded board (STM32) so I will stick to just normal pointers and arrays. (ALSO for some reason I can't use std::array in KEIL uVision, which is also why I am forced to work with pointers/arrays)

In addition, I understand that returning a pointer to a local array int arr[2][2] defined inside the function is not a good idea since it will no longer be valid after the function returns, which is why I creating test_array, declaring it inside a class and defining it in a function (acting as a global variable) so I assume this shouldn't be a problem. What do you guys think? However, doing it this way gives an error "Excess elements in scalar initializer"

#include <iostream>
#include "file.hpp"

int main() {

  myClass class_object;

  class_object.value = class_object.foo();


}

//file.hpp

#include <stdio.h>

class myClass{

  int array[2][2];
  int (*foo())[2];
  int (*value)[2];

  int test_array[2][2];   //declaring here! 

}; 

//file.cpp 

#include "file.hpp" 

int (*myClass::foo())[2]{

    test_array[2][2]={ {10,20}, {30, 40} }; //defining here - ERROR!!

    int arr[2][2]= {
        {1, 10},
        {20, 30}
    };


return arr;


}

Upvotes: 0

Views: 809

Answers (2)

user4581301
user4581301

Reputation: 33931

The immediate problem:

test_array[2][2]={ {10,20}, {30, 40} }; //defining here - ERROR!!

is not defining. test_array was defined up in myClass. This is attempting to assign to a single element of test_array, specifically [2][2] which does not exist. What particularly offends the compiler is not the out of bounds access, but that ={ {10,20}, {30, 40} }; is trying to stuff an array into a single array element. The compiler is expecting a single number, so four numbers is definitely in excess.

Unfortunately I don't know of a good way to do what you want to do. You can initialize an array with an initializer list, but you can't assign from one.

So

class myClass{
public:

    myClass();
    void foo();

    int test_array[2][2];   //declaring here!
};

// you can do this:
myClass::myClass(): test_array{ {10,20}, {30, 40} }
{

}

void myClass::foo()
{
    // but you can't do this:
    test_array = { {10,20}, {30, 40} };
}

Depending on what you do with test_array, initializing in the constructor may work for you. If you have to reset the array on every call to foo, perhaps an Automatic variable is a better fit for you

void myClass::foo()
{
    int temp_array[2][2] = { {10,20}, {30, 40} };

    // use temp_array

    // maybe copy temp_array to test_array with good ol' memcpy here if you 
    // need to carry the state for some reason.
}

To silence the elephant in the room and gain access to std::array, give this a try. Note: I've never done this. It could be an utter freaking disaster for all I know, so take it with a grain of salt.

Upvotes: 1

Jarod42
Jarod42

Reputation: 217135

If you really want to work with C-Array, use typedef to have normal syntax:

class myClass{
    public:
    using array2 = int[2][2];

    myClass() {
        test_array[0][0] = 0;
        test_array[0][1] = 1;
        test_array[1][0] = 2;
        test_array[1][1] = 3;
    }

    const array2& getArray() const { return test_array; }
    array2& getArray() { return test_array; }

private:
    array2 test_array;
};

Upvotes: 0

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