Get array from object in c++ - c++

Question

This question might be simple but I never use raw pointers or arrays in C++ so...
I need to use a library function which looks like this:
void f(double a[3][3], double b[3], double c[3]);
a and b are used for input and the result is stored in c.
The computation of a is a bit complex but does never change so it makes sense to calculate it only once and save the result. In my program, I can link it to an object of type X.
class X{
public:
X(){
a = {{1,2,3},
{4,5,6},
{7,8,9}};
}
private:
double a[3][3];
}
How can I write a getter for X::a which can be used in function f?
This is how I would like to call function f:
#include "X.h"
int main(int argc, char *argv[]){
X o = X(); //create object
double a[3][3] = o.getA(); // I want to get a somehow
double b[3] = {1,2,3}; // create dummy b
double c[3] = {}; // create empty c
f(a,b,c); // call funktion to populate c
for(int i=0; i<3; ++i){
std::cout << c[i] << endl;
}
}

Answer 1

You know std::vector is the way to go for 2D arrays in C++, but if you can't bypass the obstacle you are facing, then it would be possible to pass the matrix as a parameter to the getter function, like this:
#include <iostream>
class X {
public:
void getA(double (&array)[3][3]) {
for (int i = 0; i < 3; ++i)
for (int j = 0; j < 3; ++j)
array[i][j] = a[i][j];
}
private:
double a[3][3] = {{1,2,3},
{4,5,6},
{7,8,9}};
};
int main(void) {
X o = X();
double a[3][3];
o.getA(a);
for(int i = 0; i < 3; ++i)
for(int j = 0; j < 3; ++j)
std::cout << a[i][j] << std::endl;
return 0;
}

Answer 2

This snippet should serve your purpose.
#include <iostream>
#include <string>
using namespace std;
class X {
public:
X() {
}
typedef double (*ptr_matrix)[3];
ptr_matrix getA(){
return a;
}
private:
double a[3][3] = {{ 1,2,3},
{4,5,6},
{7,8,9}};
};
void f(double a[3][3], double b[3], double c[3])
{
cout<<"Inside f function";
for(auto i = 0; i < 3;i++) {
cout<<endl;
for(auto j = 0 ; j < 3;j++)
cout<<a[i][j];
}
}
int main()
{
X o = X(); //create object
double (*a)[3] = NULL;
a = o.getA(); // I want to get a somehow
double b[3] = {0};
double c[3] = {0};
f(a,b,c);
}