Initialize all elements in matrix with variable dimensions to zero - c++

Question

I'm trying to initialize an int matrix in C++, with user-inputted dimensions, with every element set to zero. I know there are some elegant ways to do that with a one-dimensional array so I was wondering if there are any similar ways to do it with a two-dimensional array without using for loops and iterating through every element.
I found a source that gave several different ways, including std::fill (I've modified the code so that the dimensions are read with cin):
#include <iostream>
using namespace std;
int main() {
int x;
cin >> x;
int matrix[x][x];
fill(*matrix, *matrix + x * 3, 0);
for (int i = 0; i < x; i++) {
for (int j = 0; j < 3; j++) {
cout << matrix[i][j] << " ";
}
cout << endl;
}
}
But why does this work, and why would the pointer to the matrix in the arguments for fill be necessary if it's not necessary for a one-dimensional array? That source said it was because matrixes in C++ are treated like one-dimensional arrays, which would make sense, but that is why I don't understand why the pointer is needed.
I don't know if this is relevant, but in case it can help, I've described my previous attempts below.
At first I thought I could initialize all elements to zero like in a one-dimensional array. For the matrix, this worked fine when the side lengths were not read with cin (i.e. when I declared the matrix as int matrix[3][3] = {{}}; as answered here) but when I tried getting the side lengths from cin I started getting errors.
This was my code:
#include <iostream>
using namespace std;
int main() {
int x;
cin >> x;
int matrix[x][x] = {{}};
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 3; j++) {
cout << matrix[i][j] << " ";
}
cout << endl;
}
}
And when I tried to compile it, it threw this error:
matrix_test.cpp:7:14: error: variable-sized object may not be initialized
int matrix[x][x] = {{}};
^
1 error generated.

Answer 1

Why you're getting the error
c-style arrays (such as int matrix[3][3]) must have size specified at the point you declare it. They can't vary in size in C++.
What you could do instead.
If you use std::vector, there's a really elegant way to do it:
#include <vector>
#include <iostream>
int main() {
using namespace std;
int x;
cin >> x;
auto matrix = vector<vector<int>>(x, vector<int>(x, 0));
// This is how we can print it
for(auto& row : matrix) {
for(auto& elem : row) {
cout << elem << ' ';
}
cout << '\n';
}
}
In C++17, you can shorten this even further:
auto matrix = vector(x, vector(x, 0));
What vector(number, thing) means is "Create a vector of number, where each element is thing".

Answer 2

The second dimension of two-dimension array must be a compile time constant, but in your code x is not.
Actually if you write a function with a two-dimension parameter, the second dimension must also be a compile time constant. That's because the array is stored linearly in the memory and the compiler must know the second dimension to calculate the offset correctly.