Passing 2D array to a funciton in c & c++

Question

I tried same program for C & C++ i.e. passing 2D array to a function. The following program is working in C, but not working in C++, Please explain why so ?

IN C

#include<stdio.h> 
void pass(int n, int arr[][n])  // or void pass(int n, int (*arr)[n])
{
printf("%d",arr[0][0]);
//.....
}

int main()
{
int n;
scanf("%d",&n);  
int arr[n][n];
arr[0][0]=0;
pass(n,arr);
return 0;
}

IN C++

#include <bits/stdc++.h>
using namespace std;
void pass(int n, int arr[][n])  // or void pass(int n, int (*arr)[n])
{
cout << arr[0][0];
//.....
}

int main()
{
int n; 
   cin >> n;
int arr[n][n];
arr[0][0]=0;
pass(n,arr);
return 0;
}

3:29:error: use of parameter outside function body before ']' token void pass(int n, int arr[][n]) // or void pass(int n, int (*arr)[n])

6:9:error: 'arr' was not declared in this scope cout << arr[0][0];*

this shouldnt be too surprising, c and c++ are differrent langauges. The code will also not compile as fortran — 463035818_is_not_an_ai, Nov 23 '19 at 11:15
C++ is not a superset of C. There are things in C that are not valid C++ (like eg VLA as in the question); there are things in C that have a different meaning in C++ (like eg `sizeof '$'` means `sizeof (int)` in C but `sizeof (char)` in C++). — pmg, Nov 23 '19 at 11:28

score 2 · Accepted Answer · answered Nov 23 '19 at 11:05

These statements

int n;
scanf("%d",&n);  
int arr[n][n];

provide a declaration of a variable length array. A variable length array is an array dimensions of which depend on run-time values. It is a feature of C that compilers may conditionally support.

In C++ there is no such a standard feature. So the C++ compiler issues an error. In C++ dimensions of an array shall be compile-time constants. Though some C++ compilers can have their own C++ language extensions that can include the feature of variable length arrays.

In C++ use the class template std::vector instead of variable length arrays.

The advice is good, but here it seems that the problem is not the VLA declaration by itself, accepted by most compilers, but the fact that in C++ you don't have a simple and clean way to pass them as argument of a function. Another strong reason not to use them — Damien, Nov 23 '19 at 11:45

Passing 2D array to a funciton in c & c++

1 Answers1