Define Global variables from File input C++ - c++

Question

I have to define a global array in my C++ code the size of which has to be read from a file. I am using the below code
#include<iostream>
#include<string>
#include<fstream>
using namespace std;
string inputfile = "input.txt";
ifstream infile(inputfile.c_str());
infile>>N; // N = size of Array
int array[N];
// ------some code here-----
int main(){
int N;
cout << N<<endl;
return 0;
}
But if I place the 3 lines
string inputfile = "input.txt";
ifstream infile(inputfile.c_str());
infile>>N; // N = size of Array
inside the main loop this code works. Unfortunately I cant put it inside any function because I need to initialise a global array from variable N.
I have asked many people and searched different places but I cant seem to be able to figure this out. Thanks for your help.

Answer 1

The size of an array has to be a constant expression, i.e. known at compile-time.
Reading a value from a file is an inherently dynamic operation, that happens at run-time.
One option is to use dynamic allocation:
int array_size()
{
int n;
ifstream infile("input.txt");
if (infile>>n)
return n;
else
throw std::runtime_error("Cannot read size from file");
}
int* array = new int[array_size()];
However it would be better to replace the array with std::vector<int> which can be resized dynamically.

Answer 2

Use a global pointer. Define
int* array;
in the global space before your main procedure. Then later, in a loop or not, say
array = new int[N];
to allocate your array. Just remember to also say
delete[] array;
before you exit your main or re-allocate array

Answer 3

int array[N]; - N should be know at compile-time.
Instead, use int array[]=new int[N]; /*some code using array*/ delete[] array;
int *array;
int main(){
ifstream infile("input.txt");
unsigned N;
infile>>N;
array=new int[N];
//using array...
delete[] array; //when no longer needed.
//don't use array after this line
//unless it's array=new int[] instruction
//or you know what you're doing.
}