Defining an array size with a variable in a structure

Question

I have tried defining an array size with a variable in C++ before and although I do not fully understand the concept of dynamic memory, I made it work. However, I don't know how to do the same thing to the array 'point' in this case.

num=50;
struct pos
{
double x;
};

 struct pos point[num]; 

Is there anything obvious that I am overlooking?

Answer 1

These types of array sizes have to be compile time constants, so the compiler knows how much memory to reserve.

int count = 50;
int arr[count] // error!

static const int count = 50;
int arr[count]; // OK!

The other option is dynamically allocated memory in which the size is known at run time.

int count = 50;
int* arr = new int[count];
delete [] arr;

However, generally you don't want to be dealing with raw pointers and memory allocation on your own, and should instead prefer:

#include <vector>

int count = 50;
std::vector<int> arr(count);

This will also work for any custom types you have provided they are copyable (hint: your example pos structure is copyable):

#include <vector>

int count = 50;
std::vector<pos> arr(count);

arr[0].x = 1;
// ... etc
arr[49].x = 49;

std::vector has a rich interface, and all the details can be found here