局部数组变量排除不良访问细分和局部或全局？

Question

Hello I have seen this Maximum size of local array variable but I want to know why that's ok if the array is set ion global and not ok if it comes after the main.

And another question in relation : Is it a good practice to have big memory objects that are defined in a.cpp and declared in a.hpp with extern ? Or better working with big memory defined in local fonction but defined as vector or new or malloc and passing them in fonction arguments.

It's see my experience that are those questions i have to resolve...

Thank You

#include <iostream>
using namespace std;

#define N (10000000000000)
int sd[N];

int main() {
  // int sd[N];
  return 0;
}

Answer 1

Declared at global scope:

int sd[N];

int main() {
  return 0;
}

And the resulting binary just gets real big. When the process is loaded into memory, the entire set of global memory data is mapped in.

Declared within a function:

int main() {
  int sd[N];
  return 0;
}

And the memory is allocated as soon as the function as invoked - and it's allocated from the stack . The stack memory for a thread is usually initialized low, closer to a megabyte or less. Once the stack memory runs out, it's game over.

As others have pointed out in the comments, the right way to allocate a LARGE array is dynamically, using heap memory, which is typically plentiful.

int main() {
  int* sd = new int[N];

  ...

  delete [] sd; // free the allocated memory
  return 0;
}

Even better, so you don't have to remember the delete:

int main() {
  std::vector<int> sd(N);

  ...

}