在全局 Scope 中声明二维向量会导致分段错误

Question

#include <bits/stdc++.h>

using namespace std;

int n;
std::vector<bool> visited(n,false);
std::vector<std::vector<int>> g(n,std::vector<int>(n));


int main() {
    cin>>n;
    //std::vector<std::vector<int>> g(n,std::vector<int>(n));
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            cin>>g[i][j];
        }
    }
    cout<<g[0][0];

}

2D vector declared inside main function gives no error but when declared in global scope gives SIGSEV error

Answer 1

When visited and g get initialized, the value of n is 0 . ( n is declared in global namespace and will be zero-initialized .) So the vector s are empty and contain no elements. Then access to them like g[0][0] leads to UB.

On the other hand, for the vector g declared in main() , n is set to some value and then used to initialize g , then g is initialized as containing n elements.