g++ 错误：“vec”未命名类型[重复]

我正在尝试使用 C++ STL 向量。这是我的程序：

# include <iostream> 
# include <vector>

using namespace std; 

vector<int> vec; 

vec.push_back(10);
vec.push_back(5);
vec.push_back(1);

vector<int>::iterator itr1 = vec.begin();
vector<int>::iterator itr2 = vec.end();

for(vector<int>::iterator itr = itr1; itr != itr2; itr++){
    cout << *itr  << endl;  
}

我收到错误error: ‘vec’ does not name a type线上vec.push_back();。我看不出我在这里做错了什么。

诸如此类的陈述vec.push_back(42);只允许在函数内部使用。例如，

#include <iostream> 
#include <vector>

int main()
{
  using namespace std; 

  vector<int> vec; 

  vec.push_back(10);
  vec.push_back(5);
  vec.push_back(1);

  vector<int>::iterator itr1 = vec.begin();
  vector<int>::iterator itr2 = vec.end();

  for(vector<int>::iterator itr = itr1; itr != itr2; itr++){
    cout << *itr  << endl;  
  }

}