use static allocator instead

vector/data/one/code.cpp

@@ -125,6 +125,8 @@ void TestErase()
 
 int main(int argc, char const *argv[])
 {
+  std::vector<int> vvv;
+  vvv.pop_back();
 	TestConstructor();
 	TestIterators();
 	TestAccessingMethod();

vector/src/vector.hpp

@@ -16,7 +16,7 @@ namespace sjtu {
  */
 template <typename T>
 class vector {
-  std::allocator<T> alloc;
+  static std::allocator<T> alloc;
   size_t allocated_length;
   size_t current_length;
   T *raw_beg, *raw_end;
@@ -504,6 +504,8 @@ class vector {
     }
   }
 };
+template<typename T>
+std::allocator<T> vector<T>::alloc;
 
 }  // namespace sjtu