如何在'{'标记'之前不允许修改功能定义

Question

在我的代码中出现“ {”令牌”之前，错误“在此不允许函数定义”。 那我该如何解决呢？

  void strongconnect(std::unordered_set<vertex>& v, const 
                  directed_graph<vertex>& d, std::vector<std::vector<vertex>>& 
                  scc, std::stack<vertex>& S, std::unordered_set<vertex, 
                  bool>& on_stack,std::unordered_map<vertex, int>& index, 
                  std::unordered_map<vertex, int>& low, int& count) 
  { // the error occured this line
    int index = 0;
    d = d.vertices;
    vector<int> s;
    for (auto v : d)
    {
      if(index[v] is undefined) // I don't know how to write this 'index[v] is undefined' code
      {
        strongconnect(v, d, scc, S, on_stack, index, low, count);
      }
    }  
  }