C++中map和vector作形参时如何给定默认参数?

struct node{int a{5};};
int main() {
  map<string,node> m1;
  cout<<m1["s"].a<<endl;
  map<string,int> m2;
  cout<<m2["s"]<<endl;
  vector<node> v1(3);//需要指定vector大小，否则不能在没有push_back的情况用下标访问 
  cout<<v1[0].a<<endl;
  vector<int> v2(3);
  cout<<v2[0];
}

#include <iostream>
#include <vector>
using namespace std;
int main ()
{
 int a = int();
 cout << a << endl;
 vector<int> v = vector<int>();
 for(vector<int>::iterator it = v.begin(); it != v.end(); ++it)
 {
 cout << *it << endl;
 }
 return 0;
}

#include <iostream>
#include <vector>
using namespace std;
void fun(int a, int b = 1, const vector<int> &v=vector<int>()) // 此处的const不能少，v也必须初始化(因为左边参数已经默认初始化)
{ 
}
int main ()
{
 fun(1);
 cout << "to end" << endl;
 return 0;
}