不可以这样使用吗？该如何解决

不可以这样使用吗？？
为什么编译通不过呀！
#include <iostream>
#include <vector>

using namespace std;

int main(){
  vector<int> ivec(10,1);
  if( !ivec.empty() ){
  vector<int>::reference rval = *ivec.begin();
  vector<int>::reference rval2 = ivec.front();
  vector<int>::reference rval3 = ivec[0];
  }
  cout << " rval: " << rval << endl;
  cout << "rval2: " << rval2 << endl;
  cout << "rval3: " << rval3 << endl;
   
  system("pause");
  return 0;
}

错误：13 rval' undeclared (first use this function) 
  14 rval2' undeclared (first use this function)
  15 rval3' undeclared (first use this function) 
这是什么原因？？？？？？？？？？？？

[解决办法]
if( !ivec.empty() ){
vector<int>::reference rval = *ivec.begin();
vector<int>::reference rval2 = ivec.front();
vector<int>::reference rval3 = ivec[0];
}
cout << " rval: " << rval << endl;
cout << "rval2: " << rval2 << endl;
cout << "rval3: " << rval3 << endl;

rval,rval2,rval3的生命在 if的 {}里面已经结束.
在作用域外面,没有 rval等几个的声明定义.
[解决办法]
你把这三句放到if语句里，不就可以了吗，局部变量的问题
 cout << " rval: " << rval << endl;
cout << "rval2: " << rval2 << endl;
cout << "rval3: " << rval3 << endl;