关于与ios::sync_with_stdio(false);有关的问题

#include<bits/stdc++.h>
using namespace std;
int a,b;
int main(){
    ios::sync_with_stdio(false);
    freopen("plus.in","r",stdin);
    freopen("plus.out","w",stdout);
    cin>>a>>b;
    cout<<a+b;
    return 0;
}

为什么这么写会
terminate called after throwing an instance of 'std::ios_base::failure'
what(): basic_filebuf::underflow error reading the file

你这文件在哪个目录下呢？应该是文件打开没有成功