Segmentation Fault with `std::pair

I am using a std::vector of std::pair<int, int>.

std::vector< std::pair<int, int> > vec(n);

Where, n is the size of the std::vector.

I am taking input from stdin in this manner:

for(int i=0; i<n; ++i) std::cin>>vec[i].first>>vec[i].second;

Doing this, I get a segmentation fault. However, if I change my code to this:

for(int i=0; i<n; ++i){
    int x, y;
    cin>>x>>y;
    vec[i] = std::make_pair(x, y);
}

It works just fine. I am unable to figure out what's wrong with the first one, since it is working in my PC on small inputs, but giving a SIGSEGV on large random input. Also, I get the same behavior on replacing the vector with a static array of pair.

Also, I have faced errors while using a for loop on a vector of pair. If I do this:

for(int i=0; i<n; ++i){
    std::cout<<vec[i].first<<" "<<vec[i].second<<"\n"
}

However, if I change it to:

for(auto x: vec){
    std::cout<<x.first<<" "<<x.second<<"\n"
}

It works just fine. What and where is the problem here?