c++ 11 typeid of string and sizeof nullptr

I'm a newbie in c++11 and I'm aware of typeid().name() and nullptr. I was just having some fun with a code but i found out that sizeof(nullptr) is 4 and moreover typeid(str).name() is something complex set of characters NSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE

#include<iostream>

#include<string>
#include<typeinfo>

int main(){
    std::string str;
    std::cout<<sizeof(nullptr)<<std::endl;
    std::cout<<typeid(str).name();
    return 0;
}

I'm a bit confused about the output. Can someone kindly explain me ??