c++ 11 typeid of string and sizeof nullptr

Question

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 ??

Answer 1

Both of these things are implementation defined.

sizeof(nullptr) is 4

I would assume that on that platform all pointers are size 4, and nullptr's size was chosen to match that.

typeid(str).name()

std::string is an alias for an instantiation of a template, std::basic_string<char, std::char_traits<char>>. The name is further mangled to conform to your platform's format for executables. There may be a tool to "demangle" names provided by the platform.

Answer 2

nullptr has type nullptr_t which is

typedef decltype(nullptr) nullptr_t;

so, it is implementation defined.

The complex string which you see looks like this because of name mangling C++ compiler is doing. Mangling is done in C++ because of overload, as same name can be used for different functions or methods; or a template is used for type declaration. More about it you can read in https://en.wikipedia.org/wiki/Name_mangling#C++

It is also implementation defined and other compilers may use other schemes for name mangling.