Why can't std::string* pstr; be used to hold a string in C++ while only std::string str; can?

Look at this code:

#include <iostream> #include <string> int main(void) {         std::string str;         std::cout << "Please enter string:" << std::endl;         std::cin >> str;         std::cout << str << std::endl;         return 0; }

It works fine.

Now I want to experiment with pointer to string.

#include <iostream> #include <string> int main(void) {         std::string* pstr;         std::cout << "Please enter string:" << std::endl;         std::cin >> *pstr;         std::cout << *pstr << std::endl;         return 0; }

It compiles and runs.

But after I type something and enter, the error message appears

Segmentation fault  (core dumped) ./a.out

I have no idea why.

Why can't std::string* pstr; be used to hold a string in C++ while only std::string str; can?