关于嵌套结构C ++

Question

I want to learn c++, and here is my beginner question.

#include <iostream>
#include <string>

using std::cout;
using std::endl;

struct student{
    struct address{
        int no;
        std::string city;    //problem is here
    };

    char grade;
    int num;
    address *addr;
};

int main(){

    student st1;
    student *pSt1 = &st1;

    pSt1->grade = 'A';
    pSt1->num = 1234;

    pSt1->addr->no = 123;
    pSt1->addr->city = "Imladris";

    return 0;
}

It crashes, but if I change std::string city like this:

struct student{
    struct address{
        std::string city;   // here
        int no;
    };

    char grade;
    int num;
    address *addr;
};

It doesn't crash and returns 0... No errors nor warnings in either case. Maybe, there is no need to use a pointer here but as I said, I am learning c++. This is a sample code about pointers/structures.

I know how to fix it but I'd like to understand why it breaks. I change the declaration order of string city and it doesn't crash. Why?

Answer 1

You just came across the term Undefined behavior . The end result of your code is compiler and platform dependent. Dereferencing a pointer which value is not specified is (as mentioned here ) considered to be one of these cases where you are not able to predict the outcome of an operation.

Answer 2

Keep your nested structure but add this to your member variable declaration:

address *addr = new address();

Or a better way is to use it as user @tobi303 mentioned: Don't declare it as a pointer, just an instance of address instead:

address addr;

But use it this way:

pSt1->addr.no = 123; // set
pSt1->addr.city = "Imladris"; // set