How to Access an array Struct Inside an array Struct in C++?

Question

I'm trying to access the variable name of the Reservation struct like this hotel[SomeIndex].reservations[AnotherIndex].name but it's not working.

How can I access those variables to fill the structure?

PS: It compiles,but it shows in debugger Segmentation Fault.

struct Reservation{
string name;
};

struct Hotel {
string name;
Reservation *reservations;
};



int main()
{
    struct Hotel *hotel;
    hotel = new Hotel[20];
    hotel->reservations=new Reservation[10];


    hotel[9].name="Olympus Plaza";
    hotel[9].reservations[5].name="John Doe";

    cout<<"Hotel: "<<hotel[9].name<<" Name: "<<hotel[9].reservations[5].name<<endl;

return 0;   
}

Answer 1

hotel->reservations=new Reservation[10]; is equivalent to hotel[0].reservations=new Reservation[10]; . You've initialised hotel[0] but none of the other elements of hotel - specifically not hotel[9] .

It looks like what you need is to define constructors for Hotel and Reservation that initialise all their members to well-defined values.

And I would strongly suggest you use std::vector rather than raw arrays; arrays are an advanced feature and very easy to go wrong with.

Answer 2

You do not correctly initialize the reservations. Doing this correctly with raw pointers is hard and error-prone, and absolutely not recommended in C++.

First of all, use an std::vector<Hotel> instead of a raw array Hotel * . Vectors are the normal C++ "array" objects.

You can then replace the raw Reservation * pointer inside the Hotel struct with a std::vector<Reservation> as well.

This makes it much easier to fix the actual error: The missing initialization.

What you did was create 20 hotels, then create 10 reservations for the first hotel ! Then you try to access reservations for the 9th hotel, where there is an uninitialized pointer which points to random data. This means the behaviour is undefined: In this case, a segmentation fault is the way that your system shows you that you are accessing data which doesn't belong to you.

You need a loop to create reservations for each of the hotels, or if you just want to create reservations in the 9th hotel, you need to specify its index.

Using std::vector is very simple:

#include <vector>

struct Reservation {
    string name;
};

struct Hotel {
    string name;
    vector<Reservation> reservations;
    // if you have no "using namespace std", then it's "std::vector".
};

And then you can just create reservations for the correct hotel:

int main()
{
    vector<Hotel> hotel(20);
    hotel[9].reservations.resize(10);

    hotel[9].name="Olympus Plaza";
    hotel[9].reservations[5].name="John Doe";

    cout<<"Hotel: "<<hotel[9].name<<" Name: "<<hotel[9].reservations[5].name<<endl;

    return 0;   
}