cin and char array pointer in struct - c++

Question

As a c++ beginner I've written the following code:
int main(void){
struct car{
char * make[200];
int manfYear;
};
int num=0;
cout << "How many cars do you wish to catalogue? ";
cin >> num;
car * Cars = new car [num];
for (int i=1;i<=num;i++){
cout << "Car #" << i << ":" << endl << "Please enter the make: ";
cin.getline(*Cars->make,200);
cout << "Please enter the year made: ";
cin >> Cars->manfYear;
}
My problem is that I can't get my head round a problem where I get a segfault when running the program at the point I need to enter the model of the car. Can someone please explain what I'm doing wrong?
As far as I understand it I'm passing a pointer to the array "make" which should make it work. Is my understanding way off?
Thanks in advance
Dan

Answer 1

Four issues I see right away:
Issue 1
In your struct, you have:
char * make[200];
In English, this is saying, "create an array of 200 pointers to character", when I think you want to say, "create an array of 200 characters." So you should have instead:
char make[200].
Issue 2
You are looping by starting at 1. This will skip the first car in the array - remember arrays are zero-indexed. So you should have instead:
for (int i = 0 ; i < num ; i++)
and for display purposes, you could say:
cout << "Car #" << (i+1) << ":" << endl << "Please enter the make: ";
Issue 3
Where you say:
cin.getline(*Cars->make,200);
and
cin >> Cars->manfYear;
Where in these lines are you specifying which car the user is populating? Nowhere. If you are looping with i, then you need to actually mention i. These should work:
cin.getline(Cars[i].make,200);
and
cin >> Cars[i].manfYear;
Notice that we are using ., not ->. This is because the items in the Cars array are actual instances, not pointers. The Cars array is itself a pointer, but not its contents.
Issue 4
All credit to #Ben C who pointed this out first: mixing the >> operator with getline() function on cin can lead to strange behavior, with leftover CR's from >> going into the getline() call. You could use either all >> (disadvantage: you don't have the 200 limit enforced when reading the make) or all cin.getline() (disadvantage: you will have to use string buffers and then convert them for number of cars and year), or put cin.ignore() after each invocation of >>, like so:
cin >> num;
cin.ignore();
and
cin >> Cars[i].manfYear;
cin.ignore();
Again, all credit to #Ben C for noticing this first.
Last But Not Least
By convention, classes/structs have capital names, and variables have lowercase / camelcase names. Consider renaming the struct from car to Car, and the array from Cars to cars. In other words, the reverse of the capitalization you have right now.
Finally, I concur with all the other posters here: you should consider using string instead of char arrays.

Answer 2

First, use string instead of poor old C char[].
Next: you don't want char * make[200];. You want char make[200];. char * make[200] is an array of 200 pointers to chars, which can be used as 200 null-terminated strings - but then, you would have to new[] each of them. Just use char make[200]; and cin.getline(Cars->make, 200);.

Answer 3

char * make[200] declares an array of 200 pointers; I'm guessing this isn't what you're after.
If you're simply looking to store a string, I'd recommend taking a look at the C++ string type instead.
#include <iostream>
#include <string>
int main()
{
using namespace std;
struct car
{
string make;
int manfYear;
};
int num=0;
cout << "How many cars do you wish to catalogue? ";
cin >> num;
car * Cars = new car [num];
for (int i=1;i<=num;i++)
{
cout << "Car #" << i << ":" << endl << "Please enter the make: ";
std::cin.ignore();
getline(cin, Cars[i-1].make);
cout << "Please enter the year made: ";
cin >> Cars[i-1].manfYear;
}
}
There were also a couple of other minor niggles with your code.
1) You had been using Cars->manfYear - this will only ever point you to the first element of your array. I'm assuming you don't want that; using the subscripting syntax as per Cars[i-1].manfYear will access an individual car object in your array. (Remember that array indexes start from zero! - it would actually make more idiomatic sense for your for loop variable to start at zero as well really)
2) Be wary of the way std::getline and the >> symbol work together. the >> (stream extraction operator) often leaves any newline characters, which means you might see "odd" behaviour with your calls to getline. If you're mixing the two together, then using something like std::cin.ignore() will help you discard the newline character.

Answer 4

First of all, arrays in C++ are indexed from 0..n-1, so your loop needs to run from
for (int i = 0; i < num; i++) { ... }
Secondly, you've declared make as a 200-element array of pointer to char; this is most likely not what you want. If make is supposed to store a character string, declare it as a plain array of char:
struct car{
char make[200];
int manfYear;
};
Finally, rewrite your getline call as
cin.getline(Cars[i].make, sizeof Cars[i].make); // fixed per comment below
Even though Cars is declared as a pointer, you can use the subscript operator on it as though it were an array; by doing so, you implicitly dereference Cars, since a[i] is interpreted as *(a + i). This also means that you will use the . component selection operator as opposed to the -> operator, since the type of Cars[i] is car, not car *.

Answer 5

First of all char *make[200] is not a string of max 200 characters but 200 pointers to char.
Second thing: you are dereferencing the pointer in cin.getline with *: what happens is that you obtain the value contained in first cell of the 200 char* pointers. But you didn't initialize the single pointers, just the higher level one, so you get a segfault.
Just change char* make[200] to char make[200] and *Cars->make to Cars[i].make.