Accessing members of an array of structures in C++

Question

Working through C++ Primer Plus and am trying to cin data to a dynamically allocated array of structures. One of the items is a char array. How do I write to these struct members? Posting code of my wrong attempt so you can see what I'm trying to do.

    #include <iostream>
using namespace std;

struct contributions
{
    char name[20];
    double dollars;
};

int donors;

int main()
{
    cout << "How many contributors will there be?\n";
    cin >> donors;
    contributions * ptr = new contributions[donors];
    for(int i = 0; i <= donors; i++)
    {
        cout << "Enter donor name #" << i+1 << ": \n";
        cin >> ptr->contributions[i].name;
        cout << "Enter donation amount: \n";
        cin >> ptr->contributions[i].dollars;
    }

Thanks in advance!

Answer 1

Try using std::string instead of char[20] for name and the sample should work just fine.

struct contributions
{
    std::string name;
    double dollars;
};

also change the access to

ptr[i].name

Answer 2

cin >> ptr[i].name;

ptr is the name of the variable, it is of type contributions*. It is an array of contributions, so to access the ith member, use ptr[i]. Then access the name field of that member via ptr[i].name. Also, cin >> char[] may not work (I don't recall for sure), as char[] is more of a C-ish thing, whereas cin is C++. So you might need to change the type of name to std::string.

As an aside, convention is to name your structs/classes with a singular noun. Thus contribution would be a more correct name; every instance represents a single contribution.

Answer 3

Also, using a std::vector of contributions w2ill make the code much simpler. as it is, you have a memory leak. If this is direct from C++ Primer Plus I'd seriously suggest changing to a text book that will teach you modern, correct C++, such as Accelerated C++ by Koenig & Moo.

Answer 4

cin >> ptr[i].name; (the correct form) would stop at the first whitespace character (and risk a buffer overflow if no such character comes before the 20 spaces in the array are exhausted). Use cin.getline(ptr[i].name, 20) instead.