Container Class vs Class - C++

Question

I'm new to programming and just come across this assignment

Create a container class family that holds an array of 20 person objects.

I have been looking on the internet as well as in my book, but I still can't figure out the difference between a Container Class and a Class in C++.

How could I create a family class and 20 person objects at the same time?

Answer 1

C++ is an object oriented language that encourages you to "encapsulate". The first step of this is to group concepts into objects described in terms of their data values and the operations that can be performed on that data.

So one could define an Account class consisting of an id and balance and functions to deposit or withdraw currency. This definition forms the type, but when you "instantiate" (create an instance of) this type, i.e.

Account a;

then the variable a refers to an object of type Account in this scope.

Sometimes you need a class that can store and track objects of other types. This is what is sometimes referred to as a "container class" and typically it combines a store and a count.

Say we want to store some floats. We could just write:

float store[64];

std::cout << "Enter the first number: ";
std::cin >> store[0];

But how would we track how many floats we have? We would probably need a counter.,

float store[64];
int stored = 0;

std::cout << "Enter the first number: ";
std::cin >> store[0];
stored++;

std::cout << "Enter the second number: ";
std::cin >> store[1];
stored++;

This works, and it's not terribly difficult, but if you are writing a function that expects to take a store and it's size, how do you express that?

void myFunction(std::string label, float* store, int count);

This requires two arguments and it's not exactly explicit.

C++ is about encapsulation: this idea of a "store" with a count of the contents could be encapsulated into a class:

struct Store {
    float store_[64] {};
    int count_ {0};
};

this is a container. We can now write our function that takes an object that contains other values with a single parameter:

void myFunction(std::string label, Store& store);  // & here = by reference

If this was 'C' you would write code that directly manipulated the values in the store:

store.store_[N] = 1;
store.count_++;

but that's messy, we didn't check there was room. In C++, we can encapsulate this into the class description with member functions and hide the member variables so that you have to go through our proscribed interface to manipulate the data.

#include <iostream>

class Store {
    enum { MaxCount = 64 };
    float store_[MaxCount] {};
    size_t count_ = 0;
public:
    // return the maximum number of elements we can store
    size_t capacity() const { return MaxCount; }
    // true/false: is the store empty?
    bool empty() const { return count_ == 0; }
    // return the current count
    size_t size() const { return count_; }

    bool add(float value) {
        if (count_ >= capacity()) {
            std::cerr << "store is full!\n";
            return false;
        }
        store_[count_] = value;
        ++count_;
    }
    // reset
    void clear() {
        count_ = 0;  // we don't actually need to change the store
    }
    // allow array-like usage
    const float& operator[](size_t index) const { return store_[index]; }
    float& operator[](size_t index) { return store_[index]; }
    // provide bounds-checked array-ish access
    float at(size_t index) const {
        if (index >= count_)
            throw std::invalid_argument("array index out of bounds");
        return store_[index];
    }
};

int main() {
    Store store;
    for (size_t i = 0; i < store.capacity(); ++i) {
        std::cout << "Enter number #" << i << " or -ve to stop: " << std::flush;
        float f = -1;
        std::cin >> f;
        std::cout << "\n" << f << "\n";
        if (f < 0)
            break;
        store.add(f);
    }
    std::cout << "You entered " << store.size() << " values:";
    for (size_t i = 0; i < store.size(); ++i) {
        std::cout << ' ' << store[i];
    }
    std::cout << '\n';
}

Live demo: http://ideone.com/boE3Ki

Answer 2

They are telling you to create a class that acts as a container for an array. In programming, especially when you first start, you will see many elements called containers because your teacher wants you to view variables, classes, and arrays (among other programming tools) as containers for you to store data in.

Viewing programming this way makes is much easier to conceptualize information as you get into more complex ideas like pointers.

So, long answer short, create a class with the array inside it. If you are learning about constructors and overloading make sure you are initializing it based on the data you pass in. If not, it should be only a couple lines of code to complete the project.

Answer 3

"Container class" is not some official term; it's just the word "class" with an English describing word next to it. The assignment is asking you to create a class that contains some other stuff; namely, an array of 20 person objects.

At its most basic, the result could be as simple as this:

class family
{
public:
   person people[20];
};

In real life, you might do something like this instead:

#include <array>
using family = std::array<person, 20>;

It seems unlikely that every family (or even most families) has precisely 20 people in it, so I would personally end up just going with:

#include <vector>
std::vector<person> family;

… and manipulating the vector as appropriate.