Adding element to Array of Objects in C++

Question

How do I add an element to the end of an array dynamically in C++?

I'm accustomed to using vectors to dynamically add an element. However, vectors does not seem to want to handle an array of objects.

So, my main goal is having an array of objects and then being able to add an element to the end of the array to take another object.

EDIT** Sorry, its the pushback() that causes me the problems.

class classex
{
private:
    int i;

public:
    classex() { }
    void exmethod()
    {
        cin >> i;
    }
};

void main()
{
    vector <classex> vectorarray;
    cout << vectorarray.size();
    cout << vectorarray.push_back();
}

Now I know push_back must have an argument, but What argument?

Answer 1

Arrays are fixed sized containers. So enlarging them is not possible. You work around this and copy one array in a bigger and gain space behind the old end, but that's it.

You can create a array larger than you currently need it and remember which elements are empty. Of course they are never empty (they at least contain 0's), but that's a different story.

Like arrays, there are many containers, some are able to grow, like the stl containers: lists, vectors, deques, sets and so on.

add a Constructor to set i (just to give your example a real world touch) to your example classex, like this:

class classex {
public:
    classex(int& v) : i(v) {}
private:
    int i;
};

An example for a growing container looks like this:

vector <classex> c; // c for container
// c is empty now. c.size() == 0

c.push_back(classex(1));
c.push_back(classex(2));
c.push_back(classex(3));
// c.size() == 3

Answer 2

Now I know push_back must have an argument, but What argument?

The argument is the thing that you want to append to the vector. What could be simpler or more expected?

BTW, you really, really, really do not want exmethod as an actual method of classex in 99% of cases. That's not how classes work. Gathering the information to create an instance is not part of the class's job. The class just creates the instance from that information.

Answer 3

EDIT: The question was how to add an element to an array dynamically allocated, but the OP actually mean std::vector. Below the separator is my original answer.

std::vector<int> v;
v.push_back( 5 ); // 5 is added to the back of v.

---

You could always use C's realloc and free. EDIT: (Assuming your objects are PODs.)

When compared to the requirement of manually allocating, copying, and reallocating using new and delete, it's a wonder Stroustrup didn't add a keyword like renew.