do I HAVE to use the new keyword?

Question

This isn't the code I'm working on but it's the gist of what I want to do.

object *objects; int totalObjects; 

void addObject(object o)
{

    objects[totalObjects] = o;

    totalObjects++;


}

It's giving me an access error when I try this:

Unhandled exception at 0x00e8a214 in crow.exe: 0xC0000005: Access violation writing location 0xcccccccc

Am I going to have to use 'new' and if so do I have to create a new array to copy to every time? Can I just add or take elements from the array I'm using?

Answer 1

Why don't you just use std::vector?

std::vector<object> objects;

void addObject(object o)
{
    objects.push_back(o);
}

..or

void addObject(const object &o)
{
    objects.push_back(o);
}

to remove additional copying.

---

When it comes to implementing your own dynamic array without std::vector, Yes. you need to allocate new memory, and copy your array to new memory block. Here's my example code with malloc and placement new.

#include <stdlib.h>    // for malloc/free
#include <new>         // for placement new, std::bad_alloc

object *objects = nullptr;
size_t totalObjects = 0;

void addObject(const object &o)
{
    object *old_objects = objects;
    size_t old_size = totalObjects;

    size_t new_size = totalObjects + 1;

    object *new_objects = (object *)malloc(sizeof(object) * new_size);
    if (new_objects == nullptr)
        throw std::bad_alloc();

    size_t i;
    try
    {
        for (i = 0; i < old_size; ++i)
        {
            new (&new_objects[i]) object(old_objects[i]); // placement new
        }
    }
    catch (...)
    {
        // destroy new_objects if an exception occurs during creating new_objects
        for (size_t j = 0; j < i; ++j)
        {
            new_objects[i].~object();
        }
        free(new_objects);

        throw;
    }

    objects = new_objects;
    free(old_objects);
}

(I haven't tested the code yet >o<)

Note that I used malloc and placement new, not new operator. It's impossible to call copy constructor of each element of the dynamic array with array-new.

However, if your object is TriviallyCopyable, you can use realloc. It can be more efficient, because realloc can just expand memory block, without copying - if the memory is enough.

---

..And you can select multiple lines and just press TAB in Visual Studio (..or many other editors).

Answer 2

You declared an object pointer, but not yet allocated the actual memory to store object objects. Your assignment statement merely tries to copy the input object o into an unallocated array member.

This is why you should use new before the assignment. The new operator asks the system to allocate some memory in the required size, then return the address of that memory and assign it to the pointer. Then, the pointer points to that newly allocated memory and the assignment (or copying) can be made.

When you finished using the array space, you should free the allocated memory using delete.

Answer 3

Okay, I'm going to add an answer to my own question. Let me know if this is bad etiquette. I just wanted to post some of my own code to duel with yours.

#include <vector>
std::vector<object> objects;

okay so I want to have two arrays (vectors) for the objects and double for distances so I may end up with

std::vector<double> distances;

void swap(unsigned int a, unsigned int b)
{
    objects.swap_ranges(a,b);
    distances.swap_ranges(a,b)

}

I'm going by the cplusplus.com reference for this function so let me know if I have it wrong. I'm going to go through it and completely redo my code.

Is there a type like the matrix that will let me hold data of different types so I don't have to invent a new object to handle each one individually?

If what you wrote is the most efficient and fast way to do this then I'll make a new class to hold both items.

thanks :)