Pointer to array - initialize element

Question

I have a class Pixel and a class Image with a function used to update a pixel line. I want to initialize the pixel line. My problem is to initialize the array. Actually I have this :

bool UpdateLine(Pixel line[], int nb)
{
    bool noError = true;
    line = new Pixel[nb];
    for (int r = 0; r < nb; r++)
    {
        line[r] = new Pixel(); // -> line causing troubles

        // do some stuff with my pixel
        [...]
    }
    return noError;
}

When I try this I have :

no viable overloaded '='

How can I initialize each elements for my array ?

`line` is an array of objects! They are all already constructed. C++ works differently to C# or Java. [A good book or two is a must, I think.](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) — StoryTeller - Unslander Monica, Sep 26 '17 at 09:19

score 4 · Accepted Answer · answered Sep 26 '17 at 09:20

4

You actually have two problems.

The first, regarding your error, is because new Pixel() results in a pointer to a Pixel object. In C++ you don't need new to create objects (do you come from a Java or C# background perhaps?). The initial allocation of the array creates the objects for you.

The second problem is that you assign to the pointer variable line, but line is a local variable inside the function. All modification to it will be lost once the function returns, and you will have a memory leak. You need to pass line by reference.

In the future when dealing with collections of a single type of data, I suggest you use std::vector instead. You still need to pass the vector by reference though, if you want to add elements to it.

answered Sep 26 '17 at 09:20

Some programmer dude

400,186
35
402
621

Or enjoy the beauty/power of modern C++ and just return the vector by value – StoryTeller - Unslander Monica Sep 26 '17 at 09:25
My code has been automatically generated from C# by a piece of software, I am not yet very comfortable with the C++. I will use vector instead array. Thank you for your answer. – A.Pissicat Sep 26 '17 at 09:27
Just for sure, `bool UpdateLine(std::vector* line, int nb)` will be better ? – A.Pissicat Sep 26 '17 at 09:31
@A.Pissicat I would use `std::vector&` instead. – Some programmer dude Sep 26 '17 at 09:32

score 2 · Answer 2 · answered Sep 26 '17 at 09:19

2

    line[r] = new Pixel(); // -> line causing troubles

line[r] is a Pixel object, not a pointer, so you can't assign a pointer to it.

Why aren't you using a std::vector?

answered Sep 26 '17 at 09:19

xaxxon

19,189
5
50
80

I'll use vector instead, this part has been automatically generated from C#, I had not thought of replacing it with a vector – A.Pissicat Sep 26 '17 at 09:28
@A.Pissicat most times when you're looking at dynamically sizing an array in C++, you should be using a std::vector instead. Create the vector inside your function and return it by value. No need to pass in an empty one to the function. – xaxxon Sep 26 '17 at 09:30

Pointer to array - initialize element

2 Answers2