How to initialize a list of struct with constructor in c++

Question

I have a struct with a constructor likes:

struct Rectangle
{
    int width;
    int height;

    Rectangle(int _width, int _height)
    {
        width = _width;
        height = _height;
    }
}

And I create a Rectangle that is okay: Rectangle rect = Rectangle(4, 8);

But how to create a list a Rectangle struct with constructor: Rectangle rects[10];

That is an error: no default constructor exists for class "Rectangle".

Does it not allow create a list if I define a constructor in struct?

And how do I fix it?

Thanks!

Tip: Constructor lists. Use them!

tadman
– tadman

2020-12-02 03:00:25 +00:00
Commented Dec 2, 2020 at 3:00 — tadman
– tadman, Commented Dec 2, 2020 at 3:00

tadman · Accepted Answer · 2020-12-02 03:01:27Z

2

The "no default constructor" means there's no constructor that takes no arguments. You have one that takes 2. The compiler can't just make up values, it needs to know exactly what values to supply.

You can easily make this a default constructor by adjusting the signature to include defaults:

Rectangle(int _width = 0, int _height = 0)

Now it's usable as a default.

answered Dec 2, 2020 at 3:01

tadman

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Comments

Luke Randazzo · Accepted Answer · 2020-12-02 03:14:12Z

0

I think the spirit of the question might be rooted in you thinking about arrays in a more Java or C#-like way.

When you say Rectangle[10] in C++, it's directly allocating 10 contiguous rectangle objects, in this case on the stack in memory. If you did not have a constructor, it would use the 'default' constructor, setting the x and y members to their 'default' values (which are not 0, by the way).

Since you have designated a constructor, the compiler is inferring that you want to require that the class be instantiated with those two values!

If you want to provide a default constructor for situations such as this, try this:

Rectangle(): x(0), y(0) {}

answered Dec 2, 2020 at 3:14

Luke Randazzo

1

Comments

Loki Astari · Accepted Answer · 2020-12-02 03:39:44Z

And I create a Rectangle that is okay: Rectangle rect = Rectangle(4, 8);

Sure: But I would do it like:

 Rectangle rect{4, 8};

This form will come in useful in a second.

But how to create a list a Rectangle struct with constructor: Rectangle rects[10];

Rectangle rects[10] = {{1,2}, {2,3}, {3,4}, {4,5}, {6,7}, {1,2}, {2,3}, {3,4}, {4,5}, {6,7}};

That is an error: no default constructor exists for class "Rectangle".

That is because this:

 Rectangle rects[10];  // creates 10 Rectangles.
                       // There is no constructor with zero arguments
                       // so it can not create them

Several options here:

 // This allows multiple ways to construct a rect.
 Rectangle(int _width = 0, int _height = 0)
     : width(_width)
     , height(_height)
 {}

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