Creating an array of a struct

Question

I am trying to make an array of structs, but I am getting the error no matching function for call to 'Cell::Cell()'.

Cell is the name of my struct. Here is some of my code:

struct Cell{
    int number;

    Cell(int n){
        number = n;
    }
};

class MyClass{
    public:
        int nCells;

        void inject(){
            std::cout << "Enter number:";
            string in;
            std::cin >> in;
            int amount = in.size()/3;

            Cell cells [amount]; // <-- error

            int index = 0;
            int t = in.size();
            while (t >= 3){
                cells[index] = new Cell(atoi(in.substr(t-3,3).c_str());
                t -= 3;
                index++;
            }
        }

        MyClass(int n){
            nCells = n;
        }
};

Cell cells [amount]; is giving me the error. I am new to classes, but I know how to make arrays of primitive types. int cells [amount]; would work, for instance.

But how am I supposed to make an array of type Cell ?

Where's default constructor in Cell? You does not have it.

Jepessen
– Jepessen

2014-01-29 16:04:59 +00:00
Commented Jan 29, 2014 at 16:04 — Jepessen
– Jepessen, Commented Jan 29, 2014 at 16:04

Sebastian Hoffmann · Accepted Answer · 2014-01-29 16:14:49Z

4

Cell doesnt have a default constructor (as soon as you specify another constructor the compiler will not create a default constructor anymore). However the definition Cell cells[amount] will automatically default initialize every element.

I think the best way in this particular situation is simply to implement a default constructor:

struct Cell{
    int number;

    Cell() : number(0)
    {
    }

    Cell(int n) : number(n)
    {
    }
};

Also notice as amount is not known at compile time, Cell cells[amount] is basically illegal. However some compilers have extensions to allow this. But its better if you heap allocate it:

Cell* cells = new Cell[amount];

Dont forget to destroy it however.

edited Jan 29, 2014 at 16:14

answered Jan 29, 2014 at 16:07

Sebastian Hoffmann

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11 Comments

Alan Baljeu Over a year ago

Modern C++ prefers std::shared_ptr<Cell> cells(new Cell[amount]), because then you can't forget to destroy it.

Chronicle Over a year ago

It works! I have a side question: Is there a difference between the way we wrote our constructors? You write Name(argument): number(n) {} , while I wrote Name(argument) { number(n) }

Alan Baljeu Over a year ago

It's the same story as your question. " : number(n)" calls the constructor for number with parameter n. { number = n; } calls the default constructor for number, and then AFTER does number= n. In this case, no real difference, but in general it's less efficient - and if you don't have a default constructor (like you didn't) it would actually fail your way.

James Kanze Over a year ago

No, no, no with regards to new Cell[amount]. Use std::vector<Cell> cells(amount);. (There's practically never a case where you'd use an array new.)

James Kanze Over a year ago

@Paranaix His code didn't work. Array new is almost always a bad solution, when we have std::vector.

|

BЈовић · Accepted Answer · 2014-01-29 16:08:33Z

2

If you know how long the array is, you can use c++11 initialization. This will do :

int main()
{
    Cell c[3]{ Cell(1), Cell(2), Cell(3) };
}

By the way this

Cell cells [amount];

is using VLAs, and that is not supported by c++ (only as extension for some compilers).

In c++, much better would be to use std::vector :

#include <vector>


struct Cell{
    int number;

    Cell(int n){
        number = n;
    }
};

int main()
{
    int n = 5;

    std::vector< Cell > c;

    for ( int i =0; i < n; ++ i )
    {
        c.emplace_back( Cell( i ) );
    }
}

answered Jan 29, 2014 at 16:08

BЈовић

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2 Comments

James Kanze Over a year ago

Why emplace_back here, which is only supported by C++11, when push_back will generate exactly the same code.?

BЈовић Over a year ago

@JamesKanze No idea. As you said, push_back works fine as well :)

John Dibling · Accepted Answer · 2014-01-29 16:24:52Z

0

By doing Cell cells [amount]; you are calling the Cell constructor, but in this case you don't have a default constructor for Cell, so you must use pointers instead, you are using them in the while stuff.

Just change

Cell cells [amount];

for

Cell* cells [amount];

edited Jan 29, 2014 at 16:24

John Dibling

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answered Jan 29, 2014 at 16:05

Netwave

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4 Comments

Netwave Over a year ago

@JohnDibling, it does have a constructor, but not a default one...thats why it is crashing.

crush Over a year ago

@JohnDibling It has a constructor, but not a default constructor.

John Dibling Over a year ago

Oh, it sure didn't look like it had any defined constructors at all when I viewed this question earlier. I must have just missed it.

James Kanze Over a year ago

This is about the worst solution imaginable. What's wrong with providing a default constructor, or using std::vector<Cell>? The less you allocate dynamically, the better you'll be.

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