Nested Struct and Array C++

Question

Hello I am dealing with nested structs and arrays in C++, here is some background info:

 struct Cells // a collection of data cells lines 

     cells :: [Cell] // the cells: a location and a value 

     nCells :: Integer // number of cells in the array

     capacity :: Integer // maximum size of the array end 



struct Cell 
      location :: int // a location of data cells lines 
      value :: int // the value end Cells

The code I have which won't compile (3 files, header, ADT implementation, main) How am I declaring the nested struct in struct array wrong?

// Defines the  cell.h ADT interface
struct Cell;
struct Cells;


struct Cells {
    Cell cells[];
    int nCells;
   int capacity;
};

struct Cell {
   int location;
   int value;
};

//fill cells with random numbers
void initialize(Cells *rcells);

ADT Implementation

using namespace std;

#include <iostream>
#include <cstdlib>
#include "cell.h"

void initialize(Cells *rcells){
    for(int i = 0 ; i < rcells->nCells; i++)
   {
        rcells->cells[i].location = rand() % 100;
        rcells->cells[i].value = rand() % 1000;
    }
}

main

using namespace std;

#include <iostream>
#include <cstdlib>
#include "cell.h"

int main(){
    Cells *c;
    c->cells[0].location=0;
    c->cells[0].value=0;
    c->cells[1].location=0;
    c->cells[1].value=0;
    c->nCells = 2;
    c->capacity = 2;
    initialize(c);
}

Cell cells[]; You must declare an array size. Perhaps you actually wanted a pointer here? Cell *cells;? You've got some other fundamental errors, such as in your main using c before actually allocating any memory for it. — Greg Hewgill
– Greg Hewgill, Commented Jul 27, 2014 at 23:49
Undefined behavior. Where do you allocate memory for the cells ? — quantdev
– quantdev, Commented Jul 27, 2014 at 23:53
"Won't compile" is not useful at all. Be more specific. Very specific. — Qaz
– Qaz, Commented Jul 27, 2014 at 23:54
Is this actually c++ or is it c? you should use std::vector<Cell> and cells.push_back instead of that array. — Fantastic Mr Fox
– Fantastic Mr Fox, Commented Jul 28, 2014 at 0:31

Hsi-Hung Shih · Accepted Answer · 2014-07-28 01:04:14Z

Your original declaration fails because in

struct Cells {
    Cell cells[];
    int nCells;
    int capacity;
};

"cells" defined in this way is an array, which should have fixed size (unless it is the last member and you're using C99 standard). You may think it is the same as

Cell* cells

but it won't be converted to pointer type automatically in struct definition.

The C++ way to do such things is

typedef std::vector<Cell> Cells;

Your initialize function could be

void initialize(int ncell, Cells& cells) {
    cells.resize(ncell);
    for (Cell& cell : cells)
    {
         cell.location = rand() % 100;
         cell.value = rand() % 1000;
    }
}

Your main program should change a little bit

int main(){
    Cells c;
    initialize(2, c);

    c[0].location=0;
    c[0].value=0;
    c[1].location=0;
    c[1].value=0;
}

If you want cell count information, you can call

c.size()

There is no need for capacity variable because there is no upper limit on total number of cells.

By the way, this is not the nested struct people usually talk about. When one say nested struct, he often means nested struct definition. There is nothing special about an object containing other objects.

Lærne · Accepted Answer · 2014-07-28 00:54:54Z

Unlike some other programming languages, when you declare an array in C or C++, it is created where you declare it. For example if you declare one as a function local variable, it will be created on the stack.

In you case Cell cells[]; declares an array that must be created within your class. Thus if you class have an array of four elements, the compiler needs to allocate to each instance 4*sizeof(Cell) bytes for the field so it can fit the array in the instance. If your array have 524 elements, it needs 524*sizeof(Cell) bytes.

You see the problem here : the compiler cannot guess what is the size of your object. Which is problematic to obtain the location of each field in an instance, especially if you declare two array without size. Note that this issue is not restricted to object fields : you cannot declare an array as a local variable in a function without giving a size either, for example. This because array have a fixed size determined upon creation. Thus wherever you create the array, you must provide its size.

When you write Cell array[] as a function argument, you are not creating an array, but only obtaining a pointer to it, so not giving its size is okay.

To solve your issue you must somehow make classes with a constant size. For example you can allocate you array dynamically with new[some_variable] and use pointer Cell *cells; in your class. A pointer has a fixed size, and you array is to be declared on the heap (don't forget to delete[] it).

Remark: instead of a size, giving an array initializer only is valid :

int x[] = {1, 2, 4}; //creates an array of three elements

Collectives™ on Stack Overflow

Nested Struct and Array C++

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related