How to initialize an array in a class definition or in constructor?

pablowablo · #1 18-Jun-2004, 05:05

Let's say I have a class called Foo having a constant 2D integer array. And I want to initialize its values in the class definition or constructor... How should I do this? I always get an error when I try initializing it in the declaration or in the constructor

INITIALIZING IN THE DEFINITION

CPP / C++ / C Code:


class Foo
{
    const int twoD[2][2] = { {0,1}.{1,2}};
};

I get a syntax error when I do this.

INITIALIZING IN THE CONSTRUCTOR

CPP / C++ / C Code:


class Foo
{
   int twoD[2][2];
};

Foo::Foo()
{
  twoD[2][2] = { {0,1},{1,2}};
}

I also get an error when I do this, and when I try to declare it as extern in the definition, it doesn't help... anyone can help me out?

machinated · #2 18-Jun-2004, 08:39

you can't define anything in a class. you may only declare it. that's why constructor is a good idea. However since your array has already been declared in the class, you may not use the initializer list. Here's the correct way:

CPP / C++ / C Code:

class Foo
{
   int twoD[2][2];
   Foo();
};

Foo::Foo()
{
  twoD[0][0]=0;
  twoD[0][1]=1;
  twoD[1][0]=1;
  twoD[1][1]=2;
 //or run nested for loops
}

pablowablo · #3 18-Jun-2004, 08:45

hey thanks for the reply!

I guess there's no easier way to do it... I wanted to make my code clean.

Thanks again

Quote:

Originally Posted by machinated

you can't define anything in a class. you may only declare it. that's why constructor is a good idea. However since your array has already been declared in the class, you may not use the initializer list. Here's the correct way:

CPP / C++ / C Code:

class Foo
{
   int twoD[2][2];
   Foo();
};

Foo::Foo()
{
  twoD[0][0]=0;
  twoD[0][1]=1;
  twoD[1][0]=1;
  twoD[1][1]=2;
 //or run nested for loops
}

dsmith · #4 18-Jun-2004, 08:50

Quote:

Originally Posted by pablowablo

Let's say I have a class called Foo having a constant 2D integer array. And I want to initialize its values in the class definition or constructor... How should I do this? I always get an error when I try initializing it in the declaration or in the constructor

INITIALIZING IN THE DEFINITION

CPP / C++ / C Code:


class Foo
{
    const int twoD[2][2] = { {0,1}.{1,2}};
};

I get a syntax error when I do this.

INITIALIZING IN THE CONSTRUCTOR

CPP / C++ / C Code:


class Foo
{
   int twoD[2][2];
};

Foo::Foo()
{
  twoD[2][2] = { {0,1},{1,2}};
}

I also get an error when I do this, and when I try to declare it as extern in the definition, it doesn't help... anyone can help me out?

I don't think this is possible. C++ does not allow you to initialize a function member which in turn would make that static inside a class. A class is not meant to have any static variables.

The only time that you can initialize an array is at the time that you define it. For that reason, the second one is invalid. The only work around that I can see is to assign the integers one at a time...

Someone else may have an idea though...

dabigmooish · #5 18-Jun-2004, 17:12

assigning the values using a loop, within your constructor works. I've done it before

pablowablo · #6 18-Jun-2004, 17:21

Thanks for all the help guys. I fixed it by looping in the constructor

ciacia · #7 11-Jun-2009, 02:28

actually its possible to write

CPP / C++ / C Code:

MyObject [2] = {MyObject(0, 3), MyObject (2,4)};

Howard_L · #8 11-Jun-2009, 09:47

Can you give us a complete example of how you can get that to work?

davekw7x · #9 11-Jun-2009, 12:23

Since someone decided to revive this five-year-old thread, I thought I would add my .02 euros to try to throw some light on the original question.

The Original Poster wanted to initialize a const array in his class.

Now, if an array is const, it can't be changed, therefore all objects will have the same array values, so a static member is appropriate.

Regardless of opinions of others who said it can't be done and who said things like "classes aren't supposed to have" such stuff, It can be done, and, if you need it, I'll show a way.

Bottom line: Declare the array static. Static arrays, like any other static class variables, have to be initialized somewhere outside the class definition (not in the constructor). That's all there is to it:

CPP / C++ / C Code:

//
// This is foo.h
//
// davekw7x
//
#ifndef _FOO_H__
#define _FOO_H__
class Foo
{
  private:
    static const int twoD[2][2];
  public:
    int getTwoD(const int & i, const int & j);
};
#endif

CPP / C++ / C Code:

//
// This is foo.cpp
//
// davekw7x
//
#include <iostream>
#include <cstdlib>
using namespace std;
#include "foo.h"
//
// The static member array is declared once
// It's a const, and all objects have the same
// values for this array
//
const int Foo::twoD[2][2] = { {1,2},{3,4} };

//
// Access: Bail out if illegal index value
//
int Foo::getTwoD(const int & i, const int & j)
{
    if ((i < 0) || (i >= 2) || (j < 0) || (j >= 2)) {
        cerr << "Illegal index: valid values are 0 and 1" << endl;
        exit(EXIT_FAILURE);
    }
    return twoD[i][j];
}

CPP / C++ / C Code:

//
// This is test_static_array.cpp
//
#include <iostream>
#include "foo.h"
using namespace std;
int main()
{
    Foo foo;
    int i, j;
    for (i = 0; i < 2; i++) {
        for (j = 0; j < 2; j++) {
            cout << "twoD[" << i << "][" << j << "] = " << foo.getTwoD(i,j) 
                 << endl;
        }
    }
    cout << endl;

    cout << "Enter i and j: ";
    while (cin >> i >> j) {
        cout << "Now trying twoD[" << i << "]["
             << j << "]: " << flush;
        cout << foo.getTwoD(i,j) << endl;

        cout << endl;
        cout << "Enter i and j: ";
    }
    cout << "Goodbye for now." << endl;

    return 0;

}

Here's a run

Code:

twoD[0][0] = 1
twoD[0][1] = 2
twoD[1][0] = 3
twoD[1][1] = 4

Enter i and j: 0 1
Now trying twoD[0][1]: 2

Enter i and j: 1 1
Now trying twoD[1][1]: 4

Enter i and j: 1 2
Now trying twoD[1][2]: Illegal index: valid values are 0 and 1

This simple example gracelessly bails out of the program when invalid array index values are entered by the user.

A more attractive solution would have the access function throw an exception with a message that the user program could catch and recover without aborting the program, but the approach using the static array would be the same.

Regards,

Dave

L7Sqr · #10 11-Jun-2009, 16:07

Quote:

Originally Posted by davekw7x

Now, if an array is const, it can't be changed, therefore all objects will have the same array values, so a static member is appropriate.

That is not the case; const is not static.

For example:

CPP / C++ / C Code:

#include <iostream>

class Foo {
        const int twoD[2][2];
    public:
        Foo() : twoD((int[2][2]){{1,2},{3,4}}) {}
        Foo(int a,int b,int c,int d) : twoD((int[2][2]){{a,b},{c,d}}) {}

        void show () const {
            std::cout << "[[" << twoD[0][0] << "," << twoD[0][1] << "],["
                      << twoD[1][0] << "," << twoD[1][1] << "]]" << std::endl;
        }
};

int main () {

    Foo f1;
    Foo f2(4,3,2,1);

    f1.show ();
    f2.show ();

    return 0;
}

Quote:

Originally Posted by output

[[1,2],[3,4]]
[[4,3],[2,1]]

The initializer list can be used specifically for situations like this (base constructors, const members, etc).

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How to initialize an array in a class definition or in constructor?

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