C++ default constructor and 2-d array - Computer Technical Help

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C++ default constructor and 2-d array

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02-09-2013 , 05:00 PM

Addict_x

newbie

Join Date: Mar 2007 Posts: 43

In my professor's program example, he didn't use a default constructor. I was always told it's best to provide one so that all member variables are initialized in case the overloaded constructor fails. So, I started messing around...

Code:

class foo
{
  public:
           foo(); 
           foo(int arr_in[4][4]);

  private:
            int my_array[4][4];

}

function definitions:

Code:

	foo::foo()
	{
		for(int i = 0; i < 4; i++)
		{
			for(int j = 0; j < 4; j++)
			{
				my_array[i][j] = 0;
			}
		}
	}

	foo::foo(int arr_in[4][4])
	{
		for(int i = 0; i < 4; i++)
		{
			for(int j = 0; j < 4; j++)
			{
				arr_in[i][j] = arr_in[i][j];
			}
		}
	}

What I don't understand is why I can't create a foo object with the default constructor.

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02-09-2013 , 07:57 PM

adios

Carpal \'Tunnel

Join Date: Sep 2002 Posts: 22,558

Can you describe what happens exactly? Compile error, runtime error?

Quote

02-09-2013 , 08:24 PM

jmark

Pooh-Bah

Join Date: Dec 2003 Posts: 3,790

What's the purpose of this?
arr_in[i][j] = arr_in[i][j];

Quote

02-09-2013 , 08:33 PM

Addict_x

newbie

Join Date: Mar 2007 Posts: 43

Quote:

Originally Posted by adios

Can you describe what happens exactly? Compile error, runtime error?

Yeah, sorry, should've clarified. The default constructor and its implementation compile (VS2010 Pro, I can test later on gcc). In main:

Code:

int arr1[4][4] = { {1,2,3,4}, {1,2,3,4}, {1,2,3,4), {1,2,3,4} };
foo test_1(arr1)
test_1.print(); //foo member function

works fine. But this:

Code:

foo test_2() //default, should be filled with o's

does compile, but:

Code:

test_2.print();

does not compile. The error message:

Quote:

.print must have class/struct/union

It's not a foo object?

Quote

02-09-2013 , 08:41 PM

Addict_x

newbie

Join Date: Mar 2007 Posts: 43

Quote:

Originally Posted by jmark

What's the purpose of this?
arr_in[i][j] = arr_in[i][j];

oops, sorry, mistyped.

Code:

my_array[i][j] = arr_in[i][j]; //setting foo member var my_array

that's how it is in the program...

Quote

02-09-2013 , 09:53 PM

adios

Carpal \'Tunnel

Join Date: Sep 2002 Posts: 22,558

Quote:

Originally Posted by Addict_x

Yeah, sorry, should've clarified. The default constructor and its implementation compile (VS2010 Pro, I can test later on gcc). In main:

Code:

int arr1[4][4] = { {1,2,3,4}, {1,2,3,4}, {1,2,3,4), {1,2,3,4} };
foo test_1(arr1)
test_1.print(); //foo member function

works fine. But this:

Code:

foo test_2() //default, should be filled with o's

does compile, but:

Code:

test_2.print();

does not compile. The error message:

It's not a foo object?

Is print a method of foo? You need a ; after your statement instantiating test_2

Quote

02-09-2013 , 10:38 PM

Addict_x

newbie

Join Date: Mar 2007 Posts: 43

Quote:

Originally Posted by adios

Is print a method of foo? You need a ; after your statement instantiating test_2

Yes, it's a foo member function and the semicolons are all accounted for in the program (I should have done a better job in my op).

Quote

02-09-2013 , 10:55 PM

Addict_x

newbie

Join Date: Mar 2007 Posts: 43

OK, I just figured it out, it was a basic syntax problem. Initially, I declared a foo object implementing the default constructor like this:

Code:

foo test();

instead of the correct syntax:

Code:

foo test; // no parens

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02-10-2013 , 01:05 AM

jmark

Pooh-Bah

Join Date: Dec 2003 Posts: 3,790

It's easier if you post your actual code next time.

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02-10-2013 , 05:10 AM

#10

HypersionSD

centurion

Join Date: May 2012 Posts: 166

use the this type word.

Code:

 this.arr_in[i][j] = arr_in[i][j];

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