C++ How to use functions from a cpp file in a header file

Question

I have a class in a header file called bitarray.h and a corresponding bitarray.cpp and I also have a sieve.h. The sieve.h and bitarray.cpp #includes the bitarray.h and sieve.h only has a function void Sieve(BitArray a). I would like to call Set() and Unset() which are declared in bitarray.h and defined in bitarray.cpp from the Sieve function but it will not let me. How do I fix this.

    //sieve.h
    #include "bitarray.h"
    #include <cmath>

    using namespace std;

    void Sieve(BitArray a)
    {
   //initialize all to 1
   for (int i = 0; i < (a.arraySize*a.unsChar*8); i++)
   {
    a.Set(i);
   }

   //unset 0 and 1 becasue they are never prime
   a.Unset(0);
   a.Unset(1);
   //leave 2 as a 1
       /*for (int i = 2; i < (a.arraySize*a.unsChar*8); i++)
    a.Unset(2*i);*/
    }

Aesthete · Accepted Answer · 2014-04-17T00:26:17.383

2

Those functions, unless declared in the bitarray.h header, are local to the bitarray.cpp. If you want access to them in sieve.h, declare them in the bitarray.h header, or in another file accessible by sieve.cpp/sieve.h.

// bitarray.h
class BitArray
{
public:
  void Set(); // Example.
  void Unset(); // Example.
};

// sieve.h
#include bitarray.h
void Sieve(BitArray a)
{
  a.Set();
  a.Unset();
}

EDIT:

I'm pretty certain that the issue you're having, is that you're not seeing changes to your BitArray object because you're passing a copy of the original. Any changes you make to it in the Sieve() function won't be reflected in the original object.

Try passing the BitArray object by reference:

void Sieve(BitArray& a)
{
  ...

edited Apr 17 '14 at 00:26

answered Apr 17 '14 at 00:01

Aesthete

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there is no sieve.cpp – user3543331 Apr 17 '14 at 00:02
He meant `sieve.h` :) – Elvedin Hamzagic Apr 17 '14 at 00:03
Whatever file includes `bitarray.h`. – Aesthete Apr 17 '14 at 00:04
I don't think Sieve is a class, merely a function – Mooing Duck Apr 17 '14 at 00:04
Sieve is not a class but BitArray is – user3543331 Apr 17 '14 at 00:05
that is what mine looks like and set and unset are defines in BitArray.cpp so they cannot be called from sieve.h. If I define set and unset in bitaray.h will that fix the problem? – user3543331 Apr 17 '14 at 00:07
set and unset are declared in bitarray.h as public member data – user3543331 Apr 17 '14 at 00:08
I understand now, that is what I meant to say. They are declared in bitarray.h and defined in bitarray.cpp yet I cannot access them in sieve.cpp – user3543331 Apr 17 '14 at 00:11
When you include `bitarray.h` you have access to the _declaration_ of those methods. The linking phase will link these to the _definitions_ in `bitarray.cpp` file. If what you have is the same as what I've posted - there is something else wrong. Do you have any errors? Show us the contents of `bitarray.h` and `bitarray.cpp` – Aesthete Apr 17 '14 at 00:11
no errors but when I call set or unset they do not do anything – user3543331 Apr 17 '14 at 00:13
should sieve be declared as friend function in bitarray.h? – user3543331 Apr 17 '14 at 00:15
You need to post more code, and you expected input and output, because what sounded like a problem in your original question isn't actually the problem you're having. – Aesthete Apr 17 '14 at 00:15
I added the sieve file, does that help? – user3543331 Apr 17 '14 at 00:23
it is not complete yet – user3543331 Apr 17 '14 at 00:24
Yeah I think I know what your issue is, check my edits. – Aesthete Apr 17 '14 at 00:26

C++ How to use functions from a cpp file in a header file

1 Answers1