C++ - Calling functions from other files and headers?

golmschenk · 04-11-2011, 08:45 PM

I feel like there should be a cleaner way of doing this. I have one file, for example "a.cpp", calling a function from another file, "b.cpp". Currently I have it set up so that header for "b", "b.h", has the declaration of its functions. And then I'm just including "b.h" in "a.cpp". Do I have to include the "b" header file in "a" to be able to call a function from "b"? Or is there a better way I could be doing this? Like doing something different at compile? Thanks!

graemef · 04-12-2011, 12:19 AM

That's what you need to do, although you can put b.h in either a.cpp or a.h I would say that most people would choose to put it in the a.h

golmschenk · 04-12-2011, 12:28 AM

Alright. I had put it in the header file. For some reason that just hadn't seemed right to me before. Thanks for the clarification!

dwhitney67 · 04-12-2011, 05:50 AM

Quote:

Originally Posted by graemef

That's what you need to do, although you can put b.h in either a.cpp or a.h I would say that most people would choose to put it in the a.h

Actually, b.h should be included in the file where the functions are implemented; that is, b.cpp. If the "b" functions are being called from a.cpp, then the header file should be included there too.

graemef · 04-12-2011, 07:33 AM

Quote:

Originally Posted by dwhitney67

If the "b" functions are being called from a.cpp, then the header file should be included there too.

It is not should, in that either will work. But because in practice most header files include more than just a function it may not be clearer to include it in the header.

When you have whole data structures in the header file it is often necessary for the header file to include others and then it becomes unnecessary to include it in the cpp file.

dwhitney67 · 04-12-2011, 07:41 AM

I'm not sure if you are agreeing with me, or taking a different stance.

If this is B.h:

Code:

#ifndef B_H
#define B_H

#include <map>

class B
{
public:
   B();

   void insertKV(int key, int value);

private:
   std::map<int, int> myMap;
};

#endif

Then for B.cpp:

Code:

// We include B.h here to ensure that B's API is correctly followed below...
#include "B.h"

B::B()
{
}

void
B::insertKV(int key, int value)
{
   ...
}

If A.cpp requires the use of the class B, and needs to call the method insertKV(), then it would be as follows:

Code:

// Again, B.h is included to ensure that B's API is followed correctly...
#include "B.h"

int main()
{
   B b;

   b.insertKV(10, 20);
}

graemef · 04-12-2011, 08:18 AM

What I am saying is that if you have an a.h file as well as an a.cpp then you can include the b.h file in either. If for arguments sake a.h is a class that also includes the B class then the b.h must be included in that file or in an included file that also includes b.h - but it would be cleaner to include it rather than relying on another file to bring it in because that other file might change. Because the file a.cpp will include a.h it then doesn't need to include b.h keeping the implementation file a little cleaner.

This is just a matter of style and there are exceptions to this but in my experience I would say that people tend to put the include statements in the header files.

dwhitney67 · 04-12-2011, 08:33 AM

Quote:

Originally Posted by graemef

What I am saying is that if you have an a.h file as well as an a.cpp then you can include the b.h file in either. If for arguments sake a.h is a class that also includes the B class then the b.h must be included in that file or in an included file that also includes b.h - but it would be cleaner to include it rather than relying on another file to bring it in because that other file might change. Because the file a.cpp will include a.h it then doesn't need to include b.h keeping the implementation file a little cleaner.

Yes, I agree.

Quote:

Originally Posted by graemef

This is just a matter of style and there are exceptions to this but in my experience I would say that people tend to put the include statements in the header files.

I only include files where needed. So for example, if within the file A.cpp, I required to use std::cout for debug purposes, then I would include iostream within the .cpp file, not the A.h file. Furthermore, I always include project header files before system header files; the first project header file included in the .cpp file should be for the class being implemented (of course, this is not applicable to the .cpp file containing main()).