Question

I’m currently writing a program, and couldn’t figure out why I got an error (note: I already fixed it, I’m curious about WHY the error was there and what this implies about including .h files).

Basically, my program was structured as follows:

The current file I’m working with, I’ll call Current.cc (which is an implementation of Current.h).

Current.cc included a header file, named CalledByCurrent.h (which has an associated implementation called CalledByCurrent.cc). CalledByCurrent.h contains a class definition.

There was a non-class function defined in CalledByCurrent.cc called thisFunction(). thisFunction() was not declared in CalledByCurrent.h since it was not actually a member function of the class (just a little helper function). In Current.cc, I needed to use this function, so I just redefined thisFunction() at the top of Current.cc. However, when I did this, I got an error saying that the function was duplicated. Why is this, when myFunction() wasn’t even declared in CalledByCurrent.h?

Thus, I just removed the function from Current.cc, now assuming that Current.cc had access to thisFunction() from CalledByCurrent.cc. However, when I did this, I found that Current.cc did not know what function I was talking about. What the heck? I then copied the function definition for thisFunction() to the top of my CalledByCurrent.h file and this resolved the problem. Could you help me understand this behavior? Particularly, why would it think there was a duplicate, yet it didn’t know how to use the original?

p.s – I apologize for how confusing this post is. Please let me know if there’s anything I can clear up.

Answer 1

You can’t have two global functions with the same name (even in 2 different translation units). To avoid getting the linker error define the function as static so that it is not visible outside the translation unit.

EDIT

You can use the function in the other .cpp file by using extern keyword. See this example:

//Test.cpp

void myfunc()
{

}

//Main.cpp
extern void myfunc();
int main()
{
    myfunc();
}

It will call myfunc() defined in test.cpp.