Importing header file multiple times with C function in Objective-C

Question

In my Xcode project I made a standalone header file with the following C utility function:

#ifndef FOO
#define FOO
CGFloat DistanceBetweenTwoPoints(CGPoint p1, CGPoint p2)
{
    CGFloat dx = p2.x - p1.x;
    CGFloat dy = p2.y - p1.y;
    return sqrt(dx*dx + dy*dy);
};
#endif

Even with the preprocessor directives, if I try to import or include that header file in multiple locations, I receive the following error complaining about duplicate symbols:

linker command failed with exit code 1

Is there a different way I can achieve this effect? This question is out of curiosity more than anything else.

Thanks

djromero · Accepted Answer · 2012-10-08 13:35:39Z

1

Put your function body in a .c file and the function declaration (aka prototype) in the .h with those #ifndef, etc. Then use include to import the header file where you need the function.

Remember to check the target membership of the .c file, otherwise it won't be compiled.

For a small function like that you can declare it inline and just use the header file:

inline CGFloat DistanceBetweenTwoPoints(CGPoint p1, CGPoint p2)
{
    // code as is
}

The compiler will replace your function calls with the function code without actually building and linking a new object file. No more duplicated symbols.

answered Oct 8, 2012 at 13:35

djromero

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Comments

molbdnilo · Accepted Answer · 2012-10-08 13:34:36Z

1

Since the function's definition is in the header, you get one definition of the function in each file (actually, each translation unit) where it is included.

Move the function body into a ".c" file, leaving a declaration in the header, or declare it inline.

answered Oct 8, 2012 at 13:34

molbdnilo

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