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

Answer 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.