Avoiding duplicate symbols and retaining code in header files

Question

I have a single global helper function which is used by a bunch of macros in a header file. The intention is to allow the macros to be usable by simply #include'ing the single header (in other words, I'd like to keep the function definition in the header and avoid putting it in a separate compilation unit). However, this causes problems when an application #include's this file in more than one compilation unit, as the duplicate symbols problem arises.

The helper function has enough characteristics in which it shouldn't be declared inline.

I was messing around and found that using unnamed namespaces solves the problem of duplicate symbols, i.e.:

namespace
{
    void foo(...)
    {
    }
};

#define HELPER_A foo(1, ...);
#define HELPER_B foo(2, ...);
...

Is there any downside to this approach? Are there better alternatives?

Answer 1

You are only allowed one function definition in your project unless its marked as inline. You may have as many function declarations as you wish (aka function prototypes).

Move your function definition to a .cpp file and just leave the declaration in the header file

void foo(...); // no function body makes this a declaration only

or you could mark it inline:

inline void foo(...) { /* ... */ } 

inline functions should be small and computationally fast as a general rule.