multiple definition linker error after adding a function to a previously linking file

Question

So my program is working fine. Compiling, linking, running, the works. Then, I decide to add a simple function to one of my files, like this:

#ifndef UTILITY_HPP
#define UTILITY_HPP

/* #includes here.  There's no circular include, I've checked. */

namespace yarl
{
    namespace utility
    {
        (several function declarations and definitions)

        bool isVowel(const char c)
        {
            if(c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u')
                return true;
            else return false;
        }
    }
}

#endif

That function definition is the only change I've made to my code. Everything else is exactly the same as it was. Nothing calls it yet. I compile, and it fails to link, with g++ giving one of these errors for every file that #includes this one:

./obj/Feature.o: In function `yarl::utility::isVowel(char)':
/home/max/Desktop/Development/Yarl Backup/yarl v0.27/src/Utility.hpp:130: multiple     definition of `yarl::utility::isVowel(char)'
./obj/Events.o:/home/max/Desktop/Development/Yarl Backup/yarl v0.27/src    /Utility.hpp:130: first defined here
./obj/GameData.o: In function `yarl::utility::isVowel(char)':

If I comment out isVowel, it works again. I've tried renaming it, still doesn't work. I've tried replacing it with just void randomFunctionName() {}, still doesn't work. I've tried making it non-inline and putting the function body in Utility.cpp, still doesn't work. I am extremely confused. Why would adding one simple function screw up the linker?

Answer 1

Either declare the function inline, or define it in a separate .cpp file. Otherwise every C++ file in which you include the header is trying to make its own, publicly-available definition of the function.

Edit: and fwiw, you don't need to explicitly return true or false if you're testing a conditional. Just return the conditional itself:

inline bool isVowel(const char c)
{
    return (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u');
}