 <p>You have to <code>declare</code> the overload in the header and <code>define</code> it in the cpp file. Ex.:</p> <p>MyHeader.h:</p> <pre><code>Fraction&amp; operator+(const Fraction&amp; l, const Fraction&amp; r); </code></pre> <p>MyFile.cpp:</p> <pre><code>Fraction&amp; operator+(const Fraction&amp; l, const Fraction&amp; r){ // ... </code></pre>