Hello List, I please be kind, as I know this is not boost related. Inside the header file I define the following: /* Operator + */ std::vector<std::uint8_t> operator+( const std::vector<std::uint8_t>& x, const std::vector<std::uint8_t>& y ); /* Operator - */ std::vector<std::uint8_t> operator-( const std::vector<std::uint8_t>& x, const std::vector<std::uint8_t>& y ); And then inside the implementation CPP file I do the following: /* Operator + */ std::vector<std::uint8_t> operator+( const std::vector<std::uint8_t>& x, const std::vector<std::uint8_t>& y ) { *obmitted for brievity* }; /* Operator - */ std::vector<std::uint8_t> operator-( const std::vector<std::uint8_t>& x, const std::vector<std::uint8_t>& y ) { *obmitted for brievity* }; Yet when I compile I keep on getting : undefined reference to `operator+(std::vector<unsigned char, std::allocator<unsigned char> > const&, std::vector<unsigned char, std::allocator<unsigned char> > const&)' Is there something that I am missing? Kind Regards, Etienne Pretorius