One of the many arguments against allowing patents for software (alongside the principle argument that software is made up of algorithms, which are essentially mathematics, which is pure knowledge and hence is not patentable) is the fact that software is anyway covered by copyright law. This means that others cannot simply copy your code, just as a novelist cannot simply copy large chunks of someone else's writing. But whether copyright law prevents others from copying the underlying ideas of that code by re-implementing them independently is another matter.