Member functions have an invisible argument: the pointer to the C++ object. You can access this argument using the 'this' keyword. John Carmack describes pure functions like so:
> Pure functions have a lot of nice properties. Thread safety. A pure function with value parameters is completely thread safe. With reference or pointer parameters, even if they are const, you do need to be aware of the danger that another thread doing non-pure operations might mutate or free the data, but it is still one of the most powerful tools for writing safe multithreaded code.
Emphasis on 'value parameters'. The invisible 'this' argument is not passed by value, it is passed by reference. By Carmack's definition, C++ member functions are not pure.
> Pure functions have a lot of nice properties. Thread safety. A pure function with value parameters is completely thread safe. With reference or pointer parameters, even if they are const, you do need to be aware of the danger that another thread doing non-pure operations might mutate or free the data, but it is still one of the most powerful tools for writing safe multithreaded code.
Emphasis on 'value parameters'. The invisible 'this' argument is not passed by value, it is passed by reference. By Carmack's definition, C++ member functions are not pure.