How can I "override" [] to accept two arguments in C++?

Question

I am trying to create a bit-vector class in C++ to model some hardware. In most HDLs (hardware description langauges) that I know, specific bits are referenced like this:

my_vector[bit_position]

and sub-vectors are referenced like this:

my_vector[msb:lsb]

or

my_vector[msb,lsb]

I want to be able to do something similar with my bit-vector class. Is there any way to tell operator[] to accept two arguments?

The alternatives I've considered are:

1. using a range method:

   my_vector.range(msb,lsb)

2. using a string and parsing it:

   my_vector["msb:lsb"]

But neither of them is attractive. The first, because it is too different from the way it's modeled in HDL, the second because I don't like dealing with strings when I don't have to, and it seems inelegant.

What's the best way to do this?

Answer 1

The issue:

Apart from operator() all operators have a fixed arity, which effectively precludes any kind of change

You then have several solutions:

• overload operator() instead: vector(msb, lsb)
• use two successive invocations: vector[msb][lsb]
• overload the comma operator: vector[msb,lsb]

The last solution matches the syntax you require, but is somewhat subtle:

• you first need either msb or lsb to be of a custom type (for operators cannot be overloaded on built-ins only)
• you then provide an overload of operator, for this type, returning a Range object
• you finally provide a custom operator[](Range) on your class

The real bummer is the first point: that one of msb or lsb need be of a custom type. This can be somewhat alleviated using Boost.StrongTypedef which creates a custom type that mimicks an existing one.