Const-correctness and hardware writes

Question

Say I have the following member function:

void CFoo::regWrite( int addr, int data )
{
  reg_write( addr, data ); // driver call to e.g. write a firmware register
}

Clearly, calling this function doesn't modify the internal state of the object it is called on. However, it changes the state of whatever this Foo instance represents.

In circumstances such as these, should Foo::regWrite(int addr, int data) be a const function?

Answer 1

You have to decide what the meaning is of "logically const" for the class CFoo, and that depends what the class is for.

If CFoo is construed as referring to some data, then it might make sense to be able to modify that data via a const instance of CFoo, in which case your member function would be const. For examples of this consider other types that refer to some data -- you can modify the referand of a char *const or a const std::unique_ptr<char>.

If CFoo is construed as owning some data, then it might make sense to forbid modification via a const instance of CFoo. For examples of this consider containers, where the elements are logically "part of the object's state" even when they aren't physically part of the object. So vector::operator[] has a const overload that returns a const T& rather than a T&, the insert member function is non-const, etc.

Answer 2

It is up to the programmer to define what 'const' shall mean for a class. With the specifier mutable you can even have a constobject with changing values in a member. When it comes to hardware one might consider the configuration as the target for const correctness: as long as the configuration does not change the object can be considered constant.