should C++ class "helper functions" be members, free, or anon-namespace free?

Question

So, I have a class. It's a useful class. I like a lot. Let's call it MyUsefulClass. MyUsefulClass has a public method. Let's call it processUsefulData(std::vector<int>&).

Now suppose processUsefulData really does two things and I want to refactor it from this:

std::vector<int> MyUsefulClass::processUsefulData(std::vector<int>& data)
{
    for (/*...*/)
    {
        for (/*...*/)
        {
            // a bunch of statements...
        }
    }

    for (/*...*/)
    {
        for (/*...*/)
        {
            // a bunch of other statements...
        }
    }
    return data;
}

Now, I want to split these responsibilities and rewrite the code as

std::vector<int> MyUsefulClass::processUsefulData(std::vector<int>& data)
{
    doProcessA(data, dataMember_);
    doProcessB(data, otherDataMember_);
    return data;
}

So, I don't know if I should make the two helper functions free functions or member functions, and when each would be appropriate. I also don't know if it's better to make them in an anonymous namespace or not. Does anyone know good times to do this?

Answer 1

I generally make helper routines "free" routines in an anonomous namespace if possible. That way I don't complicate the interface (off in the *.h file) with stuff clients don't need to worry about.

However, you have to be careful that you don't introduce non-reentrancy by doing that. For instance, by modifying global data objects or static locals rather than class members. If you need to do that, you are better off making it a proper class member.