C++ : Ignored qualifiers (-Wignored-qualifiers)

What is an ignored qualifier?  Well, in a class, lets have a student class:

#include <string>

class Student
{
    public:
        const std::string Name;

    private:
        bool m_Present;

    public:

        Student(const std::string& p_Name)
            :
            Name(p_Name),
            m_Present
        {
        }        

        void SetPresent() { m_Present = true; }

        void SetAbsent() { m_Present = false; }
};

Now, we may want to access the present flag, therefore provide a function to do so:

    bool Present() const { return m_Present }

This function is telling the user of our class quite a bit, it tells them that the return type is boolean and that the calling of this function makes no changes to the class  contents (const trailing the parameter list).

However, for me, this isn't quite right, I believe that we want to inform any user that the boolean returned is also constant, it does not change unless you alter the internal value with the "Set" functions, therefore I prefer and like to see code stating:

    const bool Present() const { return m_Present; }

This is perhaps overkill and most of the time completely acceptable code to present, however, some might prefer not to see it, specifically anyone defining "-Wignored-qualifiers" as with the "const bool" the const here is technically superfluous, the return type is boolean and a new instance of it, it is not a reference to the internal value, if it were the function may look something more like this:

    const bool& Present() const { return m_Present; }

Now we are intrinsically returning a reference to the internal boolean, or even:

    const bool* const Present() const { return &m_Present; }

For speed of operation we may directly drive the reference back as a constant boolean constant pointer.

I find this much more informative to the user, they know our intent, the code though more verbose communicates its meaning much more clearly.

As ever, yes I have seen questions asked of interfaces where "bool X()", or "bool X() const" is provided but then programmers have asked "How do I change X", with the const return, ignored qualifier or not, the know not to ask this function to change X and can look up elsewhere in your API.