Return a read-only double pointer

Question

I want to a member variable, which is a double pointer. The object, the double pointer points to shall not be modified from outside the class.

My following try yields an "invalid conversion from ‘std::string**’ to ‘const std::string**’"

class C{

public:
    const std::string **getPrivate(){
        return myPrivate;
    }

private:
    std::string **myPrivate;
};

• Why is the same construct valid if i use just a simple pointer std::string *myPrivate

• What can i do to return a read-only double pointer?

  Is it good style to do an explicit cast return (const std::string**) myPrivate?

Answer 1

Try this:

const std::string * const *getPrivate(){
    return myPrivate;
}

The trouble with const std::string ** is that it allows the caller to modify one of the pointers, which isn't declared as const. This makes both the pointer and the string class itself const.

Answer 2

If you want to be really picky :

class C {

public:
    std::string const* const* const getPrivate(){
        return myPrivate;
    }

private:
    std::string **myPrivate;
};

Answer 3

There are very rare cases in c++ when a raw pointer (even less for a double pointer) is really needed, and your case doesn't seams to be one of them. A proper way would be to return a value or a reference, like this :

class C{

public:
    const std::string& getPrivate() const
    {
        return myPrivate;
    }

private:
    std::string myPrivate;
};