8 replies to this topic

[Servant of the Lord]

Occasionally, I have a function that is non-const, and an 99% identical function that is const, like this:

Blah &Class::GetSomething()
{
return something;
}


const Blah &Class::GetSomething() const
{
return something;
}

(Note: My actual code, in this specific case, isn't just an accessor - it's looking through a vector to find the right element to return)

The end result is that I have a two functions with identical code, but only the function declaration is different. How can I safely use the definition of one to implement both of them?


Here's the code I'm currently wanting to use it on, but I've hit the same situation several times previously:

Layer::Ptr &Floor::GetLayer(const LayerName &layerName)
{
    for(auto &layer : this->Layers)
    {
	    if(layer->Name == layerName)
		    return layer;
    }
   
    static Layer::Ptr dummy;
    return dummy;
}

[Zipster]

I believe this is one of those rare circumstances where a const_cast was considered acceptable, and the const method calls the non-const method.

[frob]

In Effective C++, they recommended removing having the non-const version call the const version and remove it with a const_cast. This assumes it is not a virtual function.

If they are virtual, you are best with the duplication, or having both of them call a separate private method that does the actual work.

[SiCrane]

If you don't like the const_cast approach, another way to tackle it is to create a private static template function that does the work. Have one template argument be a pointer to the class and another be the return type. Ex:

class Foo {
  public:
    Data * get_data(const std::string & name) {
      return get_data_impl<Data *>(this, name);
    }
    const Data * get_data(const std::string & name) const {
      return get_data_impl<const Data *>(this, name);
    }
  private:
    template <typename R, typename T> static R get_data_impl(T * self, const std::string & name) {
      for (auto & data : self->data_) {
        if (data.name == name) return &data;
      }
      return nullptr;
    }

    std::vector<Data> data_;
};

[Servant of the Lord]

I've heard of the const_cast method, but it seemed... hackish. If this is the recommended approach, then I'm happy to adopt it.

The proper usage would then be:

Blah &Class::GetSomething()
{
return const_cast<Blah&>(this->GetSomething());
}
const Blah &Class::GetSomething() const
{
return something;
}

But wouldn't that send the non-const GetSomething() into an infinite loop, because it'd call itself recursively?

So I'd actually have to do:

const Class *const_this = const_cast<const Class*>(this);
return const_cast<Blah&>(const_this->GetSomething());

Is that the correct usage?

[wqking]

For that kind of non-trivial accessor functions, I would introduce a third non-const function to do the real accessing, then add constness in your const version accessor.

SomeType & getSomething() {
    return doGetSomething()
}
const SomeType & getSomething() const {
    return const_cast<MyClass *>(this)->doGetSomething()
}
private:
SomeType & doGetSomething() {
    // do real work here
}

This code is adding constness to return value rather than removing constness.
Removing constness may be dangerous from semantic view because it allows modification on const object. Adding constness is safe.
Though the const version of getSomething() removes const from "this", it's safe as long as doGetSomething() doesn't change the object.

[wqking]

So I'd actually have to do:

const Class *const_this = const_cast<const Class*>(this);
return const_cast<Blah&>(const_this->GetSomething());

Is that the correct usage?

Yes, for that kind of usage, you should always const_cast on "this".
Also, removing constness may be "hack", but adding constness is not hack, it's normal.

[Zipster]

I've only ever used this idiom as follows:

Blah &Class::GetSomething()
{
return something;
}
const Blah &Class::GetSomething() const
{
return const_cast<Class*>(this)->GetSomething();
}

With the single explicit cast. However the other approaches migh suit your taste better

[Servant of the Lord]

The one explicit const cast on the 'this' pointer seems like the best option. Thank you, all.