Occasionally, I have a function that is non-const, and an 99% identical function that is const, like this: Blah &Class::GetSomething()
{
return something;
}
[size=2](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;
}
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.
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;
}
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.
https://github.com/cpgf/cpgf cpgf library -- free C++ open source library for reflection, serialization, script binding, callbacks, and meta data for OpenGL Box2D, SFML and Irrlicht.
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.
https://github.com/cpgf/cpgf cpgf library -- free C++ open source library for reflection, serialization, script binding, callbacks, and meta data for OpenGL Box2D, SFML and Irrlicht.
