Question

So I have a std::vector<std::unique_ptr<Base>> vec and I’m trying to sort it dynamically, given that there are logical comparisons between Derived1 to Derivedn (Derivedn always > Derivedn-1 > … > Derived1) (say n = 10 or so) and each Derivedx has it’s own different comparison with Derivedx. As an example, think 10 digit integer > 9 digit integer > 1 digit integer, but within each derived class 53 > 32 (but I’m not sorting integers).

So I can do this:

std::sort(vec.begin(), vec.end(), 
[](std::unique_ptr<Base>& const a, std::unique_ptr<Base>& const b){
  return *a<*b;}

And then in Base, have a function Base::operator<(const Base& b) make comparisons if they are different Derived classes, and cast to Derivedx if they are the same with Derivedx::operator<(const Derivedx& d) if they are the same derived.

However, I would think that there’s a way that I can compare a to b automatically given the appropriate definitions in the derived classes, but I have been unable to implement it due to compile errors. I cannot get the lambda function to compare Derivedx < Derivedy dynamically.

I’ve tried Base::operator<(const std::unique_ptr<Base>) and then use return *a<b for a compiler error, saying that I used a deleted copy assignment operator (which I don’t understand, where is the assignment??). An abstract virtual Base::operator<(const Base& b) does practically the same thing I’m doing now with more work because I have to implement Derivedx::operator<(const Base& b) (for each Derivedx) and then cast down to (Derivedx) if they’re the same.

It may just be better that I compare everything in the base class rather than implementing n^2 comparisons in (n comparisons in n derived classes) though. But I do want to see if I can keep things “object oriented”.

Any thoughts on the design issue?
Thanks.

Answer 1

Take a look at Chapter 31, Making functions virtual with respect to more than one object in Scott Meyers, More Effective C++.

Also, try googling on the phrases double dispatch and multiple dispatch.