Question

I wonder where should we use lambda expression over functor in C++. To me, these two techniques are basically the same, even functor is more elegant and cleaner than lambda. For example, if I want to reuse my predicate, I have to copy the lambda part over and over. So when does lambda really come in to place?

Answer 1

1) It’s trivial and trying to share it is more work than benefit.

2) Defining a functor simply adds complexity (due to having to make a bunch of member variables and crap).

If neither of those things is true then maybe you should think about defining a functor.

Edit: it seems to be that you need an example of when it would be nice to use a lambda over a functor. Here you go:

typedef std::vector< std::pair<int,std::string> > whatsit_t;

int find_it(std::string value, whatsit_t const& stuff)
{
  auto fit = std::find_if(stuff.begin(), stuff.end(), [value](whatsit_t::value_type const& vt) -> bool { return vt.second == value; });

  if (fit == stuff.end()) throw std::wtf_error();

  return fit->first;
}

Without lambdas you’d have to use something that similarly constructs a functor on the spot or write an externally linkable functor object for something that’s annoyingly trivial.

BTW, I think maybe wtf_error is an extension.