Lambda expressions as class template parameters

Question

Can lambda expressions be used as class template parameters? (Note this is a very different question than this one, which asks if a lambda expression itself can be

Answer 1

@Xeo gave you the reason, so I'll give you the work around.

Often times you do not wish to name a closure, in this case, you can use std::function, which is a type:

typedef std::unordered_map<
  std::string,
  std::string,
  std::hash,
  std::function
  > map_type;

Note that it captures exactly the signature of the function, and no more.

Then you may simply write the lambda when building the map.

Note that with unordered_map, if you change the equality comparison, you'd better change the hash to match the behavior. Objects that compare equal shall have the same hash.