How to add valid key without specifying value to a std::map?

Question

I have a std::map, and I would like to add a valid key to iterate over it later, but without giving any value (it will be given later on in the course of the iterations).

Answer 1

I'm not entirely sure what you mean by "without giving any value" but if you mean without explicitly assigning a value then just do

map[valid_keys[i]];

This still works i.e. it creates a new entry in the map if there was not previously one with that key. The operator[] just returns a refernce to the value so that you can assign a new value to it but remember it's already been default constructed.

If, on the other hand, you mean you want to express that there is no meaningful value and it may or may not subsequently receive a valid value then see @UncleBens` answer.