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c++dictionaryboost-optional

What is boost::optional efficiency?

I have the following:

class Obj;
typedef std::map<string, string> StrMap;
std::map<std::string, std::pair<Obj, StrMap> > complexMap;

The thing is that for some entries in complexMap the StrMap will be empty and I won't use it at all, so for efficiency I'm considering to use boost::optional. My question is what's the efficiency of boost::optional, I'm afraid that by paying its price I will gain nothing at the end.

Solution

  • Think of optional as a container that can hold 0 or 1 values. Your map already is a container that can hold 0 to N elements. An optional map is therefore a container-in-a-container which can hold 0 to N elements. Really, there's no benefit here.

    The overhead of an empty map is quite small. Maps are really built from map nodes, internally, and an empty map just doesn´t have any nodes. (It can´t, because each node holds a value, and there´s no way by which an empty map could create a default value)