Question

I’m using Google’s sparsehashmap, and trying to work out if a value was inserted or looked up. The following works, but obviously it’s looking it up twice. How do I do it without the double lookup?

Element newElement = Element();
bool  inserted = ((*map).insert(pair<const int64, Element>(key, newElement))).second;
Element element = (*(((*map).insert(pair<const int64, Element>(key, newElement))).first)).second;
if (inserted)
    puts("INSERTED");

I can’t check the contents of Element (it’s a struct) as I want to differentiate between a default Element being found and newElement being inserted. I couldn’t work out how to assign ((*map).insert(pair<const int64, Element>(key, newElement))) to a variable as it’s of a template type that includes types private to the sparse_hash_map class.

Answer 1

Try this:

typedef sparse_hash_map<...>::iterator sh_iterator; //you already have this, haven't you?

std::pair<sh_iterator, bool> res = map->insert(std::make_pair(key, newElement));
if (res.second)
    puts("INSERTED");

If, for whatever reason you don’t like the std::make_pair function, you should consider a typedef for the pair type:

typedef pair<const int64, Element> map_pair;

Anyway, the return type of insert is pair<iterator, bool>, and AFAIK iterator is a public typedef of the class.

BTW, I don’t get why you do the second insert… to get to the inserted element? Probably you should declare element as a reference. In my suggested code:

Element &element = res.first->second;

Naturally, if you were using C++11, you could simply do:

auto res = ...;