I am looking for a simple way to create an iterator for the values of a map
in C++11.
This method should be simple and transparent: simple in that it should be easy to implement, and transparent in that the client should not know the values come from a map, not a set.
This question has been asked several times before. Many of these questions predate C++11 and use boost, which I do not want to use. Some are not simple, John Ahlgren's solution here, http://john-ahlgren.blogspot.com/2013/10/how-to-iterate-over-values-of-stdmap.html , for example requires a page of code to write a custom iterator.
The others are not transparent, i.e., clearly one can write:
map<string,foo> mymap;
for (auto it=mymap.begin();it!=mymap.end();++it){
Foo val= it->second;
...
}
However, I do not want to do this because I do not want the client to have to know of the data representation.
The problem comes up as follows.
I have a bunch of objects uniquely indexed with a long "key". Sometimes I want to manipulate sets of these objects. Other times I want to retrieve an object given its key.
I cannot use the "set" class directly for several reasons, chief among which is that it does not store mutable instances, and these instances must be mutable (except, obviously, for the key).
So, I have decided to store all my objects in a giant global hashtable:
map<long,Foo> all_the_objects;
I then do not work with set<Foo>
at all. Instead I work with set<long>
and use an adaptor to simulate a set of Foo, i.e.,
class SetOfFoo{
private: set<long> theKeys;
public:
void insert(const & Foo);
size_t size() return theKeys.size();
bool is_member(const & Foo)
{return theKeys.find(Foo.key)
!= theKeys.end;}
Foo & insert(const & Foo val){
long key=val.key;
all_the_objects[key]=val;
return all_the_objects[key];
}
...::iterator begin() {???}
}
In other words, the client of the SetOfFoo class does not know or need to know that SetOfFoo is implemented as as set of keys.
I also cannot just make a Vector myself in the adaptor class, because one cannot store references in C++ collections.
Is it really impossible to make a simple, transparent way to iterate over map<> values? I find it hard to believe, as this is a very common need and is trivial to do in every language I have seen that has hashtables. I just don't understand how this can be hard.