Question

I’m using a type Id which is defined in another part of the code I’m using:

typedef int Id;

Now I am provided many objects, each of which comes with such an Id, and I would like to use the Id as index into a std::vector that stores these objects. It may look something like this:

std::vector<SomeObj*> vec(size);
std::pair<Id, SomeObj*> p = GetNext();
vec[p.first] = p.second;

The problem is that std::vector uses its own type to index its elements: std::vector::size_type (why isn’t that templated?).

So strictly, it would be better to use std::map<Id, SomObj*>, but that would be less efficient, and an array is what I really need here (I know that the indices of all the objects are contiguous and start with 0). Another problem is that the typedef int Id might change to typedef long int Id or similar in the future … (it is part of my own code though, so I control it, but ideally I should be allowed to change the typedef at some points; that’s what a typedef is for).

How would you deal with this? Maybe use unordered_map<Id, SomeObj*>, where the hash function directly uses the Id as hash key? Would that be less memory-efficient? (I don’t completely understand how unordered_map allocates its space given that the range of the hash function is unknown in advance?)

Answer 1

You can pass any integral type as an index for std::vector. If it doesn’t match std::vector<T>::size_type (which is typically unsigned long) then the value will be implicitly converted.