A sparse array saves memory by not allocating memory for elements that are not initialized or to which you don't need access. They're quite easy using the STL map.
This example defines a two dimensional array of type SomeData and a three dimensional array of type SomeStruct:
map<unsigned, SomeData> > arr2SomeData;
map<unsigned, SomeStruct> > arr3SomeStruct;
Here's how to use it:
arr2SD = SomeData();
arr3SS = SomeStruct();
If SomeStruct and SomeData were non-integral types it would be more effective to use hash_map instead of map. However, hash_map is not a standard