2D-Plane data structure

I am writing an rts game engine in C++. Does anyone know of a good example of a 2d plane data structure for storing mobile units and fog of war?

You misunderstand. By using x+y*WIDTH you are essentially converting

 |---|---|---|
 |1,1|2,1|3,1|
 |---|---|---|
 |1,2|2,2|3,2|
 |---|---|---|
 |1,3|2,3|3,3|
 |---|---|---|
into

 |---|---|---|
 | 1 | 2 | 3 |
 |---|---|---|
 | 4 | 5 | 6 |
 |---|---|---|
 | 7 | 8 | 9 |
 |---|---|---|
which relies on one of the dimensions - the one whose range is represented by WIDTH - being finite. If both dimensions are infinite, this will fail to work.