Eigen defines several typedef shortcuts for most common 1D and 2D array types.

The general patterns are the following:

ArrayRowsColsType where Rows and Cols can be 2,3,4 for fixed size square matrices or X for dynamic size, and where Type can be i for integer, f for float, d for double, cf for complex float, cd for complex double.

For example, Array33d is a fixed-size 3x3 array type of doubles, and ArrayXXf is a dynamic-size matrix of floats.

There are also ArraySizeType which are self-explanatory. For example, Array4cf is a fixed-size 1D array of 4 complex floats.

With \cpp11, template alias are also defined for common sizes. They follow the same pattern as above except that the scalar type suffix is replaced by a template parameter, i.e.:

ArrayRowsCols<Type> where Rows and Cols can be 2,3,4, or X for fixed or dynamic size.
ArraySize<Type> where Size can be 2,3,4 or X for fixed or dynamic size 1D arrays.

See also: class Array