Const pointer in typedef?

Question

What does this line of code do': typedef const real_T * const * InputRealPtrsType; I think it defines `InputRealPtrsType` as a pointer to a constant pointer to a constant `real_T`. `real_T` is essentially a `double`, defined to give some cross platform flexibility. Why would you want to do that? This definition is in the simulink header file simstruc.h for use in C S-functions.

Answer 1

typedef const real_T * const * InputRealPtrsType;

or, equivalently:

typedef real_T const * const * InputRealPtrsType;

means that InputRealPtrsType is the name of a type, which is a:

1. pointer, to a
2. const pointer, to a
3. const real_T.

If you were to use this type to declare a variable p, as in:

InputRealPtrsType p;

Then

1. p is a pointer to a const pointer to a const real_T.
2. *p is a const pointer to a const real_T.
3. **p is a const real_T.

This sort of construction can might be used when e.g. passing the address of a vector of [constant] addresses to vectors of [constant] real_Ts, i.e. a way of representing 2d arrays in C.