Why is a doublePtr output argument actually a double?

In this page below titled "Working with Pointer Arguments", the sample function (multDoubleRef) is said to return the second output argument (xval) as a value of type double. As a matter of fact, xval indeed functions as a double variable. However, the function prototype indicates that it should be of type doublePtr. Why is there an inconsistency? Thanks.

Example:

x = 15;
xp = libpointer('doublePtr',x);
[xobj,xval] = calllib('shrlibsample','multDoubleRef',xp)
xobj =
   lib.pointer
xval =
    75

Function Prototype

[lib.pointer, doublePtr] multDoubleRef(doublePtr)

Page Link: http://www.mathworks.com/help/matlab/matlab_external/working-with-pointers.html#f50691

Answer 1

[Philip Borghesani]

This is a convenience for the user. When MATLAB knows the size and type of a simple output pointer it automatically copies the output values to MATLAB arrays. The original(possibly modified) values and pointer from the inputs are still intact and can be used when a copy is not desired.

The same conversions are done on input. This is a reasonable call:

[~,newvalue]=calllib('shrlibsample','multDoubleRef',5);

Note that in this case the first output pointer return value should not be used because it points to a temporary value that has already been deleted.