You have to refer to a field in the structure that you want to convert, if it even does need conversion, then use the proper function. Here's an example for 3 data types:
% Create the structure with 3 fields of different types.
s.anInteger = int32(42);
s.aString = '73';
s.aDouble = pi;
% Convert them to doubles.
d1 = double(s.anInteger)
d2 = str2double(s.aString)
d3 = s.aDouble
% Verify that they are actually doubles.
whos d1
whos d2
whos d3
