Let me know if my understanding of your question is incorrect.
'readprop' member function of a class is used to read a property from the instrument and then calls 'setter' to set the property value with the current reading. You would like to avoid calling the setter.
This requirement needs you to modify your code for 'readprop' function. For 'readprop' not to call setter function, you can add another boolean argument (ex: "enable") which is by default 1. Manipulate 'readprop' to call the 'setter' only if this argument is true(or the vice versa of this).
Member function 'readprop' will look like this
if nargin < 2
enable = true;