Matlab structure dismal performance when used with objects - performance

Question

Here is a simple class with two properties: PStruct is a property that will contain a structure.
classdef anobj < handle
properties
PStruct
PNum=1;
end
methods
function obj = anobj()
end
end
end
Here is a script filling the structure in an object with 1’s (pretty fast):
clear all
a = anobj(); % an object
b = anobj(); % another object for future use
ntrials=10; niterations=1000;
a.PStruct(ntrials,niterations).field1=0; % 'initialize' the struct array
for t=1:ntrials
tic;
for i=1:niterations
a.PStruct(t,i).field1=1; % store data
end
toc;
end
yielding:
Elapsed time is 0.001008 seconds.
Elapsed time is 0.000967 seconds.
Elapsed time is 0.000972 seconds.
Elapsed time is 0.001206 seconds.
Elapsed time is 0.000992 seconds.
Elapsed time is 0.000981 seconds.
Elapsed time is 0.000975 seconds.
Elapsed time is 0.001072 seconds.
Elapsed time is 0.000951 seconds.
Elapsed time is 0.000994 seconds.
When instead I use a property of another object (=1 as well), changing the line within the loops to:
a.PStruct(t,i).field1=b.PNum; % store data
I get:
Elapsed time is 0.112418 seconds.
Elapsed time is 0.107359 seconds.
Elapsed time is 0.118347 seconds.
Elapsed time is 0.127111 seconds.
Elapsed time is 0.138606 seconds.
Elapsed time is 0.152675 seconds.
Elapsed time is 0.162610 seconds.
Elapsed time is 0.172921 seconds.
Elapsed time is 0.184254 seconds.
Elapsed time is 0.190802 seconds.
Not only performance is orders of magnitude slower, but also there is a very clear trend (verified more generally) of slowing down with each trial. I don’t get it. Furthermore, if I instead use a standalone uninitialized struct array which is not an object property (this line replaces the one within the loops):
PStruct(t,i).field1=b.PNum; % store data
I get ok performance with no trends:
Elapsed time is 0.007143 seconds.
Elapsed time is 0.004208 seconds.
Elapsed time is 0.004312 seconds.
Elapsed time is 0.004382 seconds.
Elapsed time is 0.004302 seconds.
Elapsed time is 0.004545 seconds.
Elapsed time is 0.004499 seconds.
Elapsed time is 0.005840 seconds.
Elapsed time is 0.004210 seconds.
Elapsed time is 0.004177 seconds.
There is some weird interaction between struct arrays and objects. Does anybody know what is happening and how to fix this? Thanks.

Answer 1

Very strange.
I found that if you do either of the following the code returns to normal speed
c = b.PNum;
a.PStruct(t,i).field1=c; % store data
or
a.PStruct(t,i).field1=int32(b.PNum); % store data
but if you use double the code is still slow
a.PStruct(t,i).field1=double(b.PNum); % store data
and if you use both 'fast' methods at the same time
c = b.PNum;
a.PStruct(t,i).field1=c; % store data
a.PStruct(t,i).field1=int32(b.PNum); % store data
the slow speed returns.