Why does MATLAB delete breakpoints in my classdef?

Question

I am trying to debug an object-oriented application in MATLAB R2011b that I did not write myself. To do this, I want to place a breakpoint in a method in the classdef. As far as I know, this is allowed. Yet, when I run the application, the breakpoint mysteriously disappears.

I have configured a breakpoint in clear like so:

dbstop in clear

to see when clear is being called. Before running the application, I check the presence of the breakpoint with dbstatus:

>> dbstatus

Breakpoint for Processor>Processor.setResult is on line 114.
Breakpoint for inputCompute is on line 30.
Breakpoint is set for m_interpreter>clear.

(The breakpoint in question is the first one in the list.) When I run the application, the breakpoint in the classdef disappears before the first clear is called:

>> rootFunction('configuration')
417 clear('Data');
K>> dbstatus

Breakpoint for inputCompute is on line 30.
Breakpoint is set for m_interpreter>clear.

To determine where the breakpoint gets cleared, I progressively moved a breakpoint closer and closer to the start of the root function, and found the offending line of code:

cd('./configs');

And sure enough -- any invocation of cd, even indirectly, clears the breakpoints in the classdef. That means that run does exactly the same thing. This is infuriating behaviour. Why does MATLAB do this? How do I keep it from doing this?

I cannot recreate the behaviour that you describe. Could you provide an [SSCCE](http://sscce.org/)? — Florian Brucker, Jul 24 '13 at 14:54
@FlorianBrucker It will be difficult to provide a SSCCE... I'd need a classdef file and a main function, so two files. I'm also very new to OOP, I have to do this debugging to even understand what is going on. You say you cannot recreate the behaviour I'm describing. Would you be able to post your SSCCE? Then I can test it on my MATLAB installation and compare. — Stephen Bosch, Jul 24 '13 at 15:08
Is the class's m-file on your MATLAB path or in the current directory or both? Changing the directory may have weird implications when using code that MATLAB has found because it's in the current directory, and it can likewise get confused when the current directory is also on the path. — kenm, Jul 24 '13 at 15:17
@StephenBosch: It's fine to post multiple files as long as they don't contain too much unnecessary details. Posting my code wouldn't help much, since it doesn't show your problem. Make sure to check your MATLAB path (using the `path` command), as kwatford suggested. — Florian Brucker, Jul 25 '13 at 05:48
@kwatford It does appear to be path-related. If I try to set breakpoints in the file when in another directory, a dialog box appears saying "To set breakpoints in this file, you can either change the MATLAB current folder, or add its folder to the MATLAB path." In typical MATLAB fashion, there is no explanation. I fixed my breakpoint problem by replacing the offending 'cd' with an 'addpath' statement; while I don't find this particularly satisfying, it does allow me to work. Why don't you post an answer to this effect, and I'll recognise it? — Stephen Bosch, Jul 30 '13 at 10:35

score 3 · Answer 1 · answered Aug 25 '14 at 16:59

I have found the same bug and reported it to Matlab. They said:

Unfortunately, this is a known bug with the editor. Development department is aware of the problem and plans to fix it in a future release. As a workaround, you could use a line containing the single statement

keyboard

rather than set a breakpoint. You can easily comment in/out this line.

JHC

score 1 · Accepted Answer · answered Jul 30 '13 at 17:20

If the class is only on your path because it's in the current directory, then changing the current directory while the code is running may have unexpected results, including clearing breakpoints.

Use either the Set Path dialog or the addpath function to add the code's directory to your MATLAB path. This way it can be accessed by MATLAB even if you change the current directory, which should resolve your debugging issues.

Why does MATLAB delete breakpoints in my classdef?

2 Answers2