How to write this preset Copy/Paste manager without memory leak?

1

I had to change the way my copy paste manager worked for Juce 7 as the prior version didn’t anymore. At the same time I consolidated my .cpp and .h files into a .h file and I might have butchered the Singleton code.

I am now getting memory leaks upon closing the running program and I’m not sure which of these two changes are triggering it.

The copy/paste manager still works but I obviously don’t want memory leaks. Any ideas? Did I do the Singleton okay? Is the unique_ptr being used wrong? Thanks for any help.

#pragma once

#include "JuceHeader.h"

class CopyPasteManager
{
public:
    CopyPasteManager() {};
    ~CopyPasteManager() {
        DBG("DESTRUCTOR RUN");
        mCopyState.reset(); 
    };
    
    void copy(AudioProcessorValueTreeState* inState) {
        juce::ValueTree state = inState->copyState();
        std::unique_ptr<XmlElement> xmlStateCopy = state.createXml(); //is this a memory leak???
        mCopyState.reset(xmlStateCopy.release());
    }

    
    void paste(AudioProcessorValueTreeState* inState) {
        if (mCopyState->hasTagName(inState->state.getType())) {
            inState->replaceState(ValueTree::fromXml(*mCopyState));
        }
        
    }
    
    juce_DeclareSingleton (CopyPasteManager, false)
    
private:
    
    std::unique_ptr<XmlElement> mCopyState;
};
juce_ImplementSingleton(CopyPasteManager)

I presume I am not handling the unique_ptr in copy properly but have spent an hour trying different things and can’t help that using this function provokes

*** Leaked objects detected: 231 instance(s) of class XmlElement
JUCE Assertion failure in juce_LeakedObjectDetector.h:92
A breakpoint instruction (__debugbreak() statement or a similar call) was executed in SinSynth.exe.

Prior copy code which worked without leaks was as follows, but this is somehow not working in JUCE 7 because state.createXml() now makes another unique_ptr rather than the xml itself:

    auto state = inState->copyState();
    mCopyState.reset(state.createXml());

I appreciate any suggestion to fix it. Thanks again.

2

The copy method is ok, but can be improved at several points.

Instead of release and reset you can use assignment with std::move():

mCopyState = std::move (xmlStateCopy);

Or if you remove that local variable, you don’t even need to move:

mCopyState = state.createXml();

I think it is the singleton that actually leaks. A singleton is just a safety wrapper around a static variable. But static variables are destroyed after the leak detector in undefined order.
The best option is to inherit your CopyPasteManager from juce::DeletedAtShutdown. This is a cleanup helper, that will delete the singleton safely before the leak detector checks.

Out of curiosity, does the DBG("DESTRUCTOR RUN"); print before or after the leak detector? (hit continue when the leak detector stops)

3

Thanks @daniel. Looks like there were two changes needed.

Adding the inheritance of DeletedAtShutdown allowed the destructor to run before the leak assertion (wasn’t before).

Then I had to add clearSingletonInstance() to the destructor as well.

The std::move didn’t work but the other method you suggested also does so I have two methods that don’t provoke at least any reported memory leaks:

#pragma once

#include "JuceHeader.h"

class CopyPasteManager : DeletedAtShutdown 
{

public:
    CopyPasteManager() {};
    
    ~CopyPasteManager() {
        DBG("DESTRUCTOR RUN");
        //mCopyState.reset(); 
        clearSingletonInstance();
    };

    void copy(AudioProcessorValueTreeState* inState) {
        juce::ValueTree state = inState->copyState();

        //option 1:
        //std::unique_ptr<XmlElement> xmlStateCopy = state.createXml(); //is this a memory leak???
        //mCopyState.reset(xmlStateCopy.release());
        
        //option 2:
        mCopyState = state.createXml();
    }

    
    void paste(AudioProcessorValueTreeState* inState) {
        if (mCopyState->hasTagName(inState->state.getType())) {
            inState->replaceState(ValueTree::fromXml(*mCopyState));
        }
        
    }
    
    juce_DeclareSingleton (CopyPasteManager, false)
    
private:
    
    std::unique_ptr<XmlElement> mCopyState;
};

juce_ImplementSingleton(CopyPasteManager)