Cannot create QImage using smart pointers

Christian Ehrlicher

@chessking5544 said in Cannot create QImage using smart pointers:

Any ideas on what's going on?

When you don't tell us what exactly does not work (and why you need a pointer in the first place for no reason) then we can maybe help.

chessking5544

The compile error is C2440 initializing: cannot convert from 'X *' to 'T *'.
I need to allocate many QImages and I am trying to use QPointers so that memory allocated is deleted automatically when the object goes out of scope. This seems to be what QPointers are designed for so I am just try to use them on Qimages

Christian Ehrlicher

@chessking5544 said in Cannot create QImage using smart pointers:

I need to allocate many QImages and I am trying to use QPointers so that memory allocated is deleted automatically when the object goes out of scope

Then you're using the wrong pointer class and it's even more a reason to simply allocate it on the stack.

The compile error is C2440 initializing: cannot convert from 'X *' to 'T *'.

X and T? Pretty sure not.
It compiles fine for me. Make sure to include QPointer and QImage header files.

SGaist

Hi,

As stated in the QPointer documentation, the type must derive from QObject. QImage does not.

chessking5544

@SGaist that makes sense and I should have seen that in the docs. thanks. Will c++ smart pointers work, or is there just no way to do this? I'm looking for a way to get auto deletion for all QImages created.

SGaist

Can you explain exactly how many QImage do you need to generate ?

Christian Ehrlicher

@chessking5544 said in Cannot create QImage using smart pointers:

I'm looking for a way to get auto deletion for all QImages created.

~~Don't create it on the stack but on the heap.~~ Don't create it on the heap but on the stack. QImage will allocate it's internal image data on the stack anyway.

/edit: mixed up stack and heap for unknown reasons - fixed :)

chessking5544

@SGaist most likely a few per thread and maybe a dozen threads. I am coming from the C# world and trying to avoid potential memory leaks by using smart pointers for all dynamic allocation.

SGaist

As @Christian-Ehrlicher suggests: no need for heap allocation in the case of QImage. That will also free you from having to delete them manually.

chessking5544

ok. thanks

JonB

@Christian-Ehrlicher said in Cannot create QImage using smart pointers:

Don't create it on the stack but on the heap.

Do you want to rephrase this? :)

Christian Ehrlicher

@JonB said in Cannot create QImage using smart pointers:

Do you want to rephrase this? :)

What should be rephrased here? It's basic ++ stuff. You can create an object either on the heap with new or directly on the stack.

jsulm

@Christian-Ehrlicher "Don't create it on the stack but on the heap." - you should exchange stack and heap :-)

Christian Ehrlicher

@jsulm Thx, fixed

SimonSchroeder

@chessking5544 said in Cannot create QImage using smart pointers:

Will c++ smart pointers work, or is there just no way to do this?

Just for completeness I will answer this. But first I have to agree with the others that you should allocate QImage on the stack. Good C++ style avoids pointers wherever possible. The best lifetime management is variables on the stack. Containers - and in this case I will count QImage as a container - will internally allocate memory on the heap. This is a good thing as your stack is quite small. I don't know C# well (but assume it has some similarity to Java within this respect), but objects do not need to be allocated using new in C++. If you want polymorphism you can use references instead of pointers. Rarely do you actually need pointers in C++.

Now to the answer: C++ smart pointers work with every class. Just use it like this:

std::shared_ptr<QImage> image = std::make_shared<QImage>(QSize(1920,1080), QImage::Format_RGB32);

Note the use of make_shared here. This will automatically handle out-of-memory errors. Internally it will call new and hand down its parameters to the constructor of QImage. It is advised to use make_shared<QImage>(...) over shared_ptr<QImage>(new QImage(...)) (I can't remember all the reasons).