STL or Qt containers?

Question

What are the pros and cons of using Qt containers (QMap, QVector, etc.) over their STL equivalent?

I can see one reason to prefer Qt:

Answer 1

This is a difficult to answer question. It can really boil down to a philosophical/subjective argument.

That being said...

I recommend the rule "When in Rome... Do as the Romans Do"

Which means if you are in Qt land, code as the Qt'ians do. This is not just for readability/consistency concerns. Consider what happens if you store everything in a stl container then you have to pass all that data over to a Qt function. Do you really want to manage a bunch of code that copies things into/out-of Qt containers. Your code is already heavily dependent on Qt, so its not like you're making it any more "standard" by using stl containers. And whats the point of a container if everytime you want to use it for anything useful, you have to copy it out into the corresponding Qt container?

Answer 2

I guess it depends on the way you use Qt. If you use it all over your product, than it probably makes sense to use Qt containers. If you contain it only to (for instance) the UI portion, it may be better to use C++ standard containers.

Answer 3

Qt containers use copy-on-write idiom.

Answer 4

STL containers:

• Have performance guarantees
• Can be used in STL algorithms which also have performance guarantees
• Can be leveraged by third-party C++ libraries like Boost
• Are standard, and likely to outlive proprietary solutions
• Encourage generic programming of algorithms and data structures. If you write new algorithms and data structures that conform to STL you can leverage what STL already provides at no cost.

Answer 5

If the data you are working with is mostly used to drive the Qt based UI, then definitely use Qt containers.

If the data is mostly used internally in the app, and you're never likely to port away from Qt, then barring performance issues, use the Qt containers because it will make the bits of data that go to the UI easier to deal with.

If the data is mostly used in conjunction with other libraries that only know about STL containers, then use STL containers. If you have this situation you're in trouble no matter what because you're going to do a lot of porting back and forth between container types no matter what you do.

Answer 6

One of the main issues is that Qt's API expects you to provide data in Qt's containers, so you may as well simply use the Qt containers rather than transforming back and forth between the two.

Also, if you're already using the Qt containers, it might be slightly more optimal to use them exclusively, as you would not have to include the STL header files and potentially link in the STL libraries. However, depending on your toolchain, that may happen anyway. Purely from a design perspective, consistency is generally a good thing.