'for' loop vs Qt's 'foreach' in C++

Question

Which is better (or faster), a C++ for loop or the foreach operator provided by Qt? For example, the following condition

QList

        

Answer 1

First, I completely agree with the answer that "it doesn't matter". Pick the cleanest solution, and optimize if it becomes a problem.

But another way to look at it is that often, the fastest solution is the one that describes your intent most accurately. In this case, QT's foreach says that you'd like to apply some action for each element in the container.

A plain for loop say that you'd like a counter i. You want to repeatedly add one to this value i, and as long as it is less than the number of elements in the container, you would like to perform some action.

In other words, the plain for loop overspecifies the problem. It adds a lot of requirements that aren't actually part of what you're trying to do. You don't care about the loop counter. But as soon as you write a for loop, it has to be there.

On the other hand, the QT people have made no additional promises that may affect performance. They simply guarantee to iterate through the container and apply an action to each.

In other words, often the cleanest and most elegant solution is also the fastest.

Answer 2

I don't want to answer the question which is faster, but I do want to say which is better.

The biggest problem with Qt's foreach is the fact that it takes a copy of your container before iterating over it. You could say 'this doesn't matter because Qt classes are refcounted' but because a copy is used you don't actually change your original container at all.

In summary, Qt's foreach can only be used for read-only loops and thus should be avoided. Qt will happily let you write a foreach loop which you think will update/modify your container but in the end all changes are thrown away.

Answer 3

The foreach from Qt has a clearer syntax for the for loop IMHO, so it's better in that sense. Performance wise I doubt there's anything in it.

You could consider using the BOOST_FOREACH instead, as it is a well thought out fancy for loop, and it's portable (and presumably will make it's way into C++ some day and is future proof too).

Answer 4

Since Qt 5.7 the foreach macro is deprecated, Qt encourages you to use the C++11 for instead.
http://doc.qt.io/qt-5/qtglobal.html#foreach

(more details about the difference here : https://www.kdab.com/goodbye-q_foreach/)

Answer 5

It really doesn't matter. Odds are if your program is slow, this isn't the problem. However, it should be noted that you aren't make a completely equal comparison. Qt's foreach is more similar to this (this example will use QList<QString>):

for(QList<QString>::iterator it = Con.begin(); it != Con.end(); ++it) {
    QString &str = *it;
    // your code here
}

The macro is able to do this by using some compiler extensions (like GCC's __typeof__) to get the type of the container passed. Also imagine that boost's BOOST_FOREACH is very similar in concept.

The reason why your example isn't fair is that your non-Qt version is adding extra work.

You are indexing instead of really iterating. If you are using a type with non-contiguous allocation (I suspect this might be the case with QList<>), then indexing will be more expensive since the code has to calculate "where" the n-th item is.

That being said. It still doesn't matter. The timing difference between those two pieces of code will be negligible if existent at all. Don't waste your time worrying about it. Write whichever you find more clear and understandable.

EDIT: As a bonus, currently I strongly favor the C++11 version of container iteration, it is clean, concise and simple:

for(QString &s : Con) {
    // you code here
}