[Qt-interest] What is the best, "for" or "foreach"?

[Sean Harmer]

Hi,

Paul Miller wrote:
> M. Bashir Al-Noimi wrote:
>> I've wrote the following snippet for testign the speed of "for" and 
>> "forech" loops and I noticed that "foreach" faster than "for" about 2 
>> times! >:o
>>
>> *Is "foreach" faster than "for"?*
> 
> The reason your particular example is faster with the foreach is because 
> foreach is using an iterator and your for loop is looping over the 
> actual list indexes. But, since you're using a QList<>, each access of 
> the list in the for loop is linear time, since QList<> is not 
> random-accessible.

I'm sorry but that is wrong. From the docs of QList:

"Internally, QList<T> is represented as an array of pointers to items of 
type T. If T is itself a pointer type or a basic type that is no larger 
than a pointer, or if T is one of Qt's shared classes, then QList<T> 
stores the items directly in the pointer array."

So for types where sizeof( type ) <= sizeof( pointer ) a QList is just 
the same as a QVector - except that it also pre-allocates extra space at 
the beginnging whereas QVector doesn't.

Sean