Knowing when to use [ ] vs .at( ) in Qt to read vs write data to objects

Knowing when to use [ ] vs .at( ) in Qt to read vs write data to objects

In Qt c++, you can directly access members of an object, whether it’s a class or struct, by using the square brackets [ and ] after the name of the object. For example:


Using square brackets [ and ] on an object allows you to directly READ AND WRITE to the member. However, knowing the difference between WHEN to use it could help increase the performance of the data handling cycles. In order to directly access that member data, and to allow read AND write access, Qt has to create a new pointer to the object itself in memory to do so. And creating pointers costs a lot of memory. And if you’re dealing with hundreds of thousands of objects and their hundreds of members, then things can get sluggish.

Luckily, Qt has been optimized with the .at( ) function. Check this out, yo:

Using .at( ) only reads the member data, it doesn’t allow writing to it. So it is accessing the data “by value” rather than “by reference”, so it doesn’t create a new pointer in memory to the object. Whodda thunk that old concept would show up later in your programming studies? LOL. So when you’re processing kazillion objects, you’re not handling kazillion pointers and pointers to pointers. And this makes the processing cycles REALLY FAST, or at least, fast as it can be.

The added benefit of using .at( ) is that, in most cases you will not be writing data to your object members all the time, so you want to protect the data by not accidentally overwriting it. And when you do want to write data to the member, you want to make damn sure you’re not writing incorrect data that would crash your entire setup and be hard to hunt down the source of the crash. So a good rule is to only use the direct square bracket [ ] on the left of operators and .at( ) on the right of operators like so:

int i;
classObject[0].classMember = i;

int i =;

That way, you can have the confidence you’re doing things intentionally. And your program will run faster too! Hope that helps!