Question

is there a difference between:

mvt_act_idx = openCloseList.size()-1;
openCloseList[mvt_act_idx].A += a;

and

 openCloseList.back().A += a;

Besides readability?

Answer 1

No useful difference.

Using back() directly might be fractionally more efficient, but I doubt it. You could have a look at the generated assembler if you felt really keen. Might make the difference of an instruction or two, depending on how clever your compiler is.

I do note that the former way which uses only size() and [] would be a lot more familiar to people who understand arrays or use similar contructs in any other language; back() is a bit more C++ specific (though it is hardly a cryptic idiom).