Why Prefix Increment Can Do Less Work
In C++, `++i` and `i++` both advance `i` by one, but they produce different results. Prefix increment changes the value first and yields the updated variable. Postfix increment yields the value from before the change, then advances the variable.
That old value has to be preserved when the expression’s result is used. For a simple integer, a compiler can often optimize away unnecessary work when the result is discarded, so the two forms may compile to the same instructions. The difference becomes more relevant for objects with more involved increment operations.
The Cost of Preserving the Old Value
A custom type—such as an iterator, which represents a position in a collection—can implement postfix increment by copying itself, advancing the original, and returning the copy. Prefix increment can instead advance the object and return it directly. Copying a complex object may take extra work or require extra storage.
That is why C++ code commonly uses `++i` when it only needs to advance a variable, especially in loops. Use `i++` when the expression specifically needs the previous value, as in `old = i++`. The practical distinction is not that postfix is always slower: it is that postfix must preserve a value prefix does not need to produce.
