State intent in comments

Reason

Code says what is done, not what is supposed to be done. Often intent can be stated more clearly and concisely than the implementation.

Example

void stable_sort(Sortable& c)
    // sort c in the order determined by <, keep equal elements (as defined by ==) in
    // their original relative order
{
    // ... quite a few lines of non-trivial code ...
}

Note

If the comment and the code disagree, both are likely to be wrong.