All of the comments in the source and the header files must be written in the English language. Adding a lot of comments is a simple way to leave a clear code for the next programmer who will receive your work. A part of comments must be written with several rules to help the documentation of your work generating automatically with Doxygen. Choose a part of your comments to describe your work. See Section 4, “Code documentation”. It is important that a programmer can understand the using of your work and how to use your work.
Before a declaration of a class, you should write a little description to explain it. The role, the pre-requisites, the post-requisites, the return values and the parameters should be written before the declaration of the function or method. It is the minimum for an easy comprehension of your work. In the code, it is useless to comment each line because the comments are often a paraphrase of the code. It just is essential to write a comment for strong parts that you have thought.