defadvice in emacs, what's correct context to use it

Question

Could someone please have a brief explanation on (defadvice ...), I'm not getting when is the suitable context to use it, seems it's not common in C/C++/Java languages? Such as the example shown in the figure attached. Many thanks.

Answer 1

Advising a function (whether using defadvice or the more recent advice-add etc.) is an alternative to redefining it.

If you are not the author of a function, and if it might already be used in various places beyond your code, you can either redefine it or advise it, and thereafter each use of that function will use your redefinition or your advice. And this is reversible: you can disable your advice.

Redefining a function does not, in general, play well with other libraries, which themselves might want to redefine or extend it in some way.

The point of advice is that it allows multiple libraries to alter or enhance a function definition in a reasonably controlled way, so that there is less chance of the different libraries stepping on each others' toes. Or at least so that it is easy to remove the effect of any of the library changes to the function.