Use of defer in Go

Question

What is the use of defer in Go? The language documentation says it is executed when the surrounding function returns. Why not just put the code at end of given

Answer 1

Well, it's not always guaranteed that your code may reach the end of the function (e.g. an error or some other condition may force you to return well ahead of the end of a function). The defer statement makes sure that whatever function is assigned to it gets executed for sure even if the function panics or the code returns well before the end of the function.

The defer statement also helps keep the code clean esp. in cases when there are multiple return statements in a function esp. when one needs to free resources before return (e.g. imagine you have an open call for accessing a resource at the beginning of the function - for which a corresponding close must be called before the function returns for avoiding a resource leak. And say your function has multiple return statements, maybe for different conditions including error checking. In such a case, without defer, you normally would call close for that resource before each return statement). The defer statement makes sure the function you pass to it is always called irrespective of where the function returns, and thus saves you from extraenous housekeeping work.

Also defer can be called multiple times in the same function. E.g.: In case you have different resources being allocated through your function which need to be eventually freed before returning, then you can call defer for each of them after allocation and these functions are executed in the reverse order of the sequence in which they were called when the function exits.