What is your personal approach/take on commenting?

问题:

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What are your hard rules about commenting?

A Developer I work with had some things to say about commenting that were interesting to me (see below). What is your personal approach/take on commenting?

"I don't add comments to code unless its a simple heading or there's a
platform-bug or a necessary work-around that isn't obvious. Code can change and comments may become misleading. Code should be
self-documenting in its use of descriptive names and its logical
organization - and its solutions should be the cleanest/simplest way
to perform a given task. If a programmer can't tell what a program
does by only reading the code, then he's not ready to alter it.
Commenting tends to be a crutch for writing something complex or
non-obvious - my goal is to always write clean and simple code."

"I think there a few camps when it comes to commenting, the enterprisey-type who think they're writing an API and some grand code-library that will be used for generations to come, the craftsman-like programmer that thinks code says what it does clearer than a comment could, and novices that write verbose/unclear code so as to need to leave notes to themselves as to why they did something."

回答1:

There's a tragic flaw with the "self-documenting code" theory. Yes, reading the code will tell you exactly what it is doing. However, the code is incapable of telling you what it's supposed to be doing.

I think it's safe to say that all bugs are caused when code is not doing what it's supposed to be doing :). So if we add some key comments to provide maintainers with enough information to know what a piece of code is supposed to be doing, then we have given them the ability to fix a whole lot of bugs.

That leaves us with the question of how many comments to put in. If you put in too many comments, things become tedious to maintain and the comments will inevitably be out of date with the code. If you put in too few, then they're not particularly useful.

I've found regular comments to be most useful in the following places:

1) A brief description at the top of a .h or .cpp file for a class explaining the purpose of the class. This helps give maintainers a quick overview without having to sift through all of the code.

2) A comment block before the implementation of a non-trivial function explaining the purpose of it and detailing its expected inputs, potential outputs, and any oddities to expect when calling the function. This saves future maintainers from having to decipher entire functions to figure these things out.

Other than that, I tend to comment anything that might appear confusing or odd to someone. For example: "This array is 1 based instead of 0 based because of blah blah".

Well written, well placed comments are invaluable. Bad comments are often worse than no comments. To me, lack of any comments at all indicates laziness and/or arrogance on the part of the author of the code. No matter how obvious it is to you what the code is doing or how fantastic your code is, it's a challenging task to come into a body of code cold and figure out what the heck is going on. Well done comments can make a world of difference getting someone up to speed on existing code.

回答2:

I've always liked Refactoring's take on commenting:

The reason we mention comments here is that comments often are used as a deodorant. It's surprising how often you look at thickly commented code and notice that the comments are there because the code is bad.

Comments lead us to bad code that has all the rotten whiffs we've discussed in the rest of this chapter. Our first action is to remove the bad smells by refactoring. When we're finished, we often find that the comments are superfluous.

As controversial as that is, it's rings true for the code I've read. To be fair, Fowler isn't saying to never comment, but to think about the state of your code before you do.

回答3:

You need documentation (in some form; not always comments) for a local understanding of the code. Code by itself tells you what it does, if you read all of it and can keep it all in mind. (More on this below.) Comments are best for informal or semiformal documentation.

Many people say comments are a code smell, replaceable by refactoring, better naming, and tests. While this is true of bad comments (which are legion), it's easy to jump to concluding it's always so, and hallelujah, no more comments. This puts all the burden of local documentation -- too much of it, I think -- on naming and tests.

Document the contract of each function and, for each type of object, what it represents and any constraints on a valid representation (technically, the abstraction function and representation invariant). Use executable, testable documentation where practical (doctests, unit tests, assertions), but also write short comments giving the gist where helpful. (Where tests take the form of examples, they're incomplete; where they're complete, precise contracts, they can be as much work to grok as the code itself.) Write top-level comments for each module and each project; these can explain conventions that keep all your other comments (and code) short. (This supports naming-as-documentation: with conventions established, and a place we can expect to find subtleties noted, we can be confident more often that the names tell all we need to know.) Longer, stylized, irritatingly redundant Javadocs have their uses, but helped generate the backlash.

(For instance, this:

Perform an n-fold frobulation.
@param n the number of times to frobulate
@param x the x-coordinate of the center of frobulation
@param y the y-coordinate of the center of frobulation
@param z the z-coordinate of the center of frobulation

could be like "Frobulate n times around the center (x,y,z)." Comments don't have to be a chore to read and write.)

I don't always do as I say here; it depends on how much I value the code and who I expect to read it. But learning how to write this way made me a better programmer even when cutting corners.

Back on the claim that we document for the sake of local understanding: what does this function do?

def is_even(n): return is_odd(n-1)

Tests if an integer is even? If is_odd() tests if an integer is odd, then yes, that works. Suppose we had this:

def is_odd(n): return is_even(n-1)

The same reasoning says this is_odd() tests if an integer is odd. Put them together, of course, and neither works, even though each works if the other does. Change it a bit and we'd have code that does work, but only for natural numbers, while still locally looking like it works for integers. In microcosm that's what understanding a codebase is like: tracing dependencies around in circles to try to reverse-engineer assumptions the author could have explained in a line or two if they'd bothered. I hate the expense of spirit thoughtless coders have put me to this way over the past couple of decades: oh, this method looks like it has the side effect of farbuttling the warpcore... always? Well, if odd crobuncles desaturate, at least; do they? Better check all the crobuncle-handling code... which will pose its own challenges to understanding. Good documentation cuts this O(n) pointer-chasing down to O(1): e.g. knowing a function's contract and the contracts of the things it explicitly uses, the function's code should make sense with no further knowledge of the system. (Here, contracts saying is_even() and is_odd() work on natural numbers would tell us that both functions need to test for n==0.)

回答4:

My only real rule is that comments should explain why code is there, not what it is doing or how it is doing it. Those things can change, and if they do the comments have to be maintained. The purpose the code exists in the first place shouldn't change.

回答5:

the purpose of comments is to explain the context - the reason for the code; this, the programmer cannot know from mere code inspection. For example:

strangeSingleton.MoveLeft(1.06);
badlyNamedGroup.Ignite();

who knows what the heck this is for? but with a simple comment, all is revealed:

//when under attack, sidestep and retaliate with rocket bundles
strangeSingleton.MoveLeft(1.06);
badlyNamedGroup.Ignite();

seriously, comments are for the why, not the how, unless the how is unintuitive.

回答6:

While I agree that code should be self-readable, I still see a lot of value in adding extensive comment blocks for explaining design decisions. For example "I did xyz instead of the common practice of abc because of this caveot ..." with a URL to a bug report or something.

I try to look at it as: If I'm dead and gone and someone straight out of college has to fix a bug here, what are they going to need to know?

回答7:

In general I see comments used to explain poorly written code. Most code can be written in a way that would make comments redundant. Having said that I find myself leaving comments in code where the semantics aren't intuitive, such as calling into an API that has strange or unexpected behavior etc...

回答8:

I also generally subscribe to the self-documenting code idea, so I think your developer friend gives good advice, and I won't repeat that, but there are definitely many situations where comments are necessary.

A lot of times I think it boils down to how close the implementation is to the types of ordinary or easy abstractions that code-readers in the future are going to be comfortable with or more generally to what degree the code tells the entire story. This will result in more or fewer comments depending on the type of programming language and project.

So, for example if you were using some kind of C-style pointer arithmetic in an unsafe C# code block, you shouldn't expect C# programmers to easily switch from C# code reading (which is probably typically more declarative or at least less about lower-level pointer manipulation) to be able to understand what your unsafe code is doing.

Another example is when you need to do some work deriving or researching an algorithm or equation or something that is not going to end up in your code but will be necessary to understand if anyone needs to modify your code significantly. You should document this somewhere and having at least a reference directly in the relevant code section will help a lot.

回答9:

I don't think it matters how many or how few comments your code contains. If your code contains comments, they have to maintained, just like the rest of your code.

EDIT: That sounded a bit pompous, but I think that too many people forget that even the names of the variables, or the structures we use in the code, are all simply "tags" - they only have meaning to us, because our brains see a string of characters such as customerNumber and understand that it is a customer number. And while it's true that comments lack any "enforcement" by the compiler, they aren't so far removed. They are meant to convey meaning to another person, a human programmer that is reading the text of the program.

回答10:

If the code is not clear without comments, first make the code a clearer statement of intent, then only add comments as needed.

Comments have their place, but primarily for cases where the code is unavoidably subtle or complex (inherent complexity is due to the nature of the problem being solved, not due to laziness or muddled thinking on the part of the programmer).

Requiring comments and "measuring productivity" in lines-of-code can lead to junk such as:

/*****
 *
 * Increase the value of variable i,
 * but only up to the value of variable j.
 *
 *****/

if (i < j) {
    ++i;
} else {
    i = j;
}

rather than the succinct (and clear to the appropriately-skilled programmer):

i = Math.min(j, i + 1);

YMMV

回答11:

The vast majority of my commnets are at the class-level and method-level, and I like to describe the higher-level view instead of just args/return value. I'm especially careful to describe any "non-linearities" in the function (limits, corner cases, etc) that could trip up the unwary.

Typically I don't comment inside a method, except to mark "FIXME" items, or very occasionally some sort of "here be monsters" gotcha that I just can't seem to clean up, but I work very hard to avoid those. As Fowler says in Refactoring, comments tend to indicate smally code.

回答12:

Comments are part of code, just like functions, variables and everything else - and if changing the related functionality the comment must also be updated (just like function calls need changing if function arguments change).

In general, when programming you should do things once in one place only.

Therefore, if what code does is explained by clear naming, no comment is needed - and this is of course always the goal - it's the cleanest and simplest way.

However, if further explanation is needed, I will add a comment, prefixed with INFO, NOTE, and similar...
An INFO: comment is for general information if someone is unfamiliar with this area.
A NOTE: comment is to alert of a potential oddity, such as a strange business rule / implementation.
If I specifically don't want people touching code, I might add a WARNING: or similar prefix.

What I don't use, and am specifically opposed to, are changelog-style comments - whether inline or at the head of the file - these comments belong in the version control software, not the sourcecode!

回答13:

I prefer to use "Hansel and Gretel" type comments; little notes in the code as to why I'm doing it this way, or why some other way isn't appropriate. The next person to visit this code will probably need this info, and more often than not, that person will be me.

回答14:

As a contractor I know that some people maintaining my code will be unfamiliar with the advanced features of ADO.Net I am using. Where appropriate, I add a brief comment about the intent of my code and a URL to an MSDN page that explains in more detail.

I remember learning C# and reading other people's code I was often frustrated by questions like, "which of the 9 meanings of the colon character does this one mean?" If you don't know the name of the feature, how do you look it up?! (Side note: This would be a good IDE feature: I select an operator or other token in the code, right click then shows me it's language part and feature name. C# needs this, VB less so.)

As for the "I don't comment my code because it is so clear and clean" crowd, I find sometimes they overestimate how clear their very clever code is. The thought that a complex algorithm is self-explanatory to someone other than the author is wishful thinking.

And I like @17 of 26's comment (empahsis added):

... reading the code will tell you exactly what it is doing. However, the code is incapable of telling you what it's supposed to be doing.

回答15:

I very very rarely comment. MY theory is if you have to comment it's because you're not doing things the best way possible. Like a "work around" is the only thing I would comment. Because they often don't make sense but there is a reason you are doing it so you need to explain.

Comments are a symptom of sub-par code IMO. I'm a firm believer in self documenting code. Most of my work can be easily translated, even by a layman, because of descriptive variable names, simple form, and accurate and many methods (IOW not having methods that do 5 different things).

回答16:

Comments are part of a programmers toolbox and can be used and abused alike. It's not up to you, that other programmer, or anyone really to tell you that one tool is bad overall. There are places and times for everything, including comments.

I agree with most of what's been said here though, that code should be written so clear that it is self-descriptive and thus comments aren't needed, but sometimes that conflicts with the best/optimal implementation, although that could probably be solved with an appropriately named method.

回答17:

I agree with the self-documenting code theory, if I can't tell what a peice of code is doing simply by reading it then it probably needs refactoring, however there are some exceptions to this, I'll add a comment if:

• I'm doing something that you don't normally see
• There are major side effects or implementation details that aren't obvious, or won't be next year
• I need to remember to implement something although I prefer an exception in these cases.
• If I'm forced to go do something else and I'm having good ideas, or a difficult time with the code, then I'll add sufficient comments to tmporarily preserve my mental state

回答18:

Most of the time I find that the best comment is the function or method name I am currently coding in. All other comments (except for the reasons your friend mentioned - I agree with them) feel superfluous.

So in this instance commenting feels like overkill:

/*
 * this function adds two integers
 */
int add(int x, int y)
{
    // add x to y and return it
    return x + y;
}

because the code is self-describing. There is no need to comment this kind of thing as the name of the function clearly indicates what it does and the return statement is pretty clear as well. You would be surprised how clear your code becomes when you break it down into tiny functions like this.

回答19:

When programming in C, I'll use multi-line comments in header files to describe the API, eg parameters and return value of functions, configuration macros etc...

In source files, I'll stick to single-line comments which explain the purpose of non-self-evident pieces of code or to sub-section a function which can't be refactored to smaller ones in a sane way. Here's an example of my style of commenting in source files.

If you ever need more than a few lines of comments to explain what a given piece of code does, you should seriously consider if what you're doing can't be done in a better way...

回答20:

I write comments that describe the purpose of a function or method and the results it returns in adequate detail. I don't write many inline code comments because I believe my function and variable naming to be adequate to understand what is going on.

I develop on a lot of legacy PHP systems that are absolutely terribly written. I wish the original developer would have left some type of comments in the code to describe what was going on in those systems. If you're going to write indecipherable or bad code that someone else will read eventually, you should comment it.

Also, if I am doing something a particular way that doesn't look right at first glance, but I know it is because the code in question is a workaround for a platform or something like that, then I'll comment with a WARNING comment.

回答21:

Sometimes code does exactly what it needs to do, but is kind of complicated and wouldn't be immediately obvious the first time someone else looked at it. In this case, I'll add a short inline comment describing what the code is intended to do.

I also try to give methods and classes documentation headers, which is good for intellisense and auto-generated documentation. I actually have a bad habit of leaving 90% of my methods and classes undocumented. You don't have time to document things when you're in the middle of coding and everything is changing constantly. Then when you're done you don't feel like going back and finding all the new stuff and documenting it. It's probably good to go back every month or so and just write a bunch of documentation.

回答22:

Here's my view (based on several years of doctoral research):

There's a huge difference between commenting functions (sort of a black box use, like JavaDocs), and commenting actual code for someone who will read the code ("internal commenting").

Most "well written" code shouldn't require much "internal commenting" because if it performs a lot then it should be broken into enough function calls. The functionality for each of these calls is then captured in the function name and in the function comments.

Now, function comments are indeed the problem, and in some ways your friend is right, that for most code there is no economical incentive for complete specifications the way that popular APIs are documented. The important thing here is to identify what are the "directives": directives are those information pieces that directly affect clients, and require some direct action (and are often unexpected). For example, X must be invoked before Y, don't call this from outside a UI thread, be aware that this has a certain side effect, etc. These are the things that are really important to capture.

Since most people never read full function documentations, and skim what they do read, you can actually increase the chances of awareness by capturing only the directives rather than the whole description.

回答23:

I comment as much as needed - then, as much as I will need it a year later.

回答24:

We add comments which provide the API reference documentation for all public classes / methods / properties / etc... This is well worth the effort because XML Documentation in C# has the nice effect of providing IntelliSense to users of these public APIs. .NET 4.0's code contracts will enable us to improve further on this practice.

As a general rule, we do not document internal implementations as we write code unless we are doing something non-obvious. The theory is that while we are writing new implementations, things are changing and comments are more likely than not to be wrong when the dust settles.

When we go back in to work on an existing piece of code, we add comments when we realize that it's taking some thought to figure out what in the heck is going on. This way, we wind up with comments where they are more likely to be correct (because the code is more stable) and where they are more likely to be useful (if I'm coming back to a piece of code today, it seems more likely that I might come back to it again tomorrow).

回答25:

My approach:

Comments bridge the gap between context / real world and code. Therefore, each and every single line is commented, in correct English language.

I DO reject code that doesn't observe this rule in the strictest possible sense.

Usage of well formatted XML - comments is self-evident.

Sloppy commenting means sloppy code!

回答26:

Here's how I wrote code:

if (hotel.isFull()) {
    print("We're fully booked");
} else {
    Guest guest = promptGuest();
    hotel.checkIn(guest);
}

here's a few comments that I might write for that code:

// if hotel is full, refuse checkin, otherwise 
// prompt the user for the guest info, and check in the guest.

If your code reads like a prose, there is no sense in writing comments that simply repeats what the code reads since the mental processing needed for reading the code and the comments would be almost equal; and if you read the comments first, you will still need to read the code as well.

On the other hand, there are situations where it is impossible or extremely difficult to make the code looks like a prose; that's where comment could patch in.