Is copy-and-paste coding ever acceptable?

2020-08-09 14:22发布

It's generally accepted that copy and paste programming is a bad idea, but what is the best way to handle a situation where you have two functions or blocks of code that really do need to be different in just a few ways make generalizing them extremely messy?

What if the code is substantially the same, except for a few minor variations, but those few minor variations aren't in things that are easy to factor out through either adding a parameter, template methods, or something like that?

More generally, have you ever encountered a situation where you would admit that a little copy-and-paste coding was truly justified.

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2楼-- · 2020-08-09 14:38

I avoid cut and paste like the plague. It's even worse than its cousin clone and modify. If faced with a situation like yours I'm always ready to use a macro processor or other script to generate the different variations. In my experience a single point of truth is hugely important.

Unfortunately the C macro processor is not very good for this purpose because of the annoying quoting requirements for newlines, expressions, statements, and arguments. I hate writing

#define RETPOS(E) do { if ((E) > 0) then return; } while(0)

but that quoting is a necessity. I will often use the C preprocessor despite its deficiencies because it doesn't add another item to the toolchain and so doesn't require changing the build process or Makefiles.

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劫难
3楼-- · 2020-08-09 14:44

As Martin Fowler suggests,

do it once, fine.

do it twice, starts to smell.

do it thrice, time to refactor.


EDIT: in answer to the comment, the origin of the advice is Don Roberts:

Three strikes and you refactor.

Martin Fowler describes that in Refactoring chapter 2, section The Rule of Three (page 58).

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手持菜刀,她持情操
4楼-- · 2020-08-09 14:45

Yes, and it's exactly as you say; minor but hard-to-factor variations. Don't flagellate yourself if it's really what the situation calls for.

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爷、活的狠高调
5楼-- · 2020-08-09 14:47
  1. Good code is reusable code.
  2. Don't reinvent the wheel.
  3. Examples exist for a reason: to help you learn, and ideally code better.

Should you copy and paste? Who cares! What is important is why you're copy and pasting. I'm not trying to get philosophical on anyone here, but let's think about this practically:

Is it out of laziness? "Blah blah, I've done this before... I'm only changing a few variable names.. done."

Not a problem if it was already good code before you copied and pasted it. Otherwise, you're perpetuating crappy code out of laziness which will bite your ass down the road.

Is it because you don't understand? "Damn.. I don't understand how that function works, but I wonder if it'll work in my code.." It might! This may save you time in the immediate moment when you're stressed that you have a deadline at 9 a.m. and you're staring red eyed at a clock around 4 a.m.

Will you understand this code when you return to it? Even if you comment it? No really - after thousands of lines of code, if you don't understand what the code is doing as you write it how will you understand coming back to it weeks, months later? Attempt to learn it, despite all temptation otherwise. Type it out, this will help commit it to memory. Each line you type, ask yourself what that line is doing and how it contributes to the overall purpose of that function. Even if you don't learn it inside out, you might have a chance at recognizing it at the very least when you return to it later on.

So - copying and pasting code? Fine if you're conscious of the implications of what you're doing. Otherwise? Don't do it. Also, make sure you have a copy of the license of any 3rd party code you copy and paste. Seems common sense, but you'd be surprised how many people don't.

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看我几分像从前
6楼-- · 2020-08-09 14:48

Of course it's sometimes acceptable. That's why people keep snippet files. But if you're cutting and pasting code very often, or with more than a few lines, you should think about making it a subroutine. Why? because odds on you'll have to change something, and this way, you only need to change it once.

Middle case is to use a macro if you have such available.

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beautiful°
7楼-- · 2020-08-09 14:48

I'm glad this one is tagged as subjective because it certainly is! This is an overly vague example, but I would imagine that if you have enough code that is duplicated that you could abstract those sections out and keep the different parts different. The point of not copy-pasting is so you don't end up having code that is hard to maintain and fragile.

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