何时使用无符号值而不使用有符号值？

什么时候使用无符号变量比使用有符号变量更合适? 在 for循环中怎么样？

我听到了很多关于这个问题的意见，我想看看是否存在任何类似于共识的东西。

for (unsigned int i = 0; i < someThing.length(); i++) {
SomeThing var = someThing.at(i);
// You get the idea.
}

我知道 Java 没有无符号值，这肯定是 昇阳电脑有意识的决定。

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最佳答案

I was glad to find a good conversation on this subject, as I hadn't really given it much thought before.

In summary, signed is a good general choice - even when you're dead sure all the numbers are positive - if you're going to do arithmetic on the variable (like in a typical for loop case).

unsigned starts to make more sense when:

  • You're going to do bitwise things like masks, or
  • You're desperate to to take advantage of the sign bit for that extra positive range .

Personally, I like signed because I don't trust myself to stay consistent and avoid mixing the two types (like the article warns against).

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In your example above, when 'i' will always be positive and a higher range would be beneficial, unsigned would be useful. Like if you're using 'declare' statements, such as: 

#declare BIT1 (unsigned int 1)
#declare BIT32 (unsigned int reallybignumber)

Especially when these values will never change.

However, if you're doing an accounting program where the people are irresponsible with their money and are constantly in the red, you will most definitely want to use 'signed'.

I do agree with saint though that a good rule of thumb is to use signed, which C actually defaults to, so you're covered.

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size_t is often a good choice for this, or size_type if you're using an STL class.

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C and C++ compilers will generate a warning when you compare signed and unsigned types; in your example code, you couldn't make your loop variable unsigned and have the compiler generate code without warnings (assuming said warnings were turned on).

Naturally, you're compiling with warnings turned all the way up, right?

And, have you considered compiling with "treat warnings as errors" to take it that one step further?

The downside with using signed numbers is that there's a temptation to overload them so that, for example, the values 0->n are the menu selection, and -1 means nothing's selected - rather than creating a class that has two variables, one to indicate if something is selected and another to store what that selection is. Before you know it, you're testing for negative one all over the place and the compiler is complaining about how you're wanting to compare the menu selection against the number of menu selections you have - but that's dangerous because they're different types. So don't do that.

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I would think that if your business case dictates that a negative number is invalid, you would want to have an error shown or thrown.

With that in mind, I only just recently found out about unsigned integers while working on a project processing data in a binary file and storing the data into a database. I was purposely "corrupting" the binary data, and ended up getting negative values instead of an expected error. I found that even though the value converted, the value was not valid for my business case.
My program did not error, and I ended up getting wrong data into the database. It would have been better if I had used uint and had the program fail.


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