尾随逗号和C ++ [英] Trailing commas and C++

问题描述

我读过的地方，C ++标准不允许像枚举an_enum {A，B，C，}; ，而C的更高版本（我想从90年代中期）都允许这样的声明（用逗号结尾）。如果C ++应该有使用C向后兼容性，怎么来的这个功能是被禁止的？任何特别的原因？

I've read somewhere that the C++ standard does not allow something like enum an_enum { a, b, c, };, while later versions of C(I think from mid-90s) do allow such declarations(with trailing commas). If C++ is supposed to have backwards compatibility with C, how come this feature is forbidden? Any special reason?

我也看了，这样的尾随逗号实际上是很好，所以，只是增加了混乱。

I also read that such trailing commas are actually good, so that just adds to the confusion.

推荐答案

C ++ 03（这是C ++ 98相当次要更新）立足于C89的C兼容性（也被称为C90，这取决于你'重新ANSI或ISO）。 C89不允许尾随逗号。 C99确实允许它。 C ++ 11确实允许它（7.2 / 1有一个枚举声明语法）。

C++03 (which is a fairly minor update of C++98) bases its C compatibility on C89 (also known as C90, depending on whether you're ANSI or ISO). C89 doesn't allow the trailing comma. C99 does allow it. C++11 does allow it (7.2/1 has the grammar for an enum declaration).

在事实上C ++是不完全向后兼容，即使C89，虽然这是，如果它当时在C89，你所期望的C ++允许它的那种东西。

In fact C++ isn't entirely backward-compatible even with C89, although this is the kind of thing that if had it been in C89, you'd expect C++ to permit it.

我后面的逗号的主要优点是，当你这样写：

The key advantage to me of the trailing comma is when you write this:

enum Channel {
    RED,
    GREEN,
    BLUE,
};

再后来改成这样：

and then later change it to this:

enum Channel {
    RED,
    GREEN,
    BLUE,
    ALPHA,
};

这是很好，只有一行被改变，当你差异版本。要获得相同的效果时，有没有让后面的逗号，你可以写：

It's nice that only one line is changed when you diff the versions. To get the same effect when there's no trailing comma allowed, you could write:

enum Channel {
    RED
   ,GREEN
   ,BLUE
};

但是，（一）这是疯话，和（b）它并不要在开始添加新价值（无可否认罕见）的情况下帮助。

But (a) that's crazy talk, and (b) it doesn't help in the (admittedly rare) case that you want to add the new value at the beginning.

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