int a [] = {1,2，};允许使用逗号逗号。任何特别的原因？ [英] int a[] = {1,2,}; Weird comma allowed. Any particular reason?

问题描述

也许我不是来自这个星球，但在我看来，以下应该是语法错误：

Maybe I am not from this planet, but it would seem to me that the following should be a syntax error:

int a[] = {1,2,}; //extra comma in the end

但它不是。我感到惊讶，当这段代码编译在Visual Studio，但我已经学会了不信任MSVC编译器就C ++规则，所以我检查了标准，它 允许的标准以及。如果你不相信我，你可以看到8.5.1的语法规则。

But it's not. I was surprised when this code compiled on Visual Studio, but I have learnt not to trust MSVC compiler as far as C++ rules are concerned, so I checked the standard and it is allowed by the standard as well. You can see 8.5.1 for the grammar rules if you don't believe me.

为什么允许这样做？这可能是一个愚蠢的无用的问题，但我想让你了解为什么我问。如果它是一个一般语法规则的子情形，我会理解 - 他们决定不让一般语法更困难，只是在初始化列表的末尾不允许一个冗余的逗号。但不能，额外的逗号是显式 允许。例如，不允许在函数调用参数列表的末尾有一个多余的逗号（当函数 ... 时）， which是正常的。

Why is this allowed? This may be a stupid useless question but I want you to understand why I am asking. If it were a sub-case of a general grammar rule, I would understand - they decided not to make the general grammar any more difficult just to disallow a redundant comma at the end of an initializer list. But no, the additional comma is explicitly allowed. For example, it isn't allowed to have a redundant comma in the end of a function-call argument list (when the function takes ...), which is normal.

再次，是有任何特别的原因，这个冗余的逗号是显式 允许？

So, again, is there any particular reason this redundant comma is explicitly allowed?

推荐答案

这使得生成源代码更容易，也可以编写稍后可以轻松扩展的代码。考虑为以下内容添加额外条目所需的内容：

It makes it easier to generate source code, and also to write code which can be easily extended at a later date. Consider what's required to add an extra entry to:

int a[] = {
   1,
   2,
   3
};

...您必须将逗号添加到现有行和添加新行。比较三个已经后面有逗号的情况，你只需要添加一行。同样，如果你想删除一行，你可以这样做，而不必担心它是否是最后一行，并且你可以重新排序行没有freetling用逗号。基本上，这意味着如何处理行的一致性。

... you have to add the comma to the existing line and add a new line. Compare that with the case where the three already has a comma after it, where you just have to add a line. Likewise if you want to remove a line you can do so without worrying about whether it's the last line or not, and you can reorder lines without fiddling about with commas. Basically it means there's a uniformity in how you treat the lines.

现在考虑生成代码。类似的东西（伪代码）：

Now think about generating code. Something like (pseudo-code):

output("int a[] = {");
for (int i = 0; i < items.length; i++) {
    output("%s, ", items[i]);
}
output("};");

无需担心您输出的当前项是第一个还是最后一个。更简单。

No need to worry about whether the current item you're writing out is the first or the last. Much simpler.

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