对于嵌套模板，`>>`成为标准C ++（而不是`>>`）？ [英] For nested templates, when did `>>` become standard C++ (instead of `> >`)?
问题描述
我似乎记得，在处理嵌套模板参数时，在yore的时候，警告不要将两个> 字符相邻（没有空格） 。我甚至模糊地记得宣布任何向量的向量并遇到这个编译错误。
I seem to recall, in times of yore, being warned against putting two > characters right next to each other (without a space) when dealing with nested template parameters. I even vaguely remember declaring vectors of vectors of whatever and encountering this compilation error.
但现在我发现编译可怕的>> ...
But now I find that there is absolutely nothing wrong with compiling the dreaded >>...
我的问题是：
这个公约在什么时候成为可接受的做法？
At what point did this convention become an acceptable practice?
是标准C ++的一部分吗？
Is it part of standard C++?
它是标准的一部分，我在大学使用的编译器（和我的教授）只是不支持它吗？
Was it always part of the standard and the compilers I used (and the professors I had) in college just didn't support it yet?
也许这些问题是一个
推荐答案
用嵌套<$ c关闭的模板$ c>>> 是由即将推出的标准正式支持 C ++ 0x（现在C ++ 11）。以前，你需要的空间，或一个编译器，为你的额外的英里（并没有表示的标准）。
Templates closed with nested >> are officially supported by the upcoming standard C++0x (now C++11). Previously you would need the space, or a compiler that went the extra mile for you (and did things not indicated by the standard).
这个问题源于这样的事实： C中的>> 是右移运算符，它是单个词法令牌，它与两个单独的> 令牌在经典构造的C ++编译器中的解析阶段（并且仅在模板的情况下，而不是当它实际上是右移时）将是需要的。换句话说，>> 如果允许关闭嵌套模板，在词汇上是不明确的，但是这可以（并且正在）通过解析期间的额外复杂性在现代C ++中真的没什么新鲜的）。
The issue stems from the fact that >> in C is the right-shift operator, which is a single lexical token, which conflicts with the two separate > tokens that would be needed during the parsing stage in a classically-constructed C++ compiler (and only in the case of templates, not when it actually is a right-shift). In other words, the >>, if allowed to close nested templates, is lexically ambiguous, but this can be (and is being) addressed by extra sophistication during parsing (which in modern C++ is really nothing new).
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