问题描述
我似乎记得,在过去,在处理嵌套模板参数时被警告不要将两个 >
字符放在彼此旁边(没有空格).我什至依稀记得声明了向量的向量并遇到了这个编译错误.
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:
这个约定在什么时候成为可以接受的做法?
At what point did this convention become an acceptable practice?
它是标准 C++ 的一部分吗?
Is it part of standard C++?
它是否一直是标准的一部分,而我在大学使用的编译器(以及我的教授)还不支持它?
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?
也许这些问题有点历史,但对我来说,适当的历史背景似乎使实际记忆变得微不足道.
Maybe these questions are a tad bit historical, but for me it seems that proper historical context makes actual remembering trivial.
推荐答案
使用嵌套 >>>
关闭的模板是 正式支持 即将推出的标准 C++0x(现在是 C++11).以前,您需要空间或编译器为您加倍努力(并且做了标准未指明的事情).
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).
问题源于C中的>>
是右移运算符,它是一个单独的词法标记,与两个单独的>
冲突code> 在经典构造的 C++ 编译器的解析阶段需要的标记(并且仅在模板的情况下,而不是在它实际上是右移时).换句话说,>>
,如果允许关闭嵌套模板,在词法上是模棱两可的,但是这可以(并且正在)在解析过程中通过额外的复杂性来解决(这在现代 C++ 中实际上是没什么新鲜的).
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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