C++ 模板角括号陷阱 - C++11 修复是什么?

问题描述

在 C++11 中，这是现在有效的语法:

In C++11, this is now valid syntax:

vector<vector<float>> MyMatrix;

而以前，它必须这样写(注意空格):

whereas previously, it had to be written like this (notice the space):

vector<vector<float> > MyMatrix;

我的问题是标准用于允许第一个版本的修复是什么?

My question is what is the fix that the standard uses to allow the first version?

是否可以像将 >> 变成标记而不是 >>> 一样简单?如果不是这样，那么这种方法有什么不适用的?

Could it be as simply as making > a token instead of >>? If that's not it, what does not work with this approach?

我认为像 myTemplate< 这样的形式x>>3 > 不是问题，因为您可以通过执行 myTemplate<(x>>3)> 来消除它们的歧义.

I consider that forms like myTemplate< x>>3 > are a non-problem, since you can disambiguate them by doing myTemplate<(x>>3)>.

推荐答案

通过在解析模板参数时在解析规则中添加特殊情况来修复.

It's fixed by adding a special case to the parsing rules when parsing template arguments.

C++11 14.2/3: 当解析一个template-argument-list时，第一个非嵌套的>被当作结束分隔符而不是一个大于运算符.类似地，第一个非嵌套的 >>> 被视为两个连续但不同的 > 标记，其中第一个被视为 的结尾template-argument-list 并完成template-id.

C++11 14.2/3: When parsing a template-argument-list, the first non-nested > is taken as the ending delimiter rather than a greater-than operator. Similarly, the first non-nested >> is treated as two consecutive but distinct > tokens, the first of which is taken as the end of the template-argument-list and completes the template-id.

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