C++ Templates Angle Brackets Pitfall - What is the

问题:

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

vector<vector<float>> MyMatrix;

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

vector<vector<float> > MyMatrix;

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

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

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

回答1:

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

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.