Nested templates vs shift operator

Question

I have been reading all around about be aware >> as ending of nested template and >> as shift operator...

Now I have tried it in my MSVS2010 and no problem occured.

std::map<int, std::pair<int, int>> m;

This code works exactly what I want (map of pairs) but I supposed to get some error about >>

Compiler is smarter these days?

score 6 · Accepted Answer · answered Apr 24 '11 at 14:42

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MSVC++2010 supports C++0x feature Right Angle Brackets

answered Apr 24 '11 at 14:42

Prasoon Saurav

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score 6 · Answer 2 · answered Apr 24 '11 at 15:06

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Be careful because previously good C++03 code may break with compilers supporting this feature.

MyArray< MyArray<int, 16 >> 2>, 5 > arrayInst;

This would be the fix:

MyArray< MyArray<int, (16 >> 2)>, 5 > arrayInst;

answered Apr 24 '11 at 15:06

CB Bailey

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We all would have been better off had they used `(T)` instead of `` in the first place. – shuhalo Sep 21 '20 at 13:56

score 4 · Answer 3 · answered Apr 24 '11 at 14:41

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This code works exactly what I want (map of pairs) but I supposed to get some error about >>

C++0x has fixed this. So if you're not getting any error with MSVS2010, then its no wonder, as MSVS2010 has implemented some of C++0x features.

Also, even with C++03, many compilers handle such cases, though not required by the Standard(2003).

answered Apr 24 '11 at 14:41

Nawaz

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Actually, it's not allowed for a conforming C++03 compiler to implement C++0x semantics for `>>`. There are valid C++03 constructs which can break with this change. – CB Bailey Apr 24 '11 at 15:07
@Charles: Interesting. But I would like to know what valid C++03 constructs can be broken with this change, and how C++0x handles this then? – Nawaz Apr 24 '11 at 15:10
@Nawaz: E.g. shift expressions in constant expressions for non-type template parameters. E.g. `tmpl< tmpl<2 >> 1> > instance;`. Usually valid C++03 becomes a compile error in C++0x. I haven't managed to construct any situation where this causes a silent change in behaviour. – CB Bailey Apr 24 '11 at 15:14
@Charles: How does C++0x handle this then? Is it going to interpret `>>` in `<2 >> 1>` as shift operator? – Nawaz Apr 24 '11 at 15:22
@Nawaz: As I indicated, it's a compile error. `>>` closes the template, leaving a non-sensical `1> > instance;` token sequence where a declarator is needed. – CB Bailey Apr 24 '11 at 15:36
@Charles: In other words, I can say that few valid C++03 things are invalid in C++0x, right? – Nawaz Apr 24 '11 at 15:51
@Nawaz: I'm not 100% sure I know what you are driving. Are you looking to make a wider generalisation? There are **a** few things that are valid C++03 and aren't valid C++0x. Not least there are several more keywords that can no long be used as ordinary identifiers. – CB Bailey Apr 24 '11 at 15:58
@Charles: *Are you looking to make a wider generalisation?*... what does it mean? Are we discussing some philosophy? :-/ – Nawaz Apr 24 '11 at 16:16
@Nawaz: You said "I can say that few valid C++03 things are invalid in C++0x, right?", my thoughts were that you could say that but we really only touched on one corner case and there a probable many others. – CB Bailey Apr 24 '11 at 16:25

score 3 · Answer 4 · answered Apr 24 '11 at 14:42

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C++0x now supports that syntax without errors. Compilers have already started to implement most of these features, so it wouldn't be surprising that the latest Microsoft C++ compiler supports it.

answered Apr 24 '11 at 14:42

Joseph Lisee

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Nested templates vs shift operator

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