CODEWITHSUNDEEP
c++c++11
Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385224

Global qualification in base specifier

As a follow-up to " What is this crazy C++11 syntax ==> struct : bar {} foo {};? ", I'd expect the following C++0x code to compile:

struct x {};
struct :::x {} y {};

However, GCC 4.7.0 20110731 tells me:

error: global qualification of class name is invalid before ':' token

And when I take a step back towards sanity and give the second UDT a name:

struct x {};
struct a:::x {} y{}; // remember, identical to `a::: x` or `a: ::x` or `a:: :x` etc

the error is:

error: 'a' has not been declared

It seems like the three colons are being lexed like <::> <:> rather than <:> <::>, but can this clearly be deduced from the [final draft] standard?

And might the question " Global qualification in a class declarations class-head " be related?

Upvotes: 1

Views: 665

Answers (2)

Peter Alexander
Peter Alexander

Reputation: 54280

This is just to do with parsing. From §2.5.3

If the input stream has been parsed into preprocessing tokens up to a given character, the next preprocessing token is the longest sequence of characters that could constitute a preprocessing token, even if that would cause further lexical analysis to fail.

Basically, it has to take the longest sequence of characters, so ::: is always parsed as :: : in the same way that x+++y is always parsed as x ++ + y.

This is referred to as Maximal Munch parsing.

Upvotes: 6

Armen Tsirunyan
Armen Tsirunyan

Reputation: 133044

It seems like the three colons are being lexed like <::> <:> rather than <:> <::>, but can this clearly deduced from the [final draft] standard?

It's not a [final] draft any more. It has been unanimously accepted already. And yes, it can be deduced that ::: will be parsed as :: followed by :. It's a purely lexical issue. As in C++03, the token is taken as the longest possible one. In C++03 that led template closing >>s to be considered as shift operator. In C++0x an exception had been made for this special case, but the general rule still applies (See 2.5.3). For example +++ will be parsed as ++ followed by +, not vice versa

Upvotes: 6

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