<: cannot begin a template argument list

Question

I get an error <: cannot begin a template argument list on g++ compiler. Code

template class SomeClass;
class Class;

SomeClass<::Cla         


        

Answer 1

With C++11 the answer to this question changes a bit.

Pre C++11

Previous to C++11 the maximal munch rule which is used in lexical analysis to avoid ambiguities and works by taking as many elements as it can to form a valid token caused this:

<::

to generate the following tokens as:

<: :

<: is a digraph which translates to [ and so you end up with:

SomeClass[:Class>* cls;

which is not valid code.

We can confirm that this is the case by going to the draft C++ standard section 2.4 Preprocessing tokens which says:

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.

and provides a couple of examples including the following classical maximal munch question:

[ Example: The program fragment x+++++y is parsed as x ++ ++ + y, which, if x and y are of built-in types, violates a constraint on increment operators, even though the parse x ++ + ++ y might yield a correct expression. —end example ]

C++11

In C++11 this changes, a rule was carved out for this case and the draft C++11 standard added the following:

Otherwise, if the next three characters are <:: and the subsequent character is neither : nor >, the < is treated as a preprocessor token by itself and not as the first character of the alternative token <:.

to section 2.5 Preprocessing tokens. So this code will no longer produce and error in C++11.

This change came from defect report: 1104