<: cannot begin a template argument list

Question

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

template<typename T> class SomeClass;
class Class;

SomeClass<::Class>* cls;

Answer 1

According to the Maximal Munch tokenization principle a valid C++ token must collect/have as many consecutive characters as possible.

<: is a digraph (an alternative representation of symbol [).

                           Digraph  Equivalent
                              <:          [
                              :>          ]
                              <%          {
                              %>          }
                              %:          #

So SomeClass<::Class>* cls; is interpreted as SomeClass[:Class>* cls; which doesn't make any sense.

Solution: Add a whitespace between < and :

  SomeClass< ::Class>* cls;
            ^
            | 
           White Space

Answer 2

Try the following instead:

SomeClass< ::Class>* cls;

You can find more info in this question about digraphs. This question about trigraphs could be helpful also.

Answer 3

Put spaces around the < characters:

SomeClass < ::Class > * cls;

You only actually need to separate < and :, but I like symmetry.