Why does not a template template parameter allow 'typename' after the parameter list
Template template typename?
When using template template syntax as in template class T>, it is requi
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Short answer: because the Standard says so.
Longer answer: prior to Standardization, C++ templates required the
classkeyword for all template parameters. However, to stress the fact that templates could also be of non-class (i.e. builtin) type, an alternative keywordtypenamewas introduced. However, in C++98, template-template parameters could only be of class-type, and this was the reason that thetypenamekeyword was not added in that context.Enter C++11 and its new feature template aliases, that now also introduced non-class templates, and hence non-class template-template parameters:
templatestruct A {}; template using B = int; template The above example was taken from the current C++1z proposal N4051 titled Allow
typenamein a template template parameter, and proposes to allow precisely that.Clang 3.5 SVN now supports this with the
-std=c++1zflag.
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