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:
template<typename T> struct A {}; template<typename T> using B = int; template<template<typename> class X> struct C; C<A> ca; // ok C<B> cb; // ok, not a class template template<template<typename> typename X> struct D; // error, cannot use typename hereThe 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.讨论(0) -
I'm looking for the rational behind this restriction [...]
Before C++11 was introduced, the only templates you could pass to a template template parameter were class templates. That's why the use of the keyword
classwas enforced. Additionally, the keywordtypenameimplies that the template parameter is a substitution for an arbitrary type and not a template, so usingtypenamein that context would just blur the line between the names of types and (class) templates. That's comprehensible.Nowadays, such arguments can be the names of class templates or alias templates, and since those aren't even remotely connected, the enforcement of the keyword
classis more or less obsolete. The proposal N4051 opts to change this with C++1Z.讨论(0)
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