Is it required to explicitly list default parameters when using template template parameter?
I\'d like to ask whether the following code sample should compile:
#include
#include
#include
using namespa
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Consider the similar
templatestruct A { }; template class T> struct B : T<> { }; template class B; This is clearly covered by the standard (14.3.3p3 if you're interested, I won't quote it, as GCC and clang do both implement the rule already), where the use of
Aas a template argument forBis disallowed because of the non-type template parameter. That rule makes no sense if the instantiation of a template template parameter could make use of the template template argument's default template arguments, so the behaviour of MSVC and Intel is more consistent than that of GCC and clang.Of course, the reasoning "if this were valid, the standard would have inconsistencies" doesn't actually mean it isn't valid, only that it shouldn't be valid. To actually check what the standard says:
14.1 Template parameters [temp.param]
10 The set of default template-arguments available for use with a template declaration or definition is obtained by merging the default arguments from the definition (if in scope) and all declarations in scope in the same way default function arguments are (8.3.6).
8.3.6 Default arguments [dcl.fct.default]
4 Declarations in different scopes have completely distinct sets of default arguments. That is, declarations in inner scopes do not acquire default arguments from declarations in outer scopes, and vice versa.
Although not specifically intended to address this use of default template arguments, I think it does manage to do so. Nikos Athanasiou has already included the part of the standard that says any default template arguments of
Cdo get used:14.1 Template parameters [temp.param]
14 A template-parameter of a template template-parameter is permitted to have a default template-argument. When such default arguments are specified, they apply to the template template-parameter in the scope of the template template-parameter.
Since
C's default template arguments are used,std::vector's aren't, and MSVC and Intel seem to be correct here.And to come up with an example that clearly shows that GCC and clang cannot be considered to conform here:
templatestruct A { }; template class T> struct B { using type = T<>; }; Both GCC and clang treat
B::typeasA, taking one default template argument fromT, and another fromA, even though the standard disallows merging of default arguments (and hence default template arguments) in declarations in different scopes.
A workaround for you, to avoid the need to type out the allocator argument, could be to use a template alias:
template class C> struct convert_container { using type = C; }; template using vector_default_alloc = std::vector ; int main() { std::cout << typeid(convert_container ::type).name(); } I cannot test on MSVC right now, but Intel accepts it, and I see no reason why this variant would be invalid.
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