Issue with namespace scoping

Question

Why does the following code work (Utilities is a namespace):

template<>
std::map<uint8_t, std::string> A::Utilities::EnumDescription<A::B::Tabs, uint8_t>::descMap =
{...};

while this one does not?

namespace A
{
namespace B
{

    template<>
    std::map<uint8_t, std::string> A::Utilities::EnumDescription<Tabs, uint8_t>::descMap =  // Error --> descMap
    {...};

}
}

Error C2888: symbol cannot be defined within namespace 'B'
Member "A::Utilities::EnumDescription::descMap [with E=A::B::Tabs, T=uint8_t]" cannot be specialized in the current scope

Neither posted piece of prose compiles as C++ (nothing to talk of "does work"). — Öö Tiib
– Öö Tiib, Commented Sep 16, 2019 at 13:41
@Pietro -- it's pseudo-code that doesn't compile. Don't post code that people who want to try it out have to edit. — Pete Becker
– Pete Becker, Commented Sep 16, 2019 at 15:01

Angew is no longer proud of SO · Accepted Answer · 2019-09-16 13:55:52Z

4

Full and partial specialisations of a template X can be defined in the same namespace where X is defined, or in a namespace enclosing (directly or indirectly) that namespace, but cannot be defined in other namespaces (such as child or "cousin" namespaces of X's namespace). That's just the rules of the language.

In your case, the template involved is A::Utilities::EnumDescription, so the specialisation must happen within A::Utilities, A, or the global namespace.

edited Sep 16, 2019 at 13:55

answered Sep 16, 2019 at 13:40

Angew is no longer proud of SO

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9 Comments

Pietro Over a year ago

Shouldn't namespace A { namespace B { Tabs } } and A::B::Tabs be equivalent in this context?

Pietro Over a year ago

Does the compiler look for Tabs in A::Utilities?

Angew is no longer proud of SO Over a year ago

@Pietro The problem is not with Tabs, it's with EnumDescription. That's the template you're specialising, and it's obviously defined in namespace A or A::Utilities (your code doesn't show if that's a NS or a class), so it must be specialised accordingly.

Öö Tiib Over a year ago

Note the first :: are needed for stuff being fully qualified. You can have Utilities sub-namespace in every of namespaces of your project.

Angew is no longer proud of SO Over a year ago

@Pietro The compiler may look for it, but if it doesn't find it, it will try ::A::Utilities too. The whole problem with your code is that you cannot specialise A::something inside A::B.

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