'dependent name is not a type' when using nested nested class in templated class

Question

Does anyone know why the following fails to compile (tested in MSVC2015)?

template<int N> // when this line is removed it compiles successfully
class A {
    class B {
    public:
        class C {
        };
    };
    void func(B b){} // compiles fine
    void func(B::C c){} // generates warning C4346 and error C2061
};

It generates the following on the line marked with a comment:

warning C4346: 'A<N>::B::C': dependent name is not a type
error C2061: syntax error: identifier 'C'

score 2 · Accepted Answer · edited Jun 20 '20 at 09:12

2

Let's google it for you.

The typename keyword is requiretd if a dependent name is to be treated as a type.

C++ rule.

14.6 Name resolution

A name used in a template declaration or deﬁnition and that is dependent on a template-parameter is assumed not to name a type unless the applicable name lookup ﬁnds a type name or the name is qualiﬁed by the keyword typename.

edited Jun 20 '20 at 09:12

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answered Mar 12 '18 at 01:23

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What is a 'dependent name'? – Museful Mar 12 '18 at 01:25
Edited. See above. – 273K Mar 12 '18 at 01:27
Is there a reason for the rule? – Museful Mar 12 '18 at 01:32
@Museful yes. This is because accessing a data member or a member type, and even a member template is an ambiguous syntax. Imagine `foo::bar` is that a type? is that a static data member? The compiler assumes a data member unless the `typename` keyword is there. – Guillaume Racicot Mar 12 '18 at 04:33

'dependent name is not a type' when using nested nested class in templated class

1 Answers1