Question

I’ve got a problem with type deduction in Visual Studio 2010. I need to do something like this:

class Outer {
    template<typename T> ... SomeTemplateFunction() { ... }
    class NestedClass {
        std::vector<decltype(SomeTemplateFunction<SomeOtherType>())> stuff;
    };
};

However, even after adjusting for the non-staticness of the method

class Outer {
    static Outer* null() { return nullptr; }
    template<typename T> ... SomeTemplateFunction() { ... }
    class NestedClass {
        std::vector<decltype(Outer::null()->SomeTemplateFunction<SomeOtherType>())> stuff;
    };
};

Visual Studio cries, saying that the outer class is an incomplete type. How can I modify the above snippet to deduce on the return type of the outer class’s template function?

Answer 1

You can define the nested class later

class Outer {
    static Outer* null() { return nullptr; }
    template<typename T> ... SomeTemplateFunction() { ... }
    class NestedClass;
};

class Outer::NestedClass {
    std::vector<decltype(Outer::null()->SomeTemplateFunction<SomeOtherType>())> stuff;
};

The compiler is right to moan at you. For a class member access (. and ->), unlike for ::, the class must be complete. So if it would be a static function, it would be fine, but since it’s a non-static member function, it errors out.

An exception to that rule is when the member access happens in a late specified return type of a non-static member function when using this (generally, in between after the closing ) of the parameter list and before the start of the function body, that exception to the rule is allowed. Within a member function body of the inner class, the outer class is regarded complete, so no exception to the rule is needed anymore).