CODEWITHSUNDEEP
c++templatesmetaprogramming
DrColossos
DrColossos

Reputation: 12998

Confusion with typename and therefore errors

//more code omitted that is not relevant, the IF template is not completely shown here

template <bool condition, typename ThenType, typename ElseType>
    struct IF {
         typedef typename ChooseSelector<condition>::RETURN Selector;
    };

    template <bool condition>
        struct ChooseSelector {
             typedef SelectThen RETURN;
        };

    template <>
        struct ChooseSelector<false> {
             typedef SelectElse RETURN;
        };
//SelectElse and SelectThen omitted

I get Expected nested-name-specifier before ‘ChooseSelector’. According to the often linked C++ typename description and if I get it correctly, typename is needed here. If I remove typename from the IF template, I still get the same error, so I am a little confused what is actually causing the error. I read many answers that suggest that removing typename fixes the problem, but that is not the case in this case. What am I missing?

The error comes from g++ on Linux, VS10 throws an error as well.

Upvotes: 2

Views: 276

Answers (1)

Erik
Erik

Reputation: 91270

Put your IF template after the ChooseSelector template.

When compiling the IF template, ChooseSelector need to exist as a template, you're using ChooseSelector<condition> which is parsed first pass. The typename is needed to tell the compiler that RETURN, which is fully evaluated on instantiation when specializations are known, should be considered a type for the purpose of first pass.

Upvotes: 2

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