How to set, as default template parameter, a nested template class instanciation depending on other parameters

Question

I have the following template class & sample classes :

template<typename A, typename B, typename C = typename A::Nested<B>>

struct X

{

    X()

    {

        std::cout << "A is : " << A::who() << std::endl;

        std::cout << "B is : " << B::who() << std::endl;

        std::cout << "C is : " << C::who() << std::endl;

    }

};



struct Bsample

{

    static const char* who()    { return "Bsample"; }

};



struct Asample

{

    template<typename B>

    struct Nested;

    template<>

    struct Nested<Bsample>

    {

        static const char* who()    { return "Asample::Nested<Bsample>"; }

    };

    static const char* who()    { return "Asample"; }

};

When using vc14, the above code compiles just fine and produces the expected behavior for an instanciation of

X<Asample, Bsample>

defaulting template parameter

C

to

Asample::Nested<Bsample>

.

However, when compiling with GCC 5.1, I get the following error :

prog.cpp:4:65: error: expected '>' before '<' token

 template<typename A, typename B, typename C = typename A::Nested<B>>

                                                                 ^

I tried several combinations to declare the default value for template parameter

C

, using

template

,

typename

, ... but did not succeed in compiling this code with GCC.

How to make this code compliant to C++ standard and compile with GCC ?

Thanks for help

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Answer 1

You need both typename and template here:

template<typename A, typename B, typename C = typename A::template Nested<B>>

The template says than A::Nested is a template, and the typename says that A::Nested<B> names a type.

You also need to move the specialization of Asample::Nested out of the Asample definition:

struct Asample
{
    template<typename B>
    struct Nested;

    static const char* who()    { return "Asample"; }
};

template<>
struct Asample::Nested<Bsample>
{
    static const char* who()    { return "Asample::Nested<Bsample>"; }
};

Live demo

How to set, as default template parameter, a nested template class instanciation depending on other parameters

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