Nested Template Specialization

Question

Having a brain fart... Is it possible to make something like this work?

template struct Foo
{
    template struct Bar;
};

template         


        

Answer 1

Here's what I've been doing, which I think is simpler/more inline with what you're trying to do:

With Helper

template struct Foo
{
    template struct Bar__;
    template using Bar = Bar__;
};

template template struct Foo::Bar__
{
    // general case
    const static int x = 0;
};
template template struct Foo::Bar__
{
    const static int x = 1;
};

int main(int argc, const char * argv[]) 
{   
    std::cout << Foo<1>::Bar<0>::x << "\r\n"; // general case - output 0
    std::cout << Foo<1>::Bar<1>::x << "\r\n"; // specialized case - output 1
    return 0;
}

OR - The helper can be removed if you don't mind doubling the specialization every time:

Without Helper

template struct Foo
{
    template struct Bar;
};

template template struct Foo::Bar
{
    // general case
    const static int x = 0;
};
template template struct Foo::Bar
{
    const static int x = 1;
};

int main(int argc, const char * argv[])
{   
    std::cout << Foo<1>::Bar<0, 0>::x << "\r\n"; // general case - output 0
    std::cout << Foo<1>::Bar<1, 1>::x << "\r\n"; // specialized case - output 1
    return 0;
}