(Partially) specializing a non-type template parameter of dependent type

Question

Maybe I\'m tired, but I\'m stuck with this simple partial specialization, which doesn\'t work because non-type template argument specializes a template parameter with

Answer 1

You need to pass an integral value in a template, Both, your first and second template, will not work if the type T is not an integral type.

You can pass Traits as a typed template parameter to specify the value N:

#include 

// error: ‘double’ is not a valid type for a template non-type parameter
template  struct X0;

// error: ‘double’ is not a valid type for a template non-type parameter
template  struct X1;



template 
struct IntegralTraits {
    static constexpr T Value() { return N; }
};

template 
struct X2 {
    static constexpr T Value() { return Traits::Value(); }
};

template 
struct X2 {
    static constexpr T Value() { return T(); }
};


int main() {
    // error: ‘double’ is not a valid type for a template non-type parameter
    // X0();

    // error: ‘double’ is not a valid type for a template non-type parameter
    // X1();

    X2 a;
    X2> b;

    std::cout.precision(2);
    std::cout << std::fixed  <<  a.Value() << ", "<< b.Value() << '\n';
    return 0;
}

If you limit yourself to integral types pick a large one:

template  struct X {};