how to get max using template metaprograming using variadic template class - c++

Question

Below I am trying to get a simple max value from a list of traits containing SOME_VAL at compile time:
struct A_Traits { enum { SOME_VAL=5 }; };
struct B_Traits { enum { SOME_VAL=6 }; };
struct C_Traits { enum { SOME_VAL=4 }; };
template<typename T>
T MAX(T t1, T t2)
{
return t1>t2?t1:t2;
}
template<typename T, typename... Args>
struct max_calculator
{
enum { MAX_VAL = MAX(max_calculator<typename T::SOME_VAL>::MAX_VAL, // line 65
max_calculator<Args...>::MAX_VAL) };
};
template<typename T>
struct max_calculator<T>
{
enum { MAX_VAL = T::SOME_VAL };
};
int main()
{
cout << max_calculator<A_Traits>::MAX_VAL << endl;
cout << max_calculator<A_Traits, B_Traits>::MAX_VAL << endl; // line 79
}
However I get a compile error of:
variadic.cpp: In instantiation of 'struct max_calculator<A_Traits, B_Traits>':
variadic.cpp:79:47: required from here
variadic.cpp:65:10: error: no type named 'SOME_VAL' in 'struct A_Traits'
Any idea whats wrong or better ideas to do the same at compile time?

Answer 1

std::max has a constexpr overload for std::initialiser_list<T>, you can expand your pack in one place
template<typename... Args>
struct max_calculator
{
enum { MAX_VAL = std::max({ Args::SOME_VAL... }) };
};

Answer 2

For the benefit of everyone visting this later - the fixed program based on Piotr's answers in the comments section is:
struct A_Traits { enum { SOME_VAL=5 }; };
struct B_Traits { enum { SOME_VAL=6 }; };
struct C_Traits { enum { SOME_VAL=4 }; };
template<typename T>
constexpr T MAX(T t1, T t2)
{
return t1>t2?t1:t2;
}
template<typename T, typename... Args>
struct max_calculator
{
enum { MAX_VAL =MAX<uint8_t>(max_calculator<T>::MAX_VAL,
max_calculator<Args...>::MAX_VAL) };
};
template<typename T>
struct max_calculator<T>
{
enum { MAX_VAL = T::SOME_VAL };
};
int main()
{
cout << max_calculator<A_Traits>::MAX_VAL << endl;
cout << max_calculator<A_Traits, B_Traits>::MAX_VAL << endl;
cout << max_calculator<A_Traits, B_Traits, C_Traits>::MAX_VAL << endl;
}