How to use Nested static templated structs in c++17

Question

What I am trying to achieve is to use a struct as a template parameter for another struct, without instantiation, so that it all happens at compile time. For example:

template<int v>
struct Container {
    static const int value = v;
};

template<Container a, Container b>
struct BiggerContainer {
    static const int value = a.v > b.v ? a.v : b.v;
};

int main() {
std::cout << BiggerContainer<Container<42>, Container<42>>::value << std::endl;

return 0;
}

The error I get with this code is the following:

./code.cpp: In function 'int main()':
./code.cpp:44:61: error: type/value mismatch at argument 1 in template parameter list for 'template<Container<...auto...> a, Container<...auto...> b> struct BiggerContainer'
   44 |     std::cout << BiggerContainer<Container<42>, Container<42>>::value << std::endl;
      |                                                             ^~
./code.cpp:44:61: note:   expected a constant of type 'Container<...auto...>', got 'Container<42>'
./code.cpp:44:61: error: type/value mismatch at argument 2 in template parameter list for 'template<Container<...auto...> a, Container<...auto...> b> struct BiggerContainer'
./code.cpp:44:61: note:   expected a constant of type 'Container<...auto...>', got 'Container<42>'

What am I getting wrong here? What exactly does this error mean what could I change to get at what I'm trying?

Thank you!

Answer 1

You might have:

template <int v>
struct Container {
    static const int value = v;
};
// template <int v> using Container = std::integral_constant<int, v>;

template <typename lhs, typename rhs>
struct Max
{
    static const int value = lhs::value > rhs::value ? lhs::value : rhs::value;
};

int main() {
    std::cout << Max<Container<42>, Container<42>>::value << std::endl;
}

Answer 2

In C++17 you cannot use a class or struct as a non-type template parameter. Non-type parameters have to be ints, pointers, enums, or references. This changes in C++20.

Specifically, the following declaration parameterizes BiggerContainer on values of structs, which is not possible.

template<Container a, Container b>
struct BiggerContainer

There are a few other problems with the code, but this is the main issue which will cause you to have to redesign things.

Answer 3

The problem is that in your template arguments for BiggerContainer , you need to specify the template arguments for Container ( template<Container, Container> doesn't have template arguments for Container ).

One alternative is to use a template function, like this:

template<int v1, int v2>
int BiggerContainer(Container<v1> a, Container<v2> b)
{
    return a.value > b.value ? a.value : b.value;
}

Then calling it like:

BiggerContainer(Container<42>(), Container<42>()); // No need to add '::value'

NOTE: Must call constructer, Container<val>() not just a type Container<val> . (Because we're passing a value, not a type)

The problem with that solution is that it returns an int (value type), not a Container , but it may be okay for some uses.