CODEWITHSUNDEEP
c++inheritancec++17using-directivestemplate-instantiation
glades
glades

Reputation: 4737

error: static assertion failed: template argument must be a complete class or an unbounded array

I'm deriving from a base class in which I try to define a type. That type is dependent on itself via a variant, so it would require to know the memory layout of the base class upon definition. However, I'm not prematurely defining the type? So I thought this would work but it doesnt (CompilerExplorer):

#include <variant>
#include <cstddef>
#include <array>


template <size_t V> struct container_base;
template <size_t V> struct container;

template <size_t V>
using val = std::variant<std::monostate, int, container_base<V>>;

template <size_t V>
struct container_base
{
    using iterator = typename std::array<val<V>, V>::iterator;
};

template <size_t V>
struct container : public container_base<V>
{
};

int main()
{
    container<100> A;
}

This yields:

In file included from /opt/compiler-explorer/gcc-trunk-20220726/include/c++/13.0.0/variant:37,
                 from <source>:1:
/opt/compiler-explorer/gcc-trunk-20220726/include/c++/13.0.0/type_traits: In instantiation of 'struct std::is_copy_constructible<container_base<100> >':
/opt/compiler-explorer/gcc-trunk-20220726/include/c++/13.0.0/type_traits:3202:33:   required from 'constexpr const bool std::is_copy_constructible_v<container_base<100> >'
/opt/compiler-explorer/gcc-trunk-20220726/include/c++/13.0.0/variant:329:5:   required from 'constexpr const bool std::__detail::__variant::_Traits<std::monostate, int, container_base<100> >::_S_copy_ctor'
/opt/compiler-explorer/gcc-trunk-20220726/include/c++/13.0.0/variant:1334:11:   required from 'class std::variant<std::monostate, int, container_base<100> >'
/opt/compiler-explorer/gcc-trunk-20220726/include/c++/13.0.0/array:109:55:   required from 'struct std::array<std::variant<std::monostate, int, container_base<100> >, 100>'
<source>:15:11:   required from 'struct container_base<100>'
<source>:19:8:   required from 'struct container<100>'
<source>:25:20:   required from here
/opt/compiler-explorer/gcc-trunk-20220726/include/c++/13.0.0/type_traits:1012:52: error: static assertion failed: template argument must be a complete class or an unbounded array
 1012 |       static_assert(std::__is_complete_or_unbounded(__type_identity<_Tp>{}),
      |

I understand it so that variant needs a complete type to work with when it's first used (which is in main! But the type is complete until then, so I don't know why it doesn't work. What can I do to fix this?

Upvotes: 3

Views: 2484

Answers (1)

Quuxplusone
Quuxplusone

Reputation: 26949

Answered in a comment by @IgorTandetnik:

Accessing std::array<val<V>, V>::iterator [which is necessary in order to complete the type container_base<V>] requires instantiating std::array<val<V>, V>, which requires instantiating val<V>, which requires instantiating std::variant<std::monostate, int, container_base<V>>, and container_base<V> is at that point an incomplete type.

A slightly reduced example, using stub versions of StdArray and StdVariant in place of std::array and std::variant, is here: Godbolt.

Upvotes: 0

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