C++17 optional tree, error: invalid use of incomplete type

问题

When I compile a binary tree containing optional types:

#include <optional>

class BinaryTree
{
public:
    BinaryTree();
    int value;
    std::optional<BinaryTree> left,right;
};

int main()
{
    return 0;
}

via

g++ -std=c++17 -Wfatal-errors main.cpp

I face with this error

In file included from /usr/include/c++/7/bits/move.h:54:0,
                 from /usr/include/c++/7/bits/stl_pair.h:59,
                 from /usr/include/c++/7/utility:70,
                 from /usr/include/c++/7/optional:36,
                 from main.cpp:1:
/usr/include/c++/7/type_traits: In instantiation of ‘struct std::is_trivially_copy_constructible<BinaryTree>’:
/usr/include/c++/7/optional:103:8:   required from ‘class std::_Optional_base<BinaryTree>’
/usr/include/c++/7/optional:451:11:   required from ‘class std::optional<BinaryTree>’
main.cpp:8:28:   required from here
/usr/include/c++/7/type_traits:1409:12: error: invalid use of incomplete type ‘class BinaryTree’
     struct is_trivially_copy_constructible
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated due to -Wfatal-errors.

How should I fix this error?

回答1:

Optional contains the complete instance of the type when it is engaged.

Storing two instances of a type within an instance of the type is impossible; it requires 2<=1.

Consider using a unique pointer to the type instead of an optional.

If you want to be able to copy the tree then you have to write a value pointer. Value pointer is a pointer that knows how to deep copy itself.

回答2:

@Yakk's answer is correct.

As @Dietmar Kühl points out, std::unique_ptr is what you want for nullable not-in-place storage:

#include <memory>

class BinaryTree
{
public:
    BinaryTree();
    int value;
    std::unique_ptr<BinaryTree> left,right;
};

int main()
{
    return 0;
}