boost::variant; std::unique_ptr and copy

Question

This Question Determined That a Non-Copyable Type Can\'t Be Used With Boost Variant

Tree class

template 

        

Answer 1

Concerning the call to boost::bind(), you should use boost::ref() when passing an object by reference to a function template that accepts the corresponding argument by value, otherwise a copy will be attempted (which results in a compiler error in this case, since the copy constructor is inaccessible):

boost::bind(TreeVisitor(), boost::ref(tree), val, keyIndex);
//                         ^^^^^^^^^^^^^^^^

However, there is a bigger problem here: boost::variant can only hold types which are copy-constructible. From the Boost.Variant online documentation:

The requirements on a bounded type are as follows:

• CopyConstructible [20.1.3].

• Destructor upholds the no-throw exception-safety guarantee.

• Complete at the point of variant template instantiation. (See boost::recursive_wrapper for a type wrapper that accepts incomplete types to enable recursive variant types.)

Every type specified as a template argument to variant must at minimum fulfill the above requirements. [...]