creating a shared_ptr from unique_ptr

Question

In a piece of code I reviewed lately, which compiled fine with g++-4.6, I encountered a strange try to create a std::shared_ptr from std::unique_

Answer 1

This shouldn't compile. If we disregard the uniqueness and sharedness of the pointers for a moment, it's basically trying to do this:

int *u = new int;
int *s = new int(std::move(u));

It means it's dynamically creating an int and initialising it with an rvalue reference to std::unique_ptr. For ints, that simply shouldn't compile.

For a general class Foo, it depends on the class. If it has a constructor taking a std::unique_ptr by value, const ref or rvalue ref, it will work (but maybe not do what the author intended). In other cases, it shouldn't compile.