Error while declaring a class with std::vector of structs containing std::unique_ptr

Question

Although having worked several years with C#, getting things done in C++ is sometime still difficult for me. I fully embrace the usage of smart pointers, but now I'm facing the following puzzle

I have a struct Foo, e.g.

struct Foo
{
    Foo(std::unique_ptr<Bar> bar) : m_myBar(std::move(bar)) {}

 private:
    std::unique_ptr<Bar> m_myBar;           
};

In a different class, I want to have a vector containing instances of Foo, but the following line

 std::vector<Foo> m_Foos;

yields compile errors saying that the copy constructor is deleted. In the SO thread "Why can I not push_back a unique_ptr into a vector?" an explanation as well as a remedy is given. However, there the question concerns a vector of unique pointers whereas I have a vector of structs containing a unique pointer. The suggested solution is to use move semantics, but how does that apply in my situation? Or should I be doing something else?

Answer 1

As you say, m_Foos is actually a data member of another class (I'll call it FooHolder). If you didn't provide a copy constructor for FooHolder, the compiler will generate one automatically. That copy constructor will call the copy constructors of all data members of FooHolder, including m_Foos. Of course, the copy constructor of std::vector<Foo> contains a compilation error, since Foo is not copyable. This is probably why you're getting the error.

You'll have to provide an appropriate copy constructor for FooHolder, if you are able to and want that class to be copyable. Otherwise, you can just declare a move constructor (possibly defaulted), which will make the copy constructor deleted:

struct FooHolder
{
  FooHolder(FooHolder&&) = default;

private:
  std::vector<Foo> m_Foos;
};