Question

In Exceptional C++, Herb Sutter wrote about auto_ptr:

The problem is that auto_ptr does not quite meet the requirements of a
type you can put into containers, because copies of auto_ptrs are not
equivalent.

The book was written with respect to C++03, and I wonder whether this is still valid as this code seems to compile perfectly under GCC 4.7.1:

#include <vector>

struct Foo
{
    Foo() { }
    Foo( Foo&& ) { }
    Foo( Foo& ) = delete;
    Foo& operator= (Foo&&) { return *this; }
    Foo& operator= (Foo&) = delete;
};

int main()
{
    std::vector<Foo> bar;
    bar.push_back(Foo());
}

But accepting moveable-but-not-copyable objects could also be a GCC extension. I am not sure. What are the requirements for std::vector objects?

Answer 1

To be able to use std::vector<Foo>::push_back(), the type Foo needs to be MoveInsertable or CopyInsertable (23.2.1 [container.requirements.general] paragraph 13) according to 23.2.3 [sequence.rqmts] paragraph 16 (Table 101). Clearly, providing the MoveInsertable model requires that you use push_back() with an r-value, possibly obtained using std::move(x). That is, you don’t even need the move assignment operator to be able to use std::vector<...>::push_back(). I think not all standard libraries correctly take this latter part into account and delegate to insert(): delegating to insert() doesn’t work according to the C++ 2011 standard because insert() also requires the type to be MoveAssignable or CopyAssignable.

Trying to search for push_back unfortunately fails to point to Table 101 for me because the name is split into push_ and back() by a line break…