I am using Visual Studio 2012 Update 2 and am having trouble trying to understand why std::vector is trying to use the copy constructor of unique_ptr. I have looked at similar
The push_back()
function takes its argument by value. Therefore, an attempt is made to either copy-construct the argument of push_back()
(if you are passing an lvalue), or to move-construct it (if you are passing an rvalue).
In this case, o
is an lvalue - because named objects are lvalues - and rvalue references cannot bind to lvalues. Therefore, the compiler cannot invoke your move constructor.
In order to have your object moved, you have to write:
s.push_back(std::move(o));
// ^^^^^^^^^
What surprises me in this case is that it seems VC11 generated a copy-constructor for MyObject
implicitly without defining it as deleted (judging from the error you posted). This should not be the case, since your class declares a move constructor. Per Paragraph 12.8/7 of the C++11 Standard, in fact:
If the class definition does not explicitly declare a copy constructor, one is declared implicitly. If the class definition declares a move constructor or move assignment operator, the implicitly declared copy constructor is defined as deleted; otherwise, it is defined as defaulted (8.4)
I must conclude that while the error you are getting is correct - because you are not passing an rvalue to push_back()
- VC11 is not fully compliant here.
MyObject o;
defines o
to be an object. Which means it's a l-value. Doing s.push_back(o);
then invokes the l-value overload of push_back()
(it has no other choice), which tries to create a copy.
Since your class is noncopyable, you have to move the object into the vector:
for (int i = 0; i < 5; ++i)
{
MyObject o;
s.push_back(std::move(o));
}