Iterating through vector<unique_ptr<mytype>> using C++11 for() loops

Question

I've got the following batch of code:

std::vector<std::unique_ptr<AVLTree_GeeksforGeeks>> AVLArray(100000);

/* Let's add some objects in the vector */
AVLTree_GeeksforGeeks *avl = new AVLTree_GeeksforGeeks();
avl->Insert[2]; avl->Insert[5]; AVL->Insert[0];
unique_ptr<AVLTree_GeeksforGeeks> unique_p(avl);
AVLArray[0] = move(unique_p);
/* we do this for a number of other trees, let's say another 9...
...
...
Now the vector has objects up until AVLTree[9] */

/* Let's try iterating through its valid, filled positions */
for(auto i : AVLTree )
{
   cout << "Hey there!\n";    //This loop should print 10 "Hey there"s.
}

Ruh roh. Compilation error at the last part, in the for() loop.

\DataStructures2013_2014\main.cpp||In function 'int main()':|
\DataStructures2013_2014\main.cpp|158|error: use of deleted function 'std::unique_ptr<_Tp, _Dp>::unique_ptr(const std::unique_ptr<_Tp, _Dp>&) [with _Tp = AVLTree_GeeksforGeeks; _Dp = std::default_delete<AVLTree_GeeksforGeeks>; std::unique_ptr<_Tp, _Dp> = std::unique_ptr<AVLTree_GeeksforGeeks>]'|
e:\codeblocks\mingw\bin\..\lib\gcc\mingw32\4.7.1\include\c++\bits\unique_ptr.h|256|error: declared here|
||=== Build finished: 2 errors, 0 warnings (0 minutes, 0 seconds) ===|

Any ideas on what I am doing wrong?

Answer 1

The loop

for (auto i: AVLTree) { ... }

tries to make a copy of each element of the range in AVLTree.begin() and AVLTree.end(). Of course, std::unique_ptr<T> can't be copied: there is only one std::unique_ptr<T> to each pointer. It wouldn't really copy anything but rather steal it. That would be bad.

You want to use references instead:

for (auto& i: AVLTree) { ... }

... or, if you don't modify them

for (auto const& i: AVLTree) { ... }