Why can I not push_back a unique_ptr into a vector?

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You need to move the unique_ptr:

vec.push_back(std::move(ptr2x));

unique_ptr guarantees that a single unique_ptr container has ownership of the held pointer. This means that you can’t make copies of a unique_ptr (because then two unique_ptrs would have ownership), so you can only move it.

Note, however, that your current use of unique_ptr is incorrect. You cannot use it to manage a pointer to a local variable. The lifetime of a local variable is managed automatically: local variables are destroyed when the block ends (e.g., when the function returns, in this case). You need to dynamically allocate the object:

std::unique_ptr<int> ptr(new int(1));

In C++14 we have an even better way to do so:

make_unique<int>(5);

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