why std::unique_ptr vector gets invalid pointer exception

Question

I wrote simple code to help me understand smart pointers:

string s = "str";
vector <unique_ptr<string>> pv ;

pv.push_back(unique_ptr<string>(&s));
cout<<*(pv[0])<<endl;

This code compiles fine, but gets me a runtime error:

str * Error in `...': munmap_chunk(): invalid pointer: 0x00007ffd956e57e0 * Aborted (core dumped)

What happened and what have I done wrong?

Answer 1

In the std::unique_ptr's destructor it will call delete on the &s pointer which was not allocated via new.

Just use:

std::vector<std::string> vector;
vector.emplace_back("str");
std::cout << pv[0] << std::endl;

There's no need for std::unique_ptr<std::string> there.

Answer 2

Your string is being destructed twice - once when your pv goes out of scope and is deleted, freeing all its contained unique_ptr elements, and once when s goes out of scope.

To use a vector of unique pointers (or to use unique pointers in general), it is essential that they are not aliased. So you could write:

auto *s = new std::string("str");
pv.push_back(std::unique_ptr<std::string>(s));
// do not write "delete s" anywhere...

Or, simpler and safer:

pv.push_back(std::make_unique<std::string>("str")); // make_unique is C++14

Or even:

std::unique_ptr<std::string> p{new std::string("str")};
pv.push_back(std::move(p));
// Do not attempt to use p beyond this point.