Initializing container of unique_ptrs from initializer list fails with GCC 4.7

Question

I am trying to initialise an std::vector> in a way that is equivalent to an example from Bjarne Stroustrup\'s C++11 FAQ

Answer 1

After "fixing" your example:

#include <vector>
#include <memory>
#include <string>

int main()
{
    std::vector<std::unique_ptr<std::string>> vs = { { new std::string{"Doug"} }, { new std::string{"Adams"} } }; // fails
    std::unique_ptr<std::string> ps { new std::string{"42"} }; // OK
}

I got very a clear error message:

error: converting to 'std::unique_ptr<std::basic_string<char> >' from initializer list would use explicit constructor 'std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Tp, _Dp>::pointer) [with _Tp = std::basic_string<char>, _Dp = std::default_delete<std::basic_string<char> >, std::unique_ptr<_Tp, _Dp>::pointer = std::basic_string<char>*]'

This error tells us that it is not possible to use the unique_ptr's explicit contructor!

Answer 2

unique_ptr's constructor is explicit. So you can't create one implicitly with from new string{"foo"}. It needs to be something like unique_ptr<string>{ new string{"foo"} }.

Which leads us to this

// not good
vector<unique_ptr<string>> vs {
    unique_ptr<string>{ new string{"Doug"} },
    unique_ptr<string>{ new string{"Adams"} }
};

However it may leak if one of the constructors fails. It's safer to use make_unique:

// does not work
vector<unique_ptr<string>> vs {
     make_unique<string>("Doug"),
     make_unique<string>("Adams")
};

But... initializer_lists always perform copies, and unique_ptrs are not copyable. This is something really annoying about initializer lists. You can hack around it, or fallback to initialization with calls to emplace_back.

If you're actually managing strings with smart pointers and it's not just for the example, then you can do even better: just make a vector<string>. The std::string already handles the resources it uses.