I am trying to initialise an std::vector<std::unique_ptr<std::string>>
in a way that is equivalent to an example from Bjarne Stroustrup's C++11 FAQ:
using namespace std;
vector<unique_ptr<string>> vs { new string{"Doug"}, new string{"Adams"} }; // fails
unique_ptr<string> ps { new string{"42"} }; // OK
I can see no reason why this syntax should fail. Is there something wrong with this way of initializing the container?
The compiler error message is huge; the relevant segment I find is below:
/usr/lib/gcc-snapshot/lib/gcc/i686-linux-gnu/4.7.0/../../../../include/c++/4.7.0 /bits/stl_construct.h:77:7: error: no matching function for call to
'std::unique_ptr<std::basic_string<char> >::unique_ptr(std::basic_string<char>&)'
What is the way to fix this error ?
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_list
s always perform copies, and unique_ptr
s 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 string
s 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.