C++ unique_ptr and map

Question

I'm trying to use the C++0x unique_ptr class inside a map like so:

// compile with `g++ main.cpp -std=gnu++0x`

#include <string.h>    
#include <map>
#include <memory>

using namespace std;

struct Foo {
    char *str;    
    Foo(char const *str_): str(strdup(str_)) {}
};

int main(void) {
    typedef std::map<int, unique_ptr<Foo>> Bar;
    Bar bar;
    auto a = bar.insert(Bar::value_type(1, new Foo("one")));
    return 0;
}

However GCC gives me the following error (shortened, I think this is the relevant part, please test on your own C++ compiler):

main.cpp:19:   instantiated from here
/usr/include/c++/4.4/bits/unique_ptr.h:214: error: deleted function ‘std::unique_ptr::unique_ptr(const std::unique_ptr&) [with _Tp = Foo, _Tp_Deleter = std::default_delete]’
/usr/include/c++/4.4/bits/stl_pair.h:68: error: used here

I'm really not sure what I've done wrong, this works on MSVC. I have found very similar questions, that seem alike, however their solutions do not work for me.

matt@stanley:/media/data/src/c++0x-test$ gcc --version
gcc-4.4.real (Ubuntu 4.4.3-4ubuntu5) 4.4.3

Answer 1

First: Your class Foo is really a very bad approximation of std::string. I predict lots of memory leaks. (Search for "rule of three".)

Second: A pointer is not implicitly convertible to a unique_ptr object (for safety reasons). But the templated pair constructor requires the arguments to be implicitly convertible to the respective value types. GCC seems to allow this but it is a bug. You have to create the unique_ptr object manually. Unfortunately the C++0x draft lacks the following, very useful function template that eases the creation of temporary unique_ptr objects. It's also exception-safe:

template<class T, class...Args>
std::unique_ptr<T> make_unique(Args&&... args)
{
    std::unique_ptr<T> ret (new T(std::forward<Args>(args)...));
    return ret;
}

In addition, let's use the new emplace function instead of insert:

auto a = bar.emplace(1,make_unique<Foo>("one"));

emplace forwards both arguments to the corresponding pair constructor.

The actual problem you witnessed is that the pair constructor tried to copy a unique_ptr even though such an object is only "movable". It seems that GCC's C++0x support is not yet good enough to handle this situation correctly. From what I can tell, my line from above should work according to the current standard draft (N3126).

Edit: I just tried the following as a workaround using GCC 4.5.1 in experimental C++0x mode:

map<int,unique_ptr<int> > themap;
themap[42].reset(new int(1729));

This is also supposed to work in the upcoming standard but GCC rejects it as well. Looks like you have to wait for GCC to support unique_ptr in maps and multimaps.