Trouble with std::map::emplace syntax

Question

I am trying to emplace data into a std::map. Below is what I have tried (trimmed from the original source but definitely gives the idea):

template<typename T> class trie {

private:
std::map<typename T::value_type, std::unique_ptr<trie<T>>> children;
std::unique_ptr<trie<T>> parent;

// Later
public:
trie(const trie<T>& other, trie<T>* const parent) :
parent{parent}
{
    for(auto const &it : other.children)
        children.emplace(it.first, {*it.second});
}

};

The error is as follows:

trie.h: In instantiation of ‘trie<T>::trie(const trie<T>&, trie<T>*) [with T = std::basic_string<char>]’:
main.cpp:7:23:   required from here
trie.h:90:3: error: no matching function for call to ‘std::map<char, std::unique_ptr<trie<std::basic_string<char> >, std::default_delete<trie<std::basic_string<char> > > >, std::less<char>, std::allocator<std::pair<const char, std::unique_ptr<trie<std::basic_string<char> >, std::default_delete<trie<std::basic_string<char> > > > > > >::emplace(const char&, <brace-enclosed initializer list>)’
   children.emplace(it.first, {*it.second});
   ^
trie.h:90:3: note: candidate is:
In file included from /usr/include/c++/4.8.1/map:61:0,
                 from trie.h:4,
                 from main.cpp:2:
/usr/include/c++/4.8.1/bits/stl_map.h:540:2: note: std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename _Alloc::rebind<std::pair<const _Key, _Tp> >::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::emplace(_Args&& ...) [with _Args = {}; _Key = char; _Tp = std::unique_ptr<trie<std::basic_string<char> >, std::default_delete<trie<std::basic_string<char> > > >; _Compare = std::less<char>; _Alloc = std::allocator<std::pair<const char, std::unique_ptr<trie<std::basic_string<char> >, std::default_delete<trie<std::basic_string<char> > > > > >; typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename _Alloc::rebind<std::pair<const _Key, _Tp> >::other>::iterator = std::_Rb_tree_iterator<std::pair<const char, std::unique_ptr<trie<std::basic_string<char> >, std::default_delete<trie<std::basic_string<char> > > > > >]
  emplace(_Args&&... __args)
  ^
/usr/include/c++/4.8.1/bits/stl_map.h:540:2: note:   candidate expects 0 arguments, 2 provided

So my question is:

How do I correctly initialize the map element, the goal being a deep copy of the pointed-to trie, and no needless copies/moves?

Thanks in advance!

Answer 1

By passing {*it.second} as the initialiser for the value, you're effectively trying to initialise a std::unique_ptr<trie<T>> with a trie<T>. I believe you're looking for this:

public:
trie(const trie<T>& other, trie<T>* const parent) :
parent{parent}
{
    for(auto const &it : other.children) {
        // Separate creation of unique_ptr for exception safety, thanks to @DanielFrey
        std::unique_ptr<trie<T>> p(new trie<T>(*it.second));
        children.emplace(it.first, std::move(p));
    }
}

Note that you will also have to provide a copy constructor, because the default one is deleted, as your class has non-copyable members.

---

Unrelated to the question, but you should reconsider your design: you most likely have an ownership loop. If a trie<T> stores a unique_ptr to its children and these store a unique_ptr back to their parent, you'll get double deletion errors. Turn one of these (probably the pointer to parent) into a raw pointer. Raw pointers are fine for observing without participating in ownership.

Answer 2

You need

for(auto const &it : other.children) {
    std::unique_ptr<trie<T>> element(new trie<T>(*it.second));
    children.emplace(it.first, std::move(element));
}

to prevent a resource leak in case an exception is thrown from emplace. If available (C++14), you could simplify the code to

for(auto const &it : other.children) {
    children.emplace(it.first, std::make_unique<trie<T>>(*it.second));
}

As a rule of thumb for all smart pointers, you always use std::make_* or you must use a separate line to create each of them.