Why no emplacement iterators in C++11 or C++14?

Question

C++98 has front_inserter, back_inserter, and inserter, but there don\'t seem to be any emplacement versions of these in C++11 or draft

Answer 1

Your main use case is already covered by inserter, back_inserter and front_inserter. There is already a value_type && overload of operator= that will move into the container. The only thing emplacer could do over inserter is call explicit constructors.

Compare the common overloads of container::insert, container::push_back and container::push_front to container::emplace, container::emplace_back and container::emplace_front

iterator insert( const_iterator pos, const value_type & value );
iterator insert( const_iterator pos, value_type && value );

template< class... Args > 
iterator emplace( const_iterator pos, Args&&... args );

void push_back( const value_type & value );
void push_back( value_type && value );

template< class... Args >
void emplace_back( Args&&... args );

void push_front( const value_type & value );
void push_front( value_type && value );

template< class... Args >
void emplace_front( Args&&... args );

Each of the emplace variants takes a pack of arguments with which to construct the value. operator = takes exactly one argument. You could write an emplacer that took a tuple of arguments.

template
class back_emplace_iterator : public std::iterator< std::output_iterator_tag,
                                                   void, void, void, void >
{
protected:
    Container* container;
public:
    typedef Container container_type;

    explicit back_emplace_iterator(Container& x) : container(&x) {}

    template
    back_emplace_iterator&
    operator=(std::tuple args)
    {
        std::apply(Container::emplace_back, std::tuple_cat(std::tie(*container), std::forward>(args)));
        return *this;
    }

    back_emplace_iterator& operator*() { return *this; }
    back_emplace_iterator& operator++() { return *this; }
    back_emplace_iterator& operator++(int) { return *this; }
};